Transcription
Hello and welcome back to our sport nutrition lecture series. Today's topic is going to be on micronutrients. Some of the roles that vitamins and minerals play in healthy functioning, as well as some specific situations in which micronutrients and deficiencies of micronutrients may limit performance, and a final discussion of whether or not most athletes need to take micronutrient supplements or exceed the RDAs for certain micronutrients in order to achieve sufficiency and maintain health and performance.
So, before we discuss the specific roles of vitamins and minerals, remember that these are micronutrients because the relative content of these nutrients, relative to our macronutrients, are much smaller in terms of their presence and their needed levels of intake in the diet. And before we take a look at different requirements for micronutrients, we should look at some of the ways in which micronutrient intakes are established or reported in nutrition.
And so, if we look at cross vitamins and minerals, each micronutrient has a minimal requirement. And that minimal requirement that you'll see on most food labels or in nutrition literature is usually representative of the amount of the micronutrient that's needed to prevent deficiency or inadequacy related diseases. And the first metric that typically needs to be established before we can set these criteria is known as the EAR, or the Estimated Average Requirement. And what this represents is, if we take a look at this figure, the EAR is set so that the level of intake at the EAR, if we look at the y-axis here, this is the risk of inadequacy. So the EAR is typically set at a level at which, as you can see, the EAR for a given nutrient is usually around an inadequacy risk of 0.5. So what that means is that obtaining intakes of the nutrient at this level would be enough to prevent inadequacy or deficiency related diseases in about 50% of the population.
And this is the first step in setting what is known as the RDA, or the Recommended Dietary Allowance. And we've discussed this topic previously, but it's important to understand that the recommended daily intake, so the recommended daily intake could be an RDA, which is the Recommended Dietary Allowance. And in this particular case, the RDA are set for micronutrients as well as macronutrients so that they would maintain sufficient sufficiency or prevent inadequacy in 97% of the population. So, in order to determine this, establishing the EAR is sort of a prerequisite, and then the dietary allowance, or the RDA, is set at a level that would prevent insufficiency in most people.
However, there are some cases, and especially with micronutrients, where an RDA cannot yet be established due to a lack of enough scientific research across large enough populations. And in those cases, the RDI, or the Recommended Daily Intake, is actually set as what's known as an Adequate Intake, or AI. And AIs are usually set based off of population-specific, or a number of populations-specific, scientific studies that have established safe levels of intake in order to prevent inadequacy or insufficiency, but not enough data has been collected yet to set a firm RDA or a level that would prevent it in more than 97% of individuals.
Now, what's even more interesting is that some nutrients also have tolerable upper intake levels. These are levels at which you can exceed the Recommended Dietary Allowance and avoid any risk of toxicity. So micronutrient toxicity can be an issue, hypervitaminosis, hypermineralosis. And these levels are not set for a lot of micronutrients. And we'll discuss some specific examples where we may want to increase the intake of certain micronutrients, but not exceed certain levels in order to avoid toxicity and related conditions. So these are just important things to understand as we're going through and talking some of the specifics on micronutrient intake.
Now let's start by talking about vitamins. And we intro'd this idea when we talked about nutrients at the very beginning of the quarter, and we discussed some of the roles in which nutrients play, but we spent more time on the macronutrients, which is where we've been for the majority of the quarter. So I'll start with vitamins. And vitamins are organic compounds, and again, they're needed in small quantities in the diet, mainly because they play a host of different metabolic roles and are cofactors and important constituents of various metabolic processes in the body, as well as being important for promoting normal growth and development. And we'll talk more about what some of the specific functions of each of the vitamins are.
But what's common about all of these essential vitamins is that, with the exception of two, they have to be obtained from the diet. So we don't get normal levels of intake of these vitamins from other sources. We don't produce them endogenously, and we don't get them from non-dietary sources with two exceptions. The first being vitamin D. A vast majority of vitamin D in human beings is absorbed through the skin from the sun's ultraviolet rays. And because of this, vitamin D insufficiency can be an issue that we might need to address via diet or supplementation in individuals who don't get a lot of exposure to sunlight. The other one is vitamin K. And vitamin K can actually be produced from gut bacteria. And so we do synthesize a large amount of this vitamin endogenously in the intestinal system.
So let's talk about our two types of vitamins. And this should be review from any nutrition classes that you've taken previously to this one. But vitamins are classified based off of their solubility in different substances. And so our water-soluble vitamins are going to be those which are soluble in water. And this is going to include all of the B vitamins, and we're going to talk about the B vitamins and their individual functions, especially with relation to energy metabolism, as well as vitamin C, which has some important functions that we'll discuss as well.
The fat-soluble vitamins are vitamins A, D, E, and K. And these vitamins are soluble in lipids. And as we'll see when we look at sources of these, fat-soluble vitamins are found in abundance in foods that have a lot of fats in them because of their solubility in lipid substances.
So let's start by taking a look at some of the functions of our fat-soluble vitamins A, D, E, and K. So when looking at the functions of our fat-soluble vitamins, we'll start with vitamin A. Vitamin A is an important constituent of epithelial tissues, particularly those in the integumentary system. So the formation of healthy skin cells is very dependent on vitamin A, and it's also an important constituent of things like mucosal membranes and the pigments that are found in our eyes.
Vitamin D is extremely important in promoting the absorption of calcium. And we'll see when we look at minerals that calcium absorption is pretty inefficient in human beings, and it's dependent upon other substances, particularly vitamin D. So having low levels of vitamin D can potentially impact bone formation, for example, because it will negatively impact the absorption of calcium in the gut and subsequent bone formation, of which we know calcium is an important constituent.
Vitamin E has antioxidant properties. And we'll talk a little more in depth about micronutrients as antioxidants, but these can help us eliminate some reactive oxygen species and free radicals that are produced during exercise. And we know that these free radicals can potentially have negative effects on cell structure, on DNA formation. But we also know that super high doses of these can potentially negatively impact some of the endurance-related adaptations to exercise. So we'll talk about this in the context of micronutrients, not just vitamin E, but also vitamins C and certain microminerals that may potentially serve as antioxidants, but from ingesting super high doses may negatively impact some of the adaptations to exercise training.
And then vitamin K, our final fat-soluble vitamin, is important for blood clotting. So coagulation is dependent on vitamin K, and a lack of vitamin K may be associated with some side effects that are related to improper coagulation and clotting of the blood during times when clotting of the blood might be needed in order to minimize bleeding.
So I think it's important for us to establish some food sources that are efficient and rich in one, if not multiple, of each one of these vitamins. And if we look at food sources, or we're looking for food sources that are rich in all of our fat-soluble vitamins, the most common and most abundant food sources are eggs and liver. And so this is why an old mom adage is to eat your liver. It's not exactly the tastiest food, but we'll see that liver is a really rich source of all four fat-soluble vitamins, as well as many other micronutrients.
Fish and dairy products, in particular, are very high in vitamins A and vitamin D. And with respect to dairy products, we've referenced the role of vitamin D and calcium absorption. Dairy products are also very rich in some of the best sources of calcium. So this is where the old "milk builds strong and healthy bones" kind of thing comes from, not just because of its high calcium content, but also because it contains a lot of vitamin D, and vitamin D is important for bone formation due to its effects on the absorption of calcium.
Dark leafy green vegetables, these are high in a lot of different micronutrients and of course, high in fiber. So these are, I think, really well-accepted components of a healthy diet, but they also happen to be very rich in fat-soluble vitamins A and K.
Oils, both vegetable and seed, as well as margarine and butters or animal fat-related oils, are rich in vitamin E. And it's also important to consider that vegetables that are naturally orange, yellow, or red get those colors from substances known as carotenoids. And carotenoids are not directly vitamin A, but they're what's considered pro-vitamin A substances. So when carotenoids are ingested, they actually are converted, or can be converted, to vitamin A and enhance vitamin A levels in the body indirectly because they can ultimately be converted to vitamin A.
So if we take a look at the RDAs or adequate intake levels for fat-soluble vitamins, we'll see that especially in comparison to our water-soluble vitamins, vitamin C, for example, we'll see has a pretty high RDA relative to other micronutrients. We see that the intake of fat-soluble vitamins typically are relatively lower. We see that the highest is vitamin E. And vitamin E, remember, we don't get vitamin E from sunlight or from the intestines like we can from D or K, has the highest RDA, so about 15 milligrams. And there's not an established difference between males and females. You can see that for certain RDAs of A and K for fat-soluble vitamins, there are different relative intake levels for males and females that have been established based off of the naturally occurring levels of these vitamins in males versus females and some differences in how different hormones and sex hormones impact the absorption and metabolism of these vitamins. But relatively, if we look at the content or Recommended Dietary Allowance or Adequate Intake of most of our fat-soluble vitamins, we're talking on the order of micrograms or under one milligram is required.
And if we think about why this is, it's because of the way that these are stored. They're soluble in fats or lipids, and therefore, much like the reason that many animal products and oils are rich in vitamins A, D, and or K, we store a lot of these vitamins in our fat cells. So their relative intakes are lower than the water-soluble vitamins, which are often excreted through the urine, as you might expect.
So let's start taking a look at some of the roles of our water-soluble vitamins. And we'll start with the class of vitamins known as the B vitamins. And these vitamins are essential in oxidative energy production. And we can kind of see here the outline of the Krebs cycle and the roles that some of the classes of B vitamins play in oxidative metabolism. And so we can look at these and begin to dissect where they come into play.
And the first set of vitamins are involved in either the conversion of pyruvate to acetyl-CoA. And so the important one there is thiamine, which is also known as vitamin B1. So anytime that carbohydrate is broken down oxidatively and pyruvate is formed, or pyruvate is formed from amino acid metabolism, that pyruvate needs to ultimately be converted to acetyl-CoA. And B1, or thiamine, is an important cofactor in the conversion of pyruvate to acetyl-CoA. Additionally, pantothenic acid, which is also considered a B vitamin, is a necessary part of acetyl-CoA. So it's actually part of the chemical structure of acetyl-CoA. So you need appropriate levels of pantothenic acid in order to form acetyl-CoA in the first place.
Now, our second class of B vitamins, vitamins B2 and B3, also known as riboflavin and niacin, are important components of the electron carriers that are generated, NADH and FADH. These are chemical components, much like pantothenic acid is a component of acetyl-CoA, of these electron carriers. So having insufficient levels of riboflavin, niacin, B2, and B3 are going to result in an inadequate formation of these electron carriers. And ultimately, we know if we're trying to get ATP from the constituents and from the components that are made in the Krebs cycle, these electron carriers are ultimately what is going to allow us to generate relatively large yields of ATP from oxidizing fats, carbohydrates, or amino acids in the electron transport chain.
The others that are potentially important are in relation to the breakdown of amino acids and the formation of pyruvate and oxaloacetate that can occur when certain amino acids, mainly alanine in this particular case, are being used gluconeogenically or glucose-neogenically. And the ways in which these work are that, first of all, B6, which is also known as pyridoxine, is an important constituent of the enzyme that allows for the conversion of alanine to pyruvate. And we know that in order for this amino acid to work gluconeogenically, ultimately that pyruvate also needs to be converted to oxaloacetate. So we think about during times of starvation or during times where carbohydrate availability is limited, and more oxaloacetate is being used in order to fuel gluconeogenesis, well, we know that certain amino acids can be used to help form oxaloacetate. And the B vitamin biotin is a coenzyme that is necessary for the carbon dioxide transfer that has to occur for pyruvate that is formed from alanine to be used to restore those oxaloacetate levels and allow the Krebs cycle to potentially continue.
So ultimately, a deficiency of B vitamins, because of their involvement in the oxidative energy pathways, will probably result in a shift towards carbohydrate metabolism. And the reason for this is, while we know carbohydrates can be broken down oxidatively, they can also be broken down anaerobically. So when there's a B vitamin deficiency, we may have an impaired ability to metabolize things oxidatively and shift more towards fuels that can be broken down in non-oxidative ways, such as carbohydrate.
So as you may expect, there are some common symptoms due to the impact on oxidative metabolism of B vitamin deficiency. And fatigue and lethargy would be observed in any situation where B vitamin deficiencies are occurring. And this is obviously going to cause some intolerance to exercise in athletes. If athletes are experiencing a B vitamin deficiency, and the most common form of B vitamin deficiency is known as beriberi. And beriberi, it translates to "I can't, I can't," associated obviously with the feelings of lethargy and fatigue that are hallmark symptoms of B vitamin deficiency. And this is mostly due to thiamine deficiency. And it's really noted in cultures that have white rice as a primary dietary constituent. And part of the reason for that is because whole grain foods, such as brown rice, are really rich in B1 or thiamine. And when grains are processed, so as in like white bread or white rice, the polishing of those grains or rice actually robs those nutrients of their thiamine content. So this is really common in those types of cultures.
But B1 can be found in a variety of foods that aren't just whole grain foods and brown rice. It's also found in abundance in a lot of meat products, so pork and beef, as well as fish and eggs, and dairy products, and potatoes. Nuts and legumes are also good sources of B1. So thiamine is pretty abundant in a lot of different food sources, and B1 deficiencies are pretty uncommon unless there is a real lack of these food sources, particularly whole grains and brown rice, in the diet in favor of more white grains or white rice.
We start to look at food sources that are rich in B vitamins. One thing that we'll find is that a lot of meat sources or carnivorous sources of food are abundant in many B vitamins. Beef liver, fish, and poultry are all rich in vitamins B3, B6, and B12, with beef and liver also being very rich in B2 and pantothenic acid. So as you might expect, we don't see really prevalent levels of vitamin B deficiency in individuals who consume meat or meat-related products. But also, we know that eggs are very rich in B2, B12, biotin, and pantothenic acid. So these may be an option or a good option for getting B vitamins in individuals who don't eat beef, liver, fish, or poultry for dietary reasons. But that isn't to say that they can't get some B vitamins from other sources of food. We'll talk about this when we look at our article on supplementation strategies and nutritional considerations for vegans. But one thing that vegans can definitely get, or even vegetarians who may avoid meat products, are some rich sources of B2, B6, folic acid, which is also known as B9, and pantothenic acid from our green leafy vegetables. And we know that those are obviously rich sources of other vitamins and minerals, as we previously discussed. Nuts are also a good option for B6 and folic acid. So most nuts are rich in these nutrients and provide another good source of B vitamins outside of some of our best sources of B vitamins, which are our meat and our fish and poultry sources of B vitamins.
Now, because of B vitamins' role in energy metabolism, they have also been marketed as an energy-promoting substance that when taken in excess can provide a little bit of an energy boost. The whole idea of "Red Bull gives you wings" or these "5-Hour Energy" substances. If we take a look at what these supplements, I guess you could call them, contain, well, we'll see that by and large, they contain really, really high concentrations of B vitamins. So we can see, for example, niacin and vitamin B6 in a standard Red Bull contain 106 to 104% of the RDA for niacin and vitamin B6. Now, it doesn't stop there. If we look at things like 5-Hour Energy, these contain up to 2,280% of your RDA for certain B vitamins. But we also have to consider that these drinks contain a large amount of caffeine. And we know, as we'll talk about in our final lecture on sport nutrition supplements and ergogenic aids, caffeine has powerful ergogenic and performance-enhancing effects. But the question I think that is necessary to look at is, do all these extra B vitamins, quote unquote, "give you wings" or have any performance or ergogenic benefit in the context of exercise, or is it all about the caffeine?
And so an interesting study was performed in 2013 by Petit et al. And what they attempted to do was, in eight trained men and women, they gave them two different trials of cycling at their lactate threshold. And they wanted to see if oxygen consumption or any markers of aerobic metabolism during this bout of performance was associated with just caffeine intake, or if giving an equal amount of caffeine with extra B vitamins would produce any changes in oxidative metabolism during exercise. And so what they looked at was, they gave them two different conditions. In one condition, they got Red Bull, which, as we saw, has a lot of caffeine but also has really high levels of certain B vitamins, some in excess of the RDA. Or compared to a drink that had the same amount of caffeine as a Red Bull but didn't have any of the B vitamins that Red Bull has.
And what the researchers found is that when looking at oxygen uptake, you can see that oxygen uptake, if we look at the Red Bull condition, which is the black circles, versus the control condition, which are the open circles, there was no effect. Oxygen uptake was exactly the same across these bouts of cycling whether they were given caffeine or whether they were given caffeine plus all these extra B vitamins. They also found that individuals had no difference in their heart rate response to exercise. So they had the same submaximal heart rate at their lactate threshold whether they were given B vitamins or whether they were given B vitamins and caffeine, rather, versus caffeine. And that it didn't affect their perception of how difficult performing at that workload was.
So this was one of the first studies that was done. And subsequent studies that have looked at this a little more mechanistically have really determined that the extra B vitamins in these drinks don't seem to cause any increase in oxidative energy capacity. So in reality, the majority of the performance-related benefits of things like Red Bull or 5-Hour Energy is completely due to the caffeine content of those beverages.
Given that athletes do have higher rates of oxidative metabolism, you may consider that athletes, or it may be thought that athletes need to intake more than the normal amount of B vitamins. And this is actually not the case. And the reason that this isn't the case is because if we look at the ways in which B vitamins are metabolized, we actually recycle and reuse a lot of our endogenous stores of B vitamins. And we'll see that when we look at the RDAs for the different B vitamins, they're relatively low for the most part because we do recycle and reuse them so frequently in metabolism.
So I think the other thing to ask ourselves is, will excess B vitamin intake harm you? And this is extremely relevant, right? Because we know that, I mean, we can look at Red Bull commercials and 5-Hour Energy commercials. These are being marketed and used at very high rates by athletes because of the performance benefits of caffeine. So if you're taking in lots and lots of B vitamins, are they going to be potentially harmful to you? And the answer to that is actually, probably not. And that's a good thing. And it's due to their solubility in water. So if you do intake excessive levels of B vitamins, there haven't been any documented harmful effects. You're just going to excrete them in urine. And so your urine will be slightly more expensive than it was before, but there are probably not any harmful effects of taking in B vitamins at these super high levels. Now, taking in caffeine at super high levels may have some unfortunate and uncomfortable side effects, but we'll save that for our discussion of ergogenic aids and sport nutrition supplements.
Our final water-soluble vitamin that we want to talk about is vitamin C, also known as ascorbic acid. And it has many functions. It is definitely an antioxidant. So it can be helpful in the elimination of reactive oxygen species and free radicals. And we'll talk about that a little bit more when we talk about the roles of macronutrients as antioxidants. But we also know that vitamin C is a constituent of collagen and connective tissue, as well as important hormones like catecholamines and steroid hormones. It also is related to the absorption of iron. And we're going to talk pretty in-depth about iron and iron deficiencies and what negative performance implications iron can have. But of course, if iron deficiency is an issue and giving iron sufficiency is a goal of nutrition, then adequate intake of vitamin C is also important for the absorption of iron.
And when we look at fruits, or look at food sources of vitamin C, the most well-known food source and abundant food sources of vitamin C are definitely citrus fruits. So oranges, lemons, limes, grapefruits, as well as strawberries. Strawberries are really rich in vitamin C. And then very vegetables, leafy greens again come into play here, another vitamin, and another reason to include leafy green vegetables in high content in the diet, is because of their vitamin C content. And then they're also found in the skins of potatoes, as well as peppers are really rich sources of vitamin C. We'll talk a little more about vitamin C when we talk about antioxidants.
Now, unlike the B vitamins and fat-soluble vitamins, we don't store very much vitamin C at all. And so when we look at our RDAs, or our recommended intake levels of vitamin C, we'll see that it is by far has the highest recommended intake of any of our water-soluble vitamins. And so here are your RDAs or adequate intakes for your different B vitamins, as well as vitamin C. And again, if we look at the relative content of all of these B vitamins and their recommended intakes in the diet, we can see that we're talking pretty small amounts of milligrams or micrograms of intake. And again, this is because these vitamins many times are reused and recycled metabolically, as opposed to vitamin C, where the intake levels, or recommended intake levels, are about 90 milligrams for males or 75 milligrams for females. And again, because we don't store a lot of vitamin C, we need to intake it at higher levels in the diet, as opposed to our B vitamins or fat-soluble vitamins.
So let's now begin taking a look at minerals. Which, as opposed to vitamins, are actually inorganic compounds. And these minerals are usually classified as either macro or micro depending upon how much of body mass that they make up and the subsequent amounts that are needed in the diet. And there are seven macrominerals. And each one of these macrominerals makes up at least 0.01% of body weight. And these seven macrominerals are sodium, potassium, chloride, calcium, magnesium, phosphorus, and sulfur. So each of these are found in relatively, now this is relative to how much of these are present in the body in terms of looking at macro versus microminerals, not in terms of macro versus micronutrients, but each of these make up at least 0.01% of body weight. Compare that to our microminerals, and I've given you some examples here, and we'll talk a little more specifically about some of these microminerals. But these usually constitute less than 0.01% of body weight. And if we look at their RDAs or adequate intake levels, as a result, they're usually pretty small and less than 100 milligrams per day are needed to maintain sufficiency of these microminerals.
So let's look at the function of some of our macrominerals. Some of this will be review, but I think it's good to go over. First, we'll look at calcium. And we talked about the role of calcium in promoting bone health. But we also know that it's an important component for actin-myosin crossbridge cycling and getting some of the regulatory proteins in a more advantageous position for actin sites to be exposed. But it also is important in terms of regulating membrane potentials and the activity of various enzymes that are also involved in metabolism.
Chloride is important for nerve impulse conduction. But also, if we think about digestion, hydrochloric acid. Chloride is a very important component of hydrochloric acid. We know this is important in changing the acidic environment of the stomach, or the pH in the stomach, so that various digestive enzyme activities can be turned on.
Magnesium is important for protein synthesis. And it's also an important component of bone. But it's necessary also to form ATPase. So lack of magnesium will result in inadequate ATPase formation and subsequently potentially impact our ability to hydrolyze ATP and use it for muscle contractions.
Potassium also is an important regulator of membrane potentials and nerve impulse generation. But also is important for muscle contraction and in maintaining appropriate levels of acid-base balance. We know that phosphorus is another important component of the bone matrix in addition to calcium and magnesium. But also that phosphorus is going to be related to phosphate levels. So things like ATP, phosphocreatine, as well as our high-energy phosphate formation and the formation of cell membranes that contain phospholipid bilayers is highly dependent upon adequate levels of phosphorus.
Sodium, we talked about this with hydration. And we talked about how sodium is a really important component of extracellular fluid and is important in regulating blood volume. But it also is involved in nerve impulse generation and propagation, as well as muscle contractions and acid-base balance.
And then finally, sulfur is primarily involved in acid-base balance, but is also a necessary component of healthy liver function. So all of these macrominerals make up a consistent, pretty solid amount, 0.01% or more of body weight. And therefore, there are some pretty standard levels of adequate intake that have been established for these macrominerals.
And one of the things that you can see in relation to macrominerals such as calcium and magnesium, as opposed to chloride, potassium, phosphorus, and sodium, for example, is that these are not efficiently absorbed in human beings. So in terms of calcium, that is ingested, only about 30 to 40%. And also with magnesium, 25 to 60% of ingested levels of these minerals are actually absorbed in humans. And so when we look at the RDAs or AIs for these, these RDAs and AIs are actually set so that if you intake this amount of calcium, for example, calcium absorption typically tops out somewhere around 4 or 500 milligrams, depending upon the individual. So the RDA for calcium is set high enough so that if you ingest a thousand milligrams, you should reach that sufficient level in the face of this inadequate level of absorption. And the same would be true for our recommended levels of magnesium intake.
But by and large, one of the things you can also notice is that a variety of food sources are very rich in most of these macrominerals. So achieving macromineral sufficiency for most individuals is not necessarily an issue. One issue that can occur is when limiting dairy products. Dairy products tend to be some of the richest sources of calcium. So in individuals who are abstaining from dairy consumption, if they're not getting enough calcium from either supplementation or some of their other food sources, there can be some issues with calcium sufficiency. And we also know that because fruits and vegetables are very high in these macrominerals, especially things like potassium and magnesium and calcium, that individuals who have low fruit and vegetable intake sometimes have issues with potassium and calcium sufficiency in particular. So this is kind of a way to highlight where avoiding certain foods or dietary practices that force the avoidance of certain foods may create higher potentials or risks for macromineral insufficiencies. But if you eat a relatively balanced diet, obtaining appropriate levels of macrominerals is not typically an issue.
Now, if we look at our microminerals, the levels of adequate intake and/or RDAs for these are much, much smaller. As I said before, there are none of these in which the intake level exceeds 100 milligrams or even comes close to it. Most of these are given in micrograms or on the order of very small amounts of milligrams. And really, again, if we take a look at the food sources here, it's pretty easy to get an adequate intake of most of these microminerals. And as a result, micromineral deficiencies are pretty rare. But there is one in particular that has been known to affect athletes, particularly in certain situations.
Now, this specific scenario that I'm referring to is related to the role of specific micronutrients and oxygen transport. And the primary micronutrient that we'll focus on here is iron. And the reason that iron is important is because iron is a component of hemoglobin and myoglobin, which are essential for oxygen transport and oxygen utilization. Hemoglobin is the primary protein compound that carries oxygen in the blood, whereas myoglobin is the primary protein compound that oxygen is bound to, or the limited amount of oxygen that is stored in skeletal muscle is bound to. And if we look at these two compounds, well, the globin portion of each one of these compounds is the actual protein, but the heme portion, specifically of hemoglobin, and also a small amount of myoglobin, is primarily composed of the micromineral iron. So therefore, iron is essential for both oxygen transport and oxygen utilization.
Now, it's not just iron. Iron is obviously an important component of heme, which is an important component of hemoglobin and myoglobin. But also, the microminerals copper and zinc are necessary for the formation of hemoglobin and red blood cells. Additionally, some of our B vitamins are implicated in the actual formation of hemoglobin itself, particularly vitamins B6, B12, and B9, or folic acid. And specifically, vitamin B6 is actually necessary for a component of hemoglobin and myoglobin known as the porphyrin ring. So without B6, we can't form myoglobin or hemoglobin. And then B9 and B12 actually contain the trace element cobalt. And cobalt is required for the synthesis of nucleic acids. So when you have insufficient levels of B9 and B12, you will ultimately limit the proliferation of stem cells that are coming from bone marrow that ultimately develop into red blood cells, which we know contain hemoglobin and carry oxygen.
So a deficiency in iron, copper, or zinc may negatively affect the formation of heme. Whereas deficiencies in B vitamins like B6, B9, and B12 may also negatively impact the formation of hemoglobin, whether it is in synthesis of the peripheral ring or in the synthesis of red blood cells as they proliferate from stem cells being released from the bone marrow. So these all ultimately can lead to the condition known as anemia, which functionally is representative of mostly low blood hemoglobin. So deficiencies in minerals like copper or zinc or iron specifically can cause anemia, as well as deficiencies in B vitamins such as B9 and B12.
And the results can sort of play themselves out in two different scenarios or two different types of anemia. And the first is the result of usually a B9 or B12 deficiency. And as we mentioned, these B9 or B12 are extremely important in terms of red blood cell synthesis and proliferation from stem cells. And so what happens when B9 or B12 deficiencies cause anemia is that it interferes with red blood cell division. So what ultimately happens is you have cell division that is occurring at too slow of a rate, and it results in these kind of large red blood cells that are observed here. So ideally, these larger red blood cells would proliferate and divide into multiple smaller red blood cells. And when B9 or B12 deficiencies are present, this results in these large cells. And these larger cells, because of their size, their membranes are more prone to breakage. So you end up losing some oxygen-carrying capacity due to the breakage of cells. And because this is due to the size of the cells becoming too large and cell division becoming too slow, this is termed as macrocytic anemia.
The second scenario is far more common. And this scenario is related to deficiencies in the constituents of heme. So zinc, copper, and specifically iron being the one that is the most prevalent. And what happens here is not necessarily any issues with the division of red blood cells or proliferation of red blood cells causing too large of a size, but as you can imagine, because, as we said, these minerals are important components and cofactors that are needed to form heme, heme synthesis then becomes too slow. So one of the things that we observe when this happens is because heme is a lot of what gives those their red color, we see that the cells don't grow to as large of a size. So because there's insufficient heme, which is an important structural component of red blood cells, we'll see that these cells become very small in size, as you can see in the graphic depicted here. And because they don't have as much of that heme, they also appear much more pale in color. So as opposed to macrocytic anemia, this is termed microcytic anemia.
Now, obviously, it's important to look at the big picture for anemia, regardless of whether it's microcytic or macrocytic. And as you might have guessed, the big problem is that anemia interferes with oxygen delivery. And the consequences of impaired oxygen delivery are obviously going to be related to oxidative metabolism and are going to be similar, if not related and in conjunction with, potential consequences of B vitamin insufficiency. And this ultimately is going to force us to rely more on non-oxidative energy pathways, such as non-oxidative glycolysis. And the symptoms of anemia are pretty clear. Lack of or a decrease in exercise performance might be one of the primary symptoms, or early onset of fatigue. And some of these are going to be related to obviously the lack of oxygen delivery and the need to potentially use things like glycogen faster. So anemic athletes typically become glycogen depleted earlier after the onset of exercise and therefore become fatigued earlier when looking at exercise.
And so we should also think about who is at risk for anemia. And primarily, one category of athletes that high levels or high prevalence of anemia is observed in are our endurance athletes. And this is due to the process of hemolysis. And hemolysis happens when red blood cells, specifically those that are older or more along in their life cycle, they typically get a little more frail. And one of the things that happens as the need to increase cardiac output increases during endurance exercise is that these red blood cells hit arterial walls at a higher velocity. And so they kind of crash into these walls at a higher rate, and they tend to break more rapidly during this type of exercise. So we lose some red blood cells sometimes as a result of this hemolysis. So endurance athletes have higher rates of this because of it.
The other athletes that are potentially at risk are female athletes. And this is generally, they're at higher risk than male athletes because they tend to, they tend to have lower energy contents in their diet, or there's a higher risk of energy insufficiency. So when you decrease energy content of the diet, you usually increase the risk of all nutrient deficiencies. But we also know that female athletes, or females in general, store about half as much iron as males do. So they have less stored iron and a greater risk of iron insufficiency and therefore a greater risk of anemia.
And then of course, we know that certain dietary restrictions may increase the chances for anemia. Vegans and vegetarians typically avoid meat products, and in particular, red meat, which is mostly heme iron. One serving of red meat per week is usually enough to achieve adequate intake levels of iron. So it doesn't take a whole lot, but it's a lot harder to get iron from other food sources. It's just not a real match for red meat in terms of its iron-containing properties. And even foods that are vegetables that are high in iron, such as spinach, are actually mostly non-heme iron, as opposed to red meat, which is mostly heme iron. So this kind of highlights the need for potential iron supplementation.
And so, can you correct anemia, or can you save an athlete from anemia in athletes who don't get sufficient iron from their diet by taking iron supplements? And also, is there any associated risk with taking iron supplements? And I think that's the first thing that we need to consider is that iron can be very, very toxic in high quantities. So I think the first line of defense sometimes can just be to super compensate for iron deficiencies by taking in really high quantities. But unlike taking in high quantities of B vitamins, which may be one method of treatment that you might consider for an anemic athlete, taking in iron at very high levels can be toxic. So studies have shown that there's little risk to increasing your iron content to two to three times above the RDA. But that above those levels of intake, you do run the risk for iron toxicity. So just taking more and more and more iron supplements is not a good way to try to fix this.
Now, taking in up to two to three times the RDA still might not help, but it probably won't hurt. So it's a good first option to try if getting iron from food sources, such as maybe just eating one serving of red meat per week, aren't adequate or are preferable strategies for increasing iron intake. But it could be helpful in quantities at two to three times the RDA without increasing your risk.
Now, a lot of times, iron supplementation, or increasing iron content in the diet, won't be enough to reverse anemia or to improve anemic status. So in this particular case, if increasing iron supplementation or contents of the diet doesn't work, B vitamin supplementation might be another line of defense. And it's probably a much less risky defense in terms of supplementation, because as we established, B vitamins are recycled in metabolism, but also are excreted in the urine when taken in excess. And so B vitamin toxicity is generally not thought to be an issue, as opposed to iron toxicity, which can occur pretty easily from taking in too much.
Another important role that micronutrients play is their role as antioxidants. And what antioxidants ultimately attempt to do is to eliminate substances known as free radicals, or also sometimes interchangeably termed reactive oxygen species. And these free radicals actually contain unpaired electrons. And their unpaired electrons in these free radicals can cause damage to cell membranes, cause DNA mutations, and potentially have been implicated in diseases such as cancer, where these mutations may alter cell structures and DNA transcription and translation in ways that would augment cellular growth.
Now, what antioxidants do is they prevent or limit the action of these free radicals by donating one of their electrons so that this unpaired electron now has a partner. And ultimately, this reduces the reactivity of these reactive oxygen species or free radicals. And in terms of vitamins, there are several: vitamins C, E, and then beta-carotene, which is a pro-vitamin A substance, as we've already established, these have antioxidant properties. So these vitamins actually are able to aid in the donation of electrons to these unpaired electrons. And we know that actually exercise, because it causes oxidative stress and increases the acute accumulation of free radicals, that there might be some increased need for these antioxidant vitamins due to this exercise-induced free radical formation.
We also know that several minerals, selenium, copper, and manganese in particular, are components of antioxidant enzymes. So while vitamin C, E, and beta-carotene or pro-vitamin A might be antioxidants in and of themselves, several minerals are cofactors and components of the enzymes that are needed for these antioxidant interactions to occur in the defense against free radicals.
So this might set the stage for a lot of the marketing for antioxidant substances and mega doses of antioxidants as being marketed as performance-enhancing supplements. And so when we look at the role of micronutrients as antioxidants and vitamins and antioxidants in the reduction of exercise-induced muscle damage and associated oxidative stress that occurs as a result of exercise-induced muscle damage, there has been suggested that increasing the intake would help to alleviate this damage. Although the evidence of the efficacy of these substances is rather limited.
Compound that with the fact that while this acute level of oxidative stress may increase free radical accumulation in muscle, we also know that part of the chronic adaptation to exercise training is a result of this intricate increase in oxidative stress. So reactive oxygen species and free radicals are important cellular signals that ultimately can set off a cascade of cellular events that are involved in the increase in mitochondrial biogenesis. So the actual formation of mitochondria is partially signaled by the accumulation of reactive oxygen species. And we also know that our antioxidant capacity, so our antioxidant enzymes and the activity of antioxidant buffering reactions, actually goes up as a result of these acute needs to deal with increased reactive oxygen species or free radical accumulation. So this is actually an exercise training effect and adaptation that occurs in cells that actually improves exercise performance.
And as a result, when taking in high levels of antioxidants, including vitamin C and E, when taking these antioxidants in some of the mega doses that are required or that are found in antioxidant supplements, you might actually blunt endurance training adaptations such as mitochondrial biogenesis and the up-regulation of antioxidant enzymes that occur as a result of exercise training. So when you flood the system with antioxidants, the system doesn't respond in a way that allows for the up-regulation of those enzymes and may blunt some of the mitochondrial biogenesis and antioxidant enzyme capacities that occur as a result of normal exercise training.
So in terms of the utility of antioxidant supplements that contain mega doses of these types of compounds, while absolutely meeting antioxidant needs and marginally increasing antioxidant content in the diet may be beneficial for exercise performance, taking these huge doses of antioxidant supplements ultimately
impairs exercise performance by blunting some of the cellular adaptations to exercise training. I think one of the most hotly debated issues in sport nutrition is whether or not athletes need to take micronutrient supplements in order to achieve sufficiency. This is based on some of the previous examples that we've established where increasing physical activity may increase the requirements or reliance on certain micronutrients. But typically, this is really well met by consuming a balanced diet that meets the energy requirements of the athlete's sport or training.
So if we look at studies that have looked at how well athletes are able to meet a given RDA, so that would be this kind of threshold wide here, and the two examples here given are vitamin B1 and vitamin C. There have also been studies of calcium and other nutrients where the intake may increase for athletes. But if we look at energy intakes up to 4,800 calories a day, which pretty well encompasses them but most of the energetic needs for most athletes, we see that there's a pretty linear relationship between intake of micronutrients and increasing energy intake to where most all athletes who are meeting their energy requirements and are doing so through consuming a balanced diet and paying attention to their RDAs for these micronutrients typically are meeting these requirements just as a result of increasing their energy intake.
There's also no evidence to suggest that exceeding the RDA for any micronutrient has any sort of performance benefits. So by and large, most athletes, as long as they're paying attention to getting their micronutrients from foods and meeting their overall energy intakes, don't usually need micronutrient supplements in order to achieve sufficiency. So this kind of flies in the face of the high levels of intake rates that we see in athletes of micronutrient supplements.
Now, while most athletes are able to get all the micronutrients they need from food, there are some exceptions to this. So we should take a look at athletes that are at particular risk for micronutrient deficiencies. And we've sort of established what some of those are in relation to anemia. But as you might imagine, there are some particular groups of athletes that are at risk for marginal mineral and vitamin intake. And mostly, these are in sports where low body weight is essential for success or with body weight categories. And this is going to be another place where we'll see this as inversely related to energy intake, as we've seen before. The levels of macro and micronutrient intakes tend to decrease as energy intake decreases. So the risk for becoming insufficient in any of the in any vitamins and minerals increases in sports where restricting energy intake might be a prerequisite for low body weight and subsequent success in that sport.
And then we also know, and we'll talk about this more in relation to vegetarian and vegan diets in the research article that we're going to read on sport performance or sport nutrition recommendations for vegan and vegetarian athletes, we know that certain dietary restrictions might make getting certain foods more difficult and therefore lead to micronutrient deficiencies as well as macronutrient deficiencies. But we'll save some of the details for that for the research article that we're going to read.
I want to close with some specific scenarios where micronutrient supplements might be necessary. And one is in athletes who live in conditions where a lack of sunlight is an issue. So this is really common in places that have really, really long winters, and being out in the cold is not going to be an ideal place to perform, and a lot of training and activity has to occur indoors. So this becomes an issue because, as we know, a lot of vitamin D is absorbed through the skin through ultraviolet rays from the sun. And so athletes who are exposed to these conditions may have problems with bone formation due to vitamin D's essential role in calcium absorption and bone formation. So vitamin D supplements, in the face of not being able to increase vitamin D through the diet in adequate amounts, might be a necessary strategy in in athletes who are exposed to these conditions.
Calcium supplementation is particularly useful in athletes who are undergoing amenorrhea or lack of menstruation. And this is due to the effects of amenorrhea on estrogen levels. And so not having from exposure to increased levels of estrogen in females potentially has some negative impact on bone and leads to increase of osteoporosis. So one of the strategies that can be used to combat this is to increase calcium intake, and up to 120% of the recommended dietary allowance for calcium can be effective in helping to stave off osteoporosis in athletes in female athletes who are undergoing amenorrhea.
Additionally, we have to consider that amenorrhea is a consequence of the female athlete triad, which is a host of related conditions that ultimately are associated with low energy availability and low energy intake. This is very prevalent in female athletes, and as a result, these athletes may be restricting things like dairy products or really rich sources of calcium as well. So calcium supplementation can help achieve sufficiency and potentially help to reduce some of the negative impacts on bone from these from these conditions.
And then lastly, in hot environments, and we talked about this with relevance to fluid intake, but a lot of our minerals, particularly iron, zinc, and magnesium, can be considerably lost when sweating increases. And so athletes who are consistently training and competing in hot environments may consider taking mineral supplements or increasing their intake of iron, zinc, and magnesium because they lose a lot of sweat, a lot of these minerals in sweat. And in terms of what is safe, daily supplements of about one to two times the RDA seem to be safe ways to intake these. But we want to make sure that we're not exceeding those levels because, as we saw in the context of iron, the same would be true for zinc and magnesium. Excessive levels of these and minerals can be toxic and can actually impair the absorption of other minerals.
So that wraps up our discussion of micronutrients. We'll go a little bit more in depth on micronutrient supplementation when we read our article about recommendations for sport nutrition supplementation strategies for vegan and vegetarian athletes.