Carbohydrate

Hello, and welcome back. Today, we are going to investigate the role of dietary carbohydrate in sport nutrition. We will look a little bit at some of the sources of carbohydrate and how they're used. We will also talk about some of the recommendations for carbohydrate intake leading up to exercise, during exercise, and following exercise.

We know that carbohydrate is important during exercise because it is both a preferred fuel source for the brain as well as a substrate for the muscle to use. And as a result of these two functions of carbohydrate, it is very important for the performance of prolonged exercise, also intermittent intensity exercise, and high-intensity exercise—all situations where carbohydrate availability is necessary and can become challenged by the conditions of the exercise itself.

Now, because it is the most important fuel for the brain, not only may there be physical manifestations of fatigue that are both related to its lack of availability for brain and muscle, but specific to the brain, we know that a lot of sports are going to have a strong cognitive and motor skill component that are going to be affected by carbohydrate availability. We limit the brain's source of energy; well, then anything that involves coordination or decision-making is also going to be affected, outside of the potential, any potential metabolic effects of limiting carbohydrate availability. And as a result, there have been various strategies that have been used to optimize carbohydrate availability during exercise, and we'll review what some of those strategies are in the pre, intra, and post-exercise conditions.

First, let's take a look at where we store carbohydrate, primarily in the form of glycogen. And we alluded to this previously in our first lecture, but we know that we store glycogen both in the liver—and the primary function of liver glycogen is to maintain blood glucose—as well as in the muscle, and the primary function of this muscle glycogen is to supply energy during muscle contractions. And on a subsequent slide, we'll look at physiologically what makes this distinction between muscle and liver glycogen in terms of the different roles that it plays in glucose metabolism. But let's take a look at how much of each we have.

Now, depending upon diet and training, a normal skeletal muscle glycogen concentration could be anywhere between 300 and 600 grams total in a human being. Now compare that to liver glycogen; we're in absolute terms, we have, we have much less—about 80 to 110 grams. But remember, if we're talking about concentration though, a concentration of glycogen in the liver is higher than it is in skeletal muscle, and this is because the liver is a much smaller organ than skeletal muscle is when we add up all of the contents of skeletal muscle across all the muscles of the body. So, in absolute terms, there's more in skeletal muscle, but from a relative standpoint, the liver has a higher glycogen concentration.

Now, at rest, when we look at the contribution of these two stored carbohydrate sources to energy, we see that first of all, tissues such as the brain—again, because blood glucose, which is far more limited in its capacity, and we'll take a look at that later compared to these forms of stored carbohydrate—the tissues such as the brain use blood glucose at a rate of about 0.1 grams per minute, even at rest. And so, if we look at after an overnight fast, because the breakdown of liver glycogen is one of the primary ways in which the liver is able to help maintain blood glucose, if we looked at someone after a period of sleep with no energy intake, liver glycogen levels can fall below 20 grams—so pretty close to depleted considering that in the post-absorptive state we have about 80 to 110 grams of liver glycogen available. So, just by going to sleep and not eating anything during that period of time or eating anything overnight, liver glycogen levels can be significantly impaired upon awakening after an overnight fast.

Now, when we look at the effect on muscle glycogen content, muscle glycogen content is largely unaffected at rest, so it doesn't contribute very much to, doesn't contribute at all to blood glucose maintenance, and we'll take a look at why that is. Now, the reason that liver glycogen is able to directly contribute or be broken down and released into the bloodstream to supply blood glucose has to do with the fact of what happens in the cytosol when glucose or glycogen is broken down. Now, in order for this breakdown to happen, we know that either glucose or glycogen must be phosphorylated in order to form glucose 6-phosphate. If we remember back to our overview of glycolysis and carbohydrate metabolism, we know that a molecule of glucose, via the hexokinase reaction, has to be phosphorylated and form this G6P molecule in order for it and continue to undergo the rest of the glycolytic pathway. And we also know that when we break down glycogen, it, although via a different enzyme, needs to ultimately still be converted into glucose 6-phosphate before it can undergo further breakdown.

Now, in muscle, when this G6P is formed, either from glucose or glycogen, it's irreversible, and this kind of traps this molecule inside the cell. So, this glucose 6-phosphate can't be converted back to glucose so that it can be then released into the bloodstream and used to supply blood glucose, and that's because it lacks the enzyme glucose-6-phosphatase. And this is the primary reason why muscle glycogen can't contribute to maintaining blood glucose. However, if we look at the liver, the liver does have the enzyme glucose-6-phosphatase, and again, this glucose-6-phosphatase enzyme is able to convert glucose 6-phosphate to glucose, so it could reverse the hexokinase reaction, or it can take the glucose 6-phosphate that is created from the breakdown of a molecule of glycogen and convert it to glucose so that it can then enter the bloodstream. And so, this allows liver glycogen, as opposed to muscle glycogen, to be broken down into glucose and enter the circulatory system and contribute to blood glucose level.

Now, of course, muscle glycogen, because it's located within the muscle, is a very important fuel source during exercise, and we started to investigate this a couple of weeks ago, but again, we'll remind ourselves that the contribution of muscle glycogen to energy expenditure during exercise is a function of both the intensity as well as the duration of which that exercise is performed. So, if we looked at prolonged exercises of low to moderate intensity, well, the rate of oxidative phosphorylation of both carbs and fats is able to provide a lot of the energy or most of the energy during these more prolonged bouts of low to moderate intensity exercise, and as a result, the rate of muscle glycogen utilization during these exercises is able to sustain and provide energy for between 2 and, you know, above 2 to 3 hours of performance, depending upon, of course, the specific intensity of the training session as well as the individual's diet leading up to the actual training session or competition.

Now, if we compare that to sustain higher intensities of exercise—something like an hour at 83% of your VO2 max or intermittent bouts of exercise that involves supramaximal intensities or over 100% VO2 max efforts interspersed with some recovery intervals—we see that the rate of glycogen utilization during these efforts is much more sharp than it is in our more low to moderate prolonged efforts. And the reason for this accelerated rate of muscle glycogen breakdown is because that rate of oxidative phosphorylation now can't meet that energy turnover at a high enough rate by itself. And so, what do we do? We have to dramatically increase the rate of muscle glycogen utilization because we else we now need to recruit a greater amount of energy from our anaerobic pathways, and we know that the majority of anaerobic energy delivery after PCR hydrolysis has sort of reached its capacity, which happens in a pretty short amount of time, that most of that is actually going to come down to the breakdown of carbohydrate, specifically from the breakdown of muscle glycogen.

Now, if we take a look at the utilization of liver glycogen as a fuel source during exercise, we'll see that liver glycogen is not only important at rest but also is related to exercise intensity and duration in terms of its contribution as a fuel source. So, if we take a look at this study from Golic, it looked at hepatic glucose output—so that's the amount of carbohydrate that's being output from the liver in order to help maintain blood glucose during exercise—and we'll remember that this is not just a function of glycogenolysis or the breakdown of stored carbohydrate in the liver, but it is a product of that process as well as the process of gluconeogenesis or making new glucose. And this is distinctly different from the breakdown of liver glycogen in that in during this process, the liver is making new glucose from other substrates. Now, these substrates can include lactate—so some of the lactate that is produced in skeletal muscle during exercise can be shuttled to the liver and used to make glucose—the glycerol backbone from fatty acids can be used to make glucose, pyruvate, as well as several other amino acids; alanine and glutamine are just two examples. And we'll talk in our unit on amino acid metabolism on which amino acids are specifically gluconeogenic and released into the bloodstream by the liver.

So, when we're looking at hepatic glucose output, we're looking at a combination of both liver glycogenolysis or the breakdown of liver glycogen as well as the process of making new glucose. And what we can see if we just take a look at the relationship between intensity and duration on hepatic glucose output, we can see that we don't, we don't increase the hepatic glucose output in lower or more moderate intensity exercises too much beyond that resting value of 0.1 grams per minute. We might see an increase to somewhere around 0.3 grams per minute in these more low to moderate intensity bouts, and as you can see here, this duration is only extended out for about 40 minutes. In order to see a significant increase at these intensities in hepatic glucose output, we would need to extend the duration of work at these intensities beyond a couple of hours, depending upon the intensity, so that we would then deplete our muscle glycogen, begin to dip into blood glucose, and then need a significant, we increase the output of carbohydrate coming from the liver.

However, if we look at higher intensities of exercise, i.e., those 75% of VO2 max or above, we take a look at what happens at 85% of VO2 max, we start to substantially increase the output of glucose from the liver as the intensity and the demand on carbohydrate metabolism begins to increase. And not only does the overall rate of liver glucose output change compared to rest when we're looking at high-intensity exercise, it also changes the relative proportion of which of the two components of hepatic glucose output are, are, are contributing to the overall output from the liver. So, if we look at rest, about 60% of liver glucose output at rest is coming from the breakdown of liver glycogen as compared to 40% of hepatic glucose output is coming from making new glucose or gluconeogenesis. Now, if we look at these exercise intensities above 75% of VO2 max, where we start to get some significant increases over time in the rate of hepatic glucose output, we will see that at these higher intensities, not only does this rate of hepatic glucose output increase significantly, it also taxes liver glycogen to a greater extent than liver gluconeogenesis. So, compared to rest, where 60% of liver glucose output is coming from the breakdown of glycogen, now 90% of liver glucose output at higher intensity is coming from the breakdown of liver glycogen, and significantly less so, 10% compared to 40%, is coming from the process of making new glucose or gluconeogenesis.

So, this is important because during exercise at these intensities, it doesn't take a whole lot of time—in an hour or less above 75% VO2 max—this could drop liver glycogen stores by 50%. If they were completely chopped off, then obviously this would be even more of an issue if, for some dietary reason, liver glycogen was depleted or if it was depleted due to a previous training session without being replenished.

Now, remember that one of the primary purposes of this is to regulate the concentration of glucose in the blood, and I think it's useful to use the analogy of blood glucose as a sink or reservoir that we have available to us, and that sink likes to be maintained in a pretty tight fashion. So, we see that a euglycemic blood glucose concentration would be about 4 to 5 millimoles per liter, and we like to stay in that particular range. And then, when we fall below about 3 millimoles per liter in the sink, well, the, the sink can be drained, so to speak, and the draining of this sink becomes more pronounced when tissues such as exercising muscle increase the demand and uptake of blood glucose during exercise. If we think about the way in which the liver responds to this, when blood glucose concentration drops, the liver releases glucose—either the breakdown of glycogen or the production of new glucose—and this process in the liver is primarily regulated by the hormone glucagon. So, when glucagon is released from the alpha cells of the pancreas and goes into the liver via the hepatic portal vein, that increase in the release of glucagon signifies that blood glucose concentration is dropping and that we need to produce new glucose and release it into the bloodstream or break down glycogen and convert it to glucose and release it into the bloodstream. And conversely, if the demand for glucose is less, then the release of glucagon will be blunted, and in this particular case, the liver will either produce less glucose, or if there are significant increases in blood glucose concentration following something like a meal, there might even be enough glucose available for the liver to not only begin producing and output less glucose but also to begin resynthesizing its own stores of glycogen following such a meal. Blood glucose concentrations, provided that carbohydrates were a part of that meal, are going to increase depending upon the amount and type of carbohydrate that are ingested. And so, this is a good thing because this is another way that we can kind of stop the sink from draining, so to speak, is to ingest carbohydrates, digest them, absorb them, and add them to our kind of glucose reservoir.

So, if we eat a meal, what, glucose is going to increase, and as a result of that increase, the pancreas is going to release the hormone insulin. And so, in this particular state, insulin becomes the most important gluco-regulatory hormone, and it increases following the meal, and its effects are to increase the uptake of carbohydrate in the form of glucose by target cells such as the muscle and the liver. The other effect that this has is it will increase the activity of glycogen synthase, and glycogen synthase is the primary enzyme that is responsible for regulating the storage of glycogen. So, insulin will increase the uptake of glucose by these tissues following this meal, and given that we have more carbohydrate available to us, we now have an increase in the storage of glycogen by increasing the activity of glycogen synthase. We also send signals to break down less glycogen, so the primary enzyme involved in glycogen breakdown that regulates it is glycogen phosphorylase, and the activity of this enzyme decreases in this state so that we're not only storing more glycogen but we're breaking down less glycogen.

Now, during exercise, a different situation entirely happens, and this is due mainly to the release of catecholamines that result from exercise. So, we know that exercise will increase the release of hormones such as epinephrine and norepinephrine, and that these actually have a counter-regulatory effect on insulin—i.e., when you increase catecholamine release as you do during exercise, the release or the secretion of insulin by the pancreas is reduced. Now, you might ask yourself, wouldn't this effect, wouldn't it be a bad thing during exercise? And it turns out that it's not because the uptake of glucose during exercise is enhanced dramatically independent of insulin, and we think of this process as contraction-stimulated glucose transport. So, muscle contractions in and of themselves have an insulin-like effect, and this is what we refer to many times as the insulin-like effect of exercise. So, this reduction in the secretion of insulin that is produced by the release of catecholamines is somewhat protective in that muscle glucose uptake is enhancing dramatically, and if we have the insulin-like effect of insulin occurring during this dramatic increase in contraction-stimulated transport, we may increase the risk that an acute bout of hypoglycemia could occur during that exercise. And in an inverse fashion to what happens at rest to the enzymes that regulate the storage of carbohydrate and break, storage of glycogen and breakdown of glycogen, we see the opposite happens during exercise. So, given that now we're in a state of increased glucose demands, we signal a decrease in the activity of the enzyme glycogen synthase, so we store less glycogen, and then we favor the breakdown of glycogen by increasing the activity of glycogen phosphorylase.

I think we've done a decent job of establishing all of the ways in which exercise can potentially challenge carbohydrate availability and how that could be potentially limiting to performance. And as a result of these extensive physiological mechanisms, there have been a lot of research, there has been a lot of research done on strategies to provide adequate carbohydrate or enhance its availability during exercise and performance, and we're going to review what some of these strategies are in relation to the types of exercise that provide the largest challenges to carbohydrate availability. And these strategies include high-carbohydrate diets in the time leading up to competition. We'll talk about this in relation to the days leading up as well as in the hours before exercise or bout of performance. We'll also discuss the practice of ingesting carbohydrate during these types of exercise that challenge carbohydrate availability, and then, of course, we'll also talk about ingesting carbohydrate in the period of time immediately following this type of exercise.

Let's take a look at one of the most significant challenges to carbohydrate use and replenishment, and this occurs during periods of time where consecutive days of intense training or consecutive days of intense competition, and in these types of sports and exercise that challenge carbohydrate availability need to occur. And the key here is that the main thing that's going to be the biggest challenge is replenishing all of the muscle glycogen that is getting used during these intense training or competition periods. So, let's take a look here at one of the first studies to begin investigating this issue. Now, what this study looked at in trained runners was a period of 3 days where the individuals were completing a daily run, and this was for time, so they were attempting to run 16.1 kilometers in the least amount of time possible. So, this would be something like a training camp. And in this particular study, they looked at feeding a low-carbohydrate diet. Now, this was a pretty old study, and so this would not fit our current definition of a low-carbohydrate diet; really, this would be more of a moderate intake, and the diet they were fed was about 40 to 50% of the total energy or the total amount of calories that they ate per day were coming from carbohydrate, and that's a pretty moderate or normal rate of or contribution of carbohydrate to energy intake, so we wouldn't really consider that low; we would consider it more moderate and more reflective of what an individual might normally do. And then, in the second condition—this high-carbohydrate diet—they increased the relative proportion of energy coming from carbohydrates to 70% of carbohydrates. So, this would be the high-carbohydrate, the high-carbohydrate condition. And if we look at the results in terms of muscle glycogen—so this would be the overall muscle glycogen concentration that was measured—and what we find here is if we look at what happened to the moderate carbohydrate group over this three-day period, we can see that, as we would expect, the runs cause significant usage of muscle glycogen, so we see a dip in muscle glycogen with each run, but that they weren't able to increase their muscle glycogen from this point with each run. So, in this condition, they continually depleted their muscle glycogen to lower and lower rates which, with each of these three runs, until near depletion by the second day at the end of that run. And then, each day they're able to, they're not able to replenish their muscle glycogen even to the levels that they went into the previous run with. So, there's this persistent decline in muscle glycogen concentration that can't be accounted for, can't be diminished when eating this more moderate carbohydrate diet in these individuals. If we compare that to what happened in the group who was fed a high-carbohydrate diet, we can see that while they were still using about the same amount of muscle glycogen per run as the individuals who consumed the lower carbohydrate diet, they were able to replenish back up to close to their starting values and were able to do so on each successive day following the run. So, these individuals were able to maintain their muscle glycogen and replenish it within the 24-hour period following each one of these training sessions. Now, this has also been shown in sports like soccer where there is a lot of intermittent high-intensity and moderate-intensity aerobic exercise that occurs over the course of a 45-minute half. And so, in this

Study: They put two different groups of soccer players into again a high carbohydrate diet, similar to the one that we described in the previous study on runners, and a normal carbohydrate diet. They looked at their muscle glycogen concentration. So they took a muscle biopsy of their—I believe it was their vastus lateralis—and they looked at the muscle glycogen concentration before the game. Now remember, soccer is also a sport where there could be multiple competitions happening on consecutive days, and so this would be another example of, like, an intensified training period. They looked at their glycogen concentration before the game, at halftime of the game, and at the end of the game. And we can see that a couple of things are true here. First of all, the soccer players, similar to the runners, had a lower muscle glycogen concentration to start before the game. That they were measured at halftime; they had significantly higher—the high carbohydrate group had significantly higher amounts of muscle glycogen left available to use in the second half. And by the end, they were both—they were both depleted. However, if we look at this effect in terms of their running performance in the first and second halves, well, they covered pretty similar amounts of distance; there wasn't that much difference between the carbohydrate groups, although there was a slight difference, a slight increase in distance covered in the high carbohydrate group, but not compared to when we look at what happened in the second half. The high carbohydrate group was able to cover about 1.2 miles more ground during the second half when compared to the high carbohydrate group or when compared to the normal carbohydrate group. And if we look at also the percentage of the game that they spent either walking or sprinting—now again, this is controlling for position on the field; obviously we know that the different positions in soccer cover different amounts of distances and have different requirements in terms of walking and sprinting—but we can see that the high carbohydrate group was able to spend significantly more time near their maximal running speed when compared to the normal carbohydrate group. So all in all, this kind of diet during an intense period of soccer competition was shown—the high carbohydrate diet was shown to preserve muscle glycogen. They went into the game with higher levels of muscle glycogen; they had higher levels at halftime; and they were able to cover more distance and spend more time working at high intensities later in the game.

Another practice that is common in endurance performance specifically is this idea of carbohydrate loading, or attempting to fully load up on muscle glycogen in the days leading up to an important competition. And this has been shown to be an effective strategy in improving performance in these types of sports. For example, if we were to measure someone's time to exhaustion at a given submaximal intensity after a period of glycogen loading, we could see improvements in performance up to 20 percent. So this is a potent tool in attempting to improve performance, especially for these longer duration endurance activities. Now if we look at what the model of Kawhi Kajin—super compensation or classical glycogen super compensation—looks like, we can see that there are a few different elements of this in relation to the amount of carbohydrate that is fed in the diet as well as the volume of training that is performed in the week leading up to an important competition. And so if we take a look at the very first day of this approach, we would see that the first thing that would happen is we would perform a high-volume training session that would be meant to deplete muscle glycogen—so bring those muscle glycogen stores as close down to zero as possible. Now, second phase of this would also involve—after this exhaustive bout of training—we would then feed very little carbohydrate in the three days that follow that exhaustive training bout. We would see this typically represented—again in studies that have looked at this—at this have looked at this as a percentage of energy intake. So we're talking about about 25 percent of these athletes' diet coming from carbohydrates, which is be considered pretty low. So there's a three-day period of low carbohydrate intake that follows this exhaustive bout of training. And additionally, we might perform another bout of training; ideally it's—well, it's not going to be at the same volume because they're not going to be able to perform as well given that their muscle glycogen is depleted. And the point of these—this initial phase here—is to completely deplete muscle glycogen and then kind of starve our muscle cells out of glycogen by not replenishing it or not feeding enough carbohydrate in the diet for there to be a significant increase in muscle glycogen stores. Now what we do in the final three days leading up to competition using this period or using this classical glycogen super compensation method is begin to taper training all the way down to—to rest. So in these three days leading up to the competition, we would then—we would then completely decrease the volume of training down to almost nothing or just resting energy requirements, and then begin to feed a high carbohydrate diet—so something like 70% of overall energy intake in the three days leading up to a competition. And of course, on the day of the competition, our carbohydrate intake and associated training volume would be ideally high. Now these strategies have been shown to be effective in producing super high concentrations of muscle glycogen. So if normal feeding strategies produce somewhere along the lines of 300 to 400 grams of muscle glycogen, these strategies can increase muscle glycogen stores up to 600 grams—so up towards the upper end of what we know muscle glycogen stores can go up to. But regardless of the fact that these strategies can increase muscle glycogen stores and provide significant improvements in performance, there are also several problems that athletes can encounter when using this classical model of super compensation. So I want you to take a moment and see if you can think of what some of these problems might be. Hopefully you've been able to identify what some of the potential problems that our athletes might encounter when they adopt the super compensation method might be. So I'm going to provide you with some examples of some of the most commonly reported problems that athletes have with this. And the first is due to bouts of hypoglycemia during the low carb period. If you remember, during that period of reduced carbohydrate intake down to 25% of energy intake, they're still being asked to perform an exhaustive bout of training on the day before they super compensate or increase their carbohydrate intake, and this can increase the risk of injury during training. We know that the risk of injury will increase as hypoglycemic episodes occur because of the limited supply of blood glucose for the central nervous system. And we also then, in a related concept, know that the athletes' mood could be disturbed—so they may just report feeling not well during this period of low carbohydrate intake, especially if they are not previously adapted to periods of low carbohydrate intake. The second major issue that athletes report with these diets are the extreme preparation changes, especially in this short week-long period of time that occur when they shift from a 25% carbohydrate diet to a 70% carbohydrate diet. They have some practical issues in—in going back and forth between preparing these extreme diets. One of the things that you probably thought about the most, especially given that we just talked about gastric emptying and digestion and absorption and—and training the gut specifically, are the gastrointestinal issues that could occur and that these could occur during both the low carb and high carb phase of the classic super compensation model. So during the low carb period, they might experience issues with adjusting to eating more fats and proteins in order to maintain their energy requirements. They may also have issues tolerating higher than normal carbohydrate intake during the period of time in which they're supposed to be loading up or increasing their relative intake of carbohydrate. And then potentially the most concerning factor and probably the one that is the most often reported is that as we tend to get tense when they are not training. And if we think back or look back at the volume of training during this super compensation protocol, there are several days—three days before training specifically are before competition specifically—where we're asking them to not train at all. And this idea is taboo when you're talking about individuals who are used to training nearly every day at moderate to high volumes.

So for all of these reasons, researchers sought to find out if a more moderate glycogen loading or carbohydrate loading protocol could produce similar improvements in performance without the problems of classical super compensation. And so this would be what a more moderate protocol would look like. And if we relate that protocol to the classical model of super compensation, we notice a couple of key differences here. And the first is related to the feelings of mental tenseness from not training or just all of a sudden going from training to hardly training at all. Is this gradual reduction in training volume in the days leading up to the competition. So rather than asking the individual to perform a super high depleting training session at the beginning of the week and then go down to nothing and then keep it at nothing for three days leading up to the competition, this is a little bit more easy to tolerate in that it involves a gradual reduction in training volume leading up to the competition rather than a stark change in training volume. The same could be said for the approach to carbohydrate intake. So rather than going through the periods of extremely low carbohydrate intake—around 25% of total energy—this approach favors going from a more moderate intake or around 50% of energy intake and gradually stepping that carbohydrate intake up to around 70% over the course of the given week. Now when researchers have looked at the effects of these approaches on muscle glycogen stores, it seems that this is indeed a method that can achieve a similar improvement in glycogen stores without the problems of super compensation. So while the classical method of glycogen super compensation works, it's not typically recommended because the problems seem to outweigh the benefits. And we can still get the benefits by producing or engaging in a more gradual tapering of training volume and gradual increase in carbohydrate intake over the course of the week leading up to competition. So this would definitely be the more recommended method if topping off muscle glycogen is indeed going to be important for a given competition or performance.

So given that the total energy requirements for performing a given amount of training or sport are going to be different depending upon the activity, depending upon the individual, depending upon how intense the period of their training and preparation currently is, we could be limited if we are expressing carbohydrate recommendations as a percentage of total daily energy intake, although this is still a common practice. And to kind of get around this when we look at total daily carbohydrate recommendations for athletes, modern carb recommendations are given relative to the individual's body mass. So we can see here four different types of training that the carbohydrate recommendations are given per kilogram of body weight per day, and they're also given relative to the overall volume of training. So as when we go from lower volumes of training to more moderate or high or extreme volumes of training, the recommended amount of carbohydrate per kilogram of body weight per day increases, and it tops out in these scenarios where we have four to five hours a day of daily training at moderate to high intensity exercise, where the recommendation is in upwards of 10 to 12 grams of carbohydrate per kilogram of body weight per day. So I'd like to now start looking at some specific time points relative to carbohydrate intake on the day of an important competition or a hard training session, such as the ones we've described. So let's look at a scenario here. I want you to think about a triathlete who wakes up about 5 hours before the race on race day. Let's say they got a solid eight hours of sleep the night before and that they haven't eaten anything in the last 12 hours. What I'd like you to think about is which source of carbohydrate is most likely to be depleted and why is that source of carbohydrate most likely to be depleted. So take a moment, pause the video if you need to, and think about those two particular questions. So if we think about our two sources of stored carbohydrate primarily being liver glycogen and muscle glycogen, if we think about which is most likely to be depleted in this scenario, glycogen from the liver is most likely to be depleted. And if we think about physiologically why that is, remember that the primary goal of breaking down liver glycogen is to maintain blood glucose, and we know again that the brain uses blood glucose at a rate of about 0.1 grams per minute, and that it's possible that after a period of 12 hours of fasting that that liver glycogen could be below 20 grams, or nearly depleted. And so the primary function of this meal upon awakening would be to replenish that liver glycogen. And so what in terms of the recommendations that we would make for eating a meal at this time would be to consume a fairly large meal in this time period. So in this three to five hour period before the intense competition—3 to 5 hours out—you would want to eat a fairly large meal, and this would probably be your last large meal before the race itself. Now again, in terms of how large that meal would be, it's going to be very dependent upon the individuals' energy requirements, their resting metabolism, their the metabolic costs of whatever it is they're getting ready to do, and any other individual factors that may affect their—their food intake. We obviously would want that meal to be very carb rich, and if we look at what recommendations are available in terms of how carb rich the research would suggest that the meal should have about 80% or more of its nutrients coming from carbohydrate. So for most individuals, depending upon their body weight and the size of the meal that breaks down to about 140 to 330 grams of carbohydrate. So this would be an example of a meal that would fit those criteria. So let's say we had a banana with some sort of whole grain oatmeal or granola or a porridge made with some skim milk and a glass of orange juice. We would have a mix of complex and simple carbohydrates that would—that would be about 140 to 330 grams depending on the size of the portion, and 80% of that would come from—from carbohydrates. And if this meal is eaten within this three to five hour pre-race time period or pre-performance time period, this should function to restore liver glycogen levels, and it's possible, depending upon if glycogen—muscle glycogen levels are fully loaded with the diet and the days leading up to the competition or training session, where this could slightly increase muscle glycogen levels, but primarily because liver glycogen levels are likely to be low in this case, it's going to function to restore liver glycogen.

Now let's take a moment and shift our focus to the period of time right before a bout of performance, such as the ones we've been describing. Now let's look at our friend, a half-marathoner, and let's say that he's seeking some advice on carbohydrate intake in the half an hour to two hour period before his Saturday afternoon long run. I want you to think about three physiological issues here in regards to—in taking carbohydrate during this time period. The first is what would the effect of ingesting the carbohydrate at this time be on his blood glucose levels? The second is what would the effect of the onset of his training or exercise session be on his blood glucose levels? And the third: could the effects of the meal and the effects of the exercise on his blood glucose levels lead to any potentially negative outcomes? So take a couple minutes, pause the video, and see if you can think of what the primary issues with respect to these three questions would be in relation to carbohydrate intake during this time period. And now hopefully you took some time to think about those three issues. And let's start with what the effect of ingesting the carbohydrate would be on his blood glucose levels. And we know that when we ingest exogenous carbohydrate that the—what glucose level is going to increase as those carbohydrates are broken down and absorbed into the bloodstream. And as this happens, depending upon the rate at which they are released into the bloodstream, we know that insulin is going to be secreted from the pancreas in a proportional fashion to that rise in blood glucose in order to stimulate glucose uptake. So the insulin from the pancreas will attempt to increase the glucose uptake by tissues such as the working muscle. And then when we think about what will happen as a result of exercise, well, we're also going to now see blood glucose decrease even farther as glucose uptake is now going to be stimulated by contraction-mediated transport or stimulated by the demand on blood glucose from the working muscles. We know that independent of insulin release that muscle up—muscle contraction will stimulate glucose uptake. So we think about this as the insulin-like effect of exercise. So we have a situation here in which we're going to have the insulin-like effects of insulin occurring as well as the insulin-like effects of exercise, and combined this could produce a condition known as reactive or rebound hypoglycemia. So if we think about why that's occurring again, remember we have an elevated insulin response to the meal that occurred in this period of time as well as a increased muscle glucose uptake from the contraction-mediated transport of glucose. So combined, these could produce a rapid fall in blood glucose at the start of exercise. So being that we might still need to or want to eat a meal during this time period, are there any strategies that we can use that might help us avoid these negative metabolic effects if we still want to be able to eat something that has carbohydrates in it just before an exercise session? And there are several possible strategies that have been investigated and have been found to be effective at avoiding reactive or rebound hypoglycemia when consuming a meal that's close to a bout of exercise performance or consuming carbohydrates. And the first is related to keeping the size of the meal small; keep the overall energy content of the meal as small as possible, and consider even skipping the meal if you had time to fuel hours before exercise. Like in a situation we talked about, we're eating a large meal that was rich in carbohydrates in the three to five hours prior could help in terms of restoring liver glycogen, well then if you had time to do this, then you may not need to eat something in the hours—in the hour, half an hour just before exercise. The second thing that you can do in this time period is stick to more moderate-to-low glycemic index carbohydrates. And the reason for this is that those moderate glycemic index carbohydrates are going to produce a lower glucose and insulin spike relative to a higher glycemic index carbohydrate. Now you may even choose a low glycemic index carbohydrate; however, remember that we might want to try to balance the effects of the carbohydrate intake on the insulin level and glucose spike with the fact that the lower the glycemic index carbohydrate, the slower that it might get released and absorbed from the bloodstream. Another thing that can be helpful is to actually wait until closer to the period of exercise. Research suggests that waiting until 15 minutes or less before starting the exercise session may help avoid this metabolic outcome. And if we think about why that is, well, we have kind of a balancing in this case of the hyperinsulinemic effect of ingesting that carbohydrate with those hypoinsulinemic effects of exercise. So because we are going to blunt insulin release during exercise, which is stimulated by the release and catecholamines that occurs, well then if we wait until closer to the time of the exercise start, then we might be able to balance out some of the hyperinsulinemic effects of the carbohydrate intake with the hypoinsulinemic effect of exercise. Another recommendation is—potentially eating at this time point is something that you need to do or desirable—would be to add a small amount of fat to the meal. Now adding some fat to the meal—the important effect here is that in a similar regard to how ingesting some more moderate or lower glycemic index carbohydrates might help us slow that rate of absorption and limit the glucose and insulin spike—the same thing could be said if we add some fat to the meal. This might potentially increase the overall energy content slightly, so we wouldn't want to add a large amount of fat to the meal, but adding some might help us slow down the rate of absorption of those carbohydrates and cause a little bit less of an insulin spike. And last—we probably the most important recommendation that we can make from research that has been done on this—is to consume an adequate amount of carbohydrate during exercise. Now we're going to talk about what those recommended rates of carbohydrate intake are, but…

The vast majority of studies have found that when athletes ingest this carbohydrate or eat a meal immediately before a performance, that their blood glucose response is normalized by consuming adequate amounts of carbohydrate during the exercise session. So all of these, if it is desirable, or if a meal was needed in the 30 to 60 minutes prior, would be some strategies that would help avoid reactive or rebound hypoglycemia.

Now let's run a scenario that allows us to take a look at what the benefits of consuming adequate amounts of carbohydrate during exercise would be. Let's look at an athlete who competes in Spartan races—so long distances with some obstacles intermixed that may force us to intermittently shift between high and more moderate-intensity exercise for a prolonged period of time. And so she's looking—this athlete is looking for some advice regarding carbohydrate intake during her long training sessions and during this type of performance. So why might taking carbohydrate during her training and competition improve her performance?

Let's think about that from a physiological perspective and take a few moments and see if you can think of what some of the benefits might be. So research has shown us that there are several potentially advantageous physiological effects of ingesting carbohydrate during such a bout of performance. The first is that it may help us maintain our blood glucose levels and also maintain the rate at which we're able to use carbohydrates. The second is that, because of this increased carbohydrate availability via exogenous ingestion, we might spare some of our liver and muscle glycogen stores during exercise. It's possible that we may even be able to synthesize some of that glycogen during exercise rather than just burning through it. And we know that, as a result of helping maintain blood glucose levels, we should be able to maintain or improve motor skills as well as perceptual feelings and ratings of effort during this type of performance.

So there are a lot of reasons why ingesting carbohydrate during this type of performance would be beneficial. And as a result, if we look at all of the many, many studies that have been done on this issue, we see that numerous studies have shown that when endurance exercise, or exercise that involves moderate to high intensities and intermittent fashions, or our prolonged endurance exercise lasting 45 minutes or longer, one or more of the mechanisms outlined above are implicated in improvements in performance that are seen from ingesting carbohydrate during this type of exercise. So we know that ingesting carbohydrate during this type of performance is a strategy that can improve performance.

Are there any strategies that we can use to optimize how those carbohydrates that we ingest are used? Now there have been several factors that have been proposed to influence the rate at which these carbohydrates that are ingested during exercise can be oxidized and used. And one is relative to the rate of carbohydrate intake. And if we look at the rate of carbohydrate intake during exercise and we look at studies that have investigated ingesting single-source carbohydrates—so this is a figure that is representative of multiple studies that have looked at the oxidation rates of different sources of carbohydrate and how they affect the overall rate of exogenous carbohydrate oxidation when fed—so what is their—what is their peak rate of oxidation across multiple rates of ingestion? And the primary findings from these studies would suggest that, regardless of which type of carbohydrate you're talking about, the peak rate of oxidation for these carbohydrates when fed in single source is at about 1.1 grams per minute, regardless of the type of carbohydrate.

Now ingesting carbs above this rate, as we can see, won't provide any further enhancement in the rate at which they're able to be oxidized. However, we need to remember that these studies—again, we're looking at single-source carbohydrates—none of these carbohydrates or none of these studies looked at—studies are looked at carbohydrate mixes or using multiple types of carbohydrates, such as the strategies that we discussed in relation to improving the rate of absorption of carbohydrates. So if we think back to the types of carbohydrates and their oxidation rates, we know that carbohydrates like glucose, sucrose, maltose, glucose polymers are rapidly oxidized. We know that these are rapidly oxidized mainly because they all rely on sodium-dependent glucose transporters in terms of getting from the lumen into the epithelium. And we know that fructose and galactose are oxidized at much slower rates. So if we looked at the rate of fructose oxidation, it would be around 0.6 grams per minute as opposed to one or 1.1 grams per minute. And we know that fructose competes for a specific transporter, as where glucose, sucrose, maltose, and glucose polymers are all competing for the same transporter—that sodium-derived glucose transporter.

So this led to a line of questioning about the effects of combining multiple types of carbohydrates or multiple transportable carbohydrates—for example, a glucose plus fructose mixture. And if we look at studies that have examined the effects of feeding a single source of carbohydrate—so these blue bars are the overall rates of carbohydrate ingestion and how these affect the peak rates of carbohydrate oxidation—we see that if we feed a glucose and fructose mixture—so this would be about 60% glucose or 70% glucose and 30% or 40% fructose—if we look at the effects of ingesting the same amount of carbohydrate in multiple-source format versus in this single-source format, we see that, first of all, compared to a more moderate glucose load, just feeding more glucose doesn't improve the oxidation rate of those carbohydrates beyond what feeding a more moderate glucose load would. But if we take and feed that same amount of carbohydrate in the form of glucose and fructose, we see that we can significantly increase the rate of carbohydrate oxidation when using these two sources of carbohydrate in combination. And again, this is because we're using carbohydrates with different transport mechanisms or multiple transportable carbohydrates. And by doing this and taking advantage of these two different transport pathways rather than using carbohydrates that all compete for the same transporter, then we might get a better benefit or a higher overall rate of oxidation of those carbohydrates when using these two sources in combination.

When we look at modern carbohydrate intake recommendations during exercise, we'll see that there are a couple of things at play here, but primarily the duration of that—of the exercise or performance is the primary factor that's used to determine the rate of carbohydrate ingestion during exercise. So during bouts that last 30 minutes or less, typically these bouts are not limited or this type of performance is not limited by carbohydrate availability, so we don't see any benefit to ingesting carbohydrate during this—during these types of events. If we extend the duration out to between 30 minutes and an hour of performance, the intense—the relative intensity is probably going to be a little bit lower than it would be in these particular types of events that last less than 30 minutes, so we may use up some carbohydrate and we might see a small benefit from ingesting less than 30 grams per hour of carbohydrate or even just using something known as a carbohydrate mouth rinse. Now these mouth rinses are something that have come under investigation more recently, over the last 10 to 15 years. And if you're asking yourself what is a carbohydrate mouth rinse, well, that's what we're going to investigate in the research article that we're going to read for this week.

Well, when we extend out the duration of exercise to 1 to 2 hours, well, we start to get a significantly greater challenge to carbohydrate availability. And so during this timeframe, research would suggest that between 30 and 60 grams of carbohydrates, in either single or multiple transportable form, with the emphasis being on—on rapid—ovalle oxidizable sources, are going to be recommended for these types of events—so 30 to 60 grams per hour of a rapidly oxidizable source. However, when we start to extend into more extreme durations or get into events where we're going to prolong the duration of exercise for two and a half hours and beyond, it might be necessary or desirable to increase the rate of carbohydrate intake to between 60 and 90 grams per hour. Now, as we just investigated, if we go above this 60 gram per hour recommendation, this is where we're going to max out our—our pathways in terms of absorbing and oxidizing some of those more rapidly oxidizable carbohydrate sources, such as glucose. So therefore, if we're going to ingest more than 60 grams per hour, if that's needed due to long-duration performance demands, then it would be recommended that if we go up to say 90 grams per hour, that we would use multiple transportable carbohydrates. And in terms of the amounts of multiple transportable carbohydrates, we would want to look at a carbohydrate mix that would still be rapidly oxidized, such as a mix of glucose and fructose. So at a rate of 90 grams per hour, this would look something like 60 grams of glucose to 30 grams of fructose. And by using those multiple transport pathways, we potentially can provide an increase in the overall oxidation of that exogenous carbohydrate and deliver carbohydrate at a rate that is greater than what we would normally recommend for bouts of exercise that last less than two and a half hours.

Okay, so let's shift focus now to the period of time immediately after exercise. Now let's look at a ball sport athlete, a soccer player, who routinely undergoes intense aerobic training sessions of 45 minutes or longer and games that are 90 minutes or longer with alternating bouts of both high—so intermittent high-intensity and moderate-intensity running. And so this athlete is seeking some advice on ingesting carbohydrate after these types of aerobic training sessions or their soccer games where there is a high carbohydrate demand. So let's ask ourselves first what would be the primary goal of ingesting carbohydrate following these sessions. Now take a little bit of a pause and take a moment to think about that.

So if we think about what the primary goal would be, we know that, due to the intensity and duration of this session, that replenishing muscle glycogen would be of utmost importance—or to increase glycogen synthesis in this period of time immediately following the exercise bout or the soccer game. So what physiological factors might potentially affect the rate of glycogen synthesis? So take another moment of pause and see if you can come up with what some of these factors might be. We think about this period of glycogen synthesis or this period where we need to potentially resynthesize muscle glycogen; one of the factors that's going to be important is the availability of glucose. So the availability of carbohydrate is going to increase the rate at which we can synthesize glycogen by increasing its availability; so that would be one goal with consuming carbohydrate during this time period. The second thing that is going to affect our rate of glycogen synthesis is how able we are to transport that glucose into the cell. So once that glucose goes into our bodies and into our bloodstreams, we then need to absorb it into our muscle cells so that we can store it as glycogen. And then, of course, the activity of key enzymes—the primary one being glycogen synthase—are going to dictate our ability to store glycogen. So we would want to increase glycogen synthase activity or take advantage of increased glycogen synthase activity if replenishing muscle glycogen in the period of time after exercise is desirable.

So we think about what limits glycogen synthesis after exercise; first we have to think about what the rate-limiting enzyme it is, and that enzyme is going to be glycogen synthase. Now glycogen synthase exists in both active and inactive forms, and there are several physiological criteria that may dictate how much of it is expressed in an active versus an inactive form. And as you might have guessed, more of this glycogen synthase is active when muscle glycogen is—well, when the need to replenish muscle glycogen is greater, such as it would be following a bout of performance such as the one described in this scenario. And as stores are replenished or as muscle glycogen concentration starts to increase, well, then more of it is transferred back to its inactive form. And if we look at a period of time in the hours following exercise, we can see up to 80 percent of glycogen synthase enzymes in their active form—so a pretty substantial amount of that enzyme is activated in the hours following exercise. But this doesn't matter if we don't have carbohydrate available. So glycogen can only be formed if we have high enough levels of circulating carbohydrate available to us. And this carbohydrate, for it to be stored, we need to get it across the cell membrane, and the way that we do that in muscle is by translocating these GLUT4 transport proteins to the cell membrane. And we do this abundantly during the time period following exercise in order to facilitate glucose transport into the cell. So this is kind of a lingering result of the contraction-stimulated glucose uptake pathway; contraction sends more of these glucose—or proteins—to the cell membrane of the sarcolemma, and therefore, because more of those are present, we will have an enhanced ability to uptake glucose in the period of time following exercise. And this—this increased rate of glucose uptake and glycogen synthesis will only last a few hours if we don't then eat something to help extend it and augment the release of insulin from the pancreas. So this is kind of the critical window here of time that we're looking at in this few hours, and this will last longer again when the need for replenishing muscle glycogen is greater or when we have depleted muscle glycogen to a greater degree. And this glycogen synthesis will begin to slow down as the effect of contraction-mediated glucose transport from the exercise starts to wear off, although it will still be increased. So we have a few-hour period which we call—we will think of as the rapid phase of synthesis or that kind of critical window, and then we have a slightly slower phase of glycogen synthesis that persists in the hours after the exercise bout has been performed. So we have kind of a tight period of time here to take advantage of that rapid phase of glycogen synthesis.

So let's take a look at what variables might determine how successfully a post-exercise carbohydrate feeding will be in replenishing muscle glycogen. And the first thing, as you might have guessed, is related to the timing of carbohydrate ingestion after exercise. So this is a study that looked at the rate of glycogen synthesis and compared consuming carbohydrate immediately post-exercise versus waiting until two hours following the exercise to ingest that carbohydrate. And this study looked at the effects of glycogen synthesis in both what we would think of as the rapid phase of glycogen resynthesis—so the first two hours following the exercise—and then beyond that on the rate of glycogen synthesis in a period of time two to four hours after exercise. And what we can clearly see is that nearly four times the rate of glycogen synthesis was achieved when consuming carbohydrate immediately post-exercise compared to waiting two hours in this rapid phase or two-hour period immediately following the exercise bout. So we have a dramatic ability to—to synthesize glycogen in the rapid phase of glycogen synthesis when ingesting carbohydrate immediately following the exercise session rather than delaying that intake. And those effects start to level themselves out when we get farther away from the exercise bout as the contraction-mediated glucose transport mechanisms begin to wear off, and we start to see that the rates of glycogen resynthesis following this period of time start to—start to kind of level off.

The other factor to consider is the rate at which carbohydrates are in taken following exercise. So timing matters, but so does the rate. And so if we look at studies that have tested the effects of ingesting carbohydrates or in taking carbohydrates at increasing rates of intake in grams per minute and the effect on the ability to resynthesize muscle glycogen, we see that there's kind of a breakpoint here where anything above about one to 1.2 grams of carbohydrate per minute in the period of time following exercise doesn't really result in much of an increase in terms of the maximal rate at which glycogen can be resynthesized. So ingesting carbohydrates beyond this maximal rate doesn't really offer much of a benefit in terms of being able to resynthesize muscle glycogen.

Now finally, there have been several studies that have looked at the effects of ingesting both protein and carbohydrate together on rates of glycogen synthesis following exercise. And the reason for this is that, again, certain amino acids can be used gluconeogenetically or can be used to make new glucose, and these amino acids can also exert an effect on the secretion of insulin in combination with the increase in secretion of insulin that will happen from the ingestion of carbohydrates. So this led to a line of research in which they investigated the utility of adding carbohydrates and protein—or adding protein or amino acids to carbohydrate solutions at different rates of carbohydrate ingestion. So if we take a look at the first two conditions from this particular study—again, we're looking at glycogen synthesis rates here on the y-axis and then on the x-axis we have carbohydrates fed at a rate that would be below what the optimal rate of ingestion for carbohydrate would be during the post-exercise period—both with and without the addition of a small amount—so about 0.4 grams per kilogram of body weight per hour of protein—compared to rates of optimal carbohydrate ingestion—so 1.2 grams per kilogram of body weight per hour—with the addition of 0.4 grams per kilogram of body weight per hour of protein compared to a beverage that had only carbohydrates in it. And if we look at the results from these particular—this particular study—and this has been shown over multiple studies—we'll see that at rates below the optimal rate of carbohydrate ingestion, adding protein to the solution does provide a significant benefit in terms of the rate of glycogen resynthesis. We see nearly two times the rate of glycogen resynthesis occurring when carbohydrate is being ingested at suboptimal rates. But if we then look at what happens when we feed carbohydrate at a rate that would be commensurate with what would be considered optimal during this window of time, well then we don't see really a further benefit in terms of glycogen synthesis related to adding a small amount of protein to that recovery beverage. Now, of course, there are many other benefits of ingesting protein post-exercise that we will discuss further in our specific unit on protein and amino acid metabolism.

So if we summarize the recommendations for post-exercise carbohydrate intake in terms of maximizing glycogen resynthesis, we would suggest that about one to 1.2 grams per kilogram of body weight per hour are ingested in the zero to four hours following exercise. This will allow us to take advantage of that critical window and rapid phase of glycogen resynthesis and take advantage of the—the post-exercise increase in GLUT4 concentrations as well as the insulin-like effect of ingesting those carbohydrates. We would want carbohydrate in this zero to four-hour time period to be consumed early and often in order to take advantage of this rapid phase of glycogen synthesis following exercise. We remember looking at the study that suggested that ingesting carbohydrate immediately relative to delaying carbohydrate intake took significantly greater advantage of this window. In terms of the type of carbohydrates in this period of time, the recommendation is to favor higher glycemic index meals over lower glycemic index meals during this rapid phase of synthesis. And this is again to take advantage of both the insulin-like effect of exercise as well as the insulin-like effect of insulin. And lastly, the more intense and/or long the exercise session is, the more likely glycogen is to be depleted. And so in order to resynthesize muscle glycogen, matching our total daily carbohydrate intake in the fashion described previously when we looked at recommendations for total daily carbohydrate intake and how they change with training volume, we would want to make sure that overall we're ingesting enough daily carbohydrate to ensure that we are restoring muscle glycogen in the shortest period of time possible.

So that's it for this, and what I'd like you to do now is to take a look at the study that is posted on carbohydrate mouth rinses, and we will then look at whether or not these carbohydrate mouth rinses have the potential to improve performance or if there's any practical utility for a carbohydrate mouth rinse.