Nutrients

Hello, and welcome to the first in our series of online sport nutrition lectures. Today's topic is nutrients, and the goals of this lecture are going to be to: first, define what we mean when we are talking about nutrition; determine what the individual classifications of nutrients are; and start investigating what some of the functions that nutrients perform.

Now, when we are referring to nutrition, we're referring to the sum of a few different processes. The first process in nutrition is the ingestion of foods. Now, the ingestion of food is an activity that brings people together; it brings a lot of pleasure; it tastes good. But the physiological purpose of ingesting food is, of course, to digest that food into individual nutrients, which can then be absorbed and metabolized in order to carry out a variety of different functions.

Now, specifically, a nutrient is a substance that can perform one or more of the following functions: the first is that nutrients are used to promote healthy growth and development. Now, we think about certain nutrients, such as protein and calcium, as being essential to building strong, healthy tissues like skeletal muscle and bone. We also know that many nutrients provide energy to the body, and this is where we're gonna spend a lot of time in this course, because energy metabolism is one of the primary concepts that we need to understand in order to determine how nutrients are being taken in and used by exercisers and athletes in order to provide—I must provide—energy to the muscles that are going to be performing the work needed to carry out those activities. And then some nutrients are involved in the regulation of these metabolic processes. Several vitamins and minerals serve regulatory functions in metabolism, and we'll talk a little more later in the presentation and in the course about how some of those micronutrients contribute to the regulation of metabolism.

When we think about our main categories of nutrients, we usually divide nutrients into two main categories that are reflective of the amount in which they are present in the human diet. The first category are our macronutrients, and this includes our three energy-yielding nutrients: carbohydrate, fat, and protein, as well as water. The second category of nutrients are micronutrients, and compared to macronutrients, these are present in the human diet in relatively miniscule amounts. Micronutrients include our vitamins, our minerals, and certain trace elements that are found in foods.

So let's start with talking about our energy-yielding nutrients, and the first one that we'll discuss are carbohydrates. And if we think about what the primary functions of carbohydrate are, first and foremost, carbohydrate—specifically glucose—is the energy preference for the brain and the central nervous system. So we know that the brain prefers to use carbohydrate metabolism when it's providing energy to its cells, and as a result, when carbohydrate availability is limited, one of the classic signs of low carbohydrate availability or low blood glucose is that the central nervous system—the brain—begins to slow down, and we feel feelings of lethargy and fatigue. The second major function of carbohydrate is to provide energy for the working muscle. Now, we can see here depicted the oxidative pathway by which carbohydrates can be used as well as fats, but carbohydrate can be used both in the presence and without the presence of oxygen to provide a source of energy for the working muscle. And as a result, we also store carbohydrate, and we'll talk more in depth about these forms of stored carbohydrate when we discuss our unit specifically about carbohydrate and carbohydrate metabolism and how they relate to sport nutrition. But primarily, carbohydrate is stored in two places: both in the liver, where it's used and broken down to help maintain blood glucose, and then, of course, we store a decent amount of carbohydrate in our skeletal muscle and as one of the primary supplies and fuel sources that helped fuel muscle contractions during exercise, physical activity, and sport performance.

Now let's begin talking about our basic classifications of carbohydrates, and before we do that, let's revisit what the chemical structure of a carbohydrate is. A carbohydrate is a molecule that is comprised of three elements, and those elements are carbon, hydrogen, and oxygen. Now, these three elements—one of the things that signifies that this molecule was indeed a carbohydrate—is that all carbohydrates have these three elements arranged in a one to two to one ratio. The classic example that you're all familiar with at this point would be the chemical formula for glucose, and if we take a look at that formula, which is C6H12O6, we can see that indeed it follows this 1 to 2 to 1 ratio, which is a classic chemical hallmark of a carbohydrate.

Now, glucose is one of three what are known as monosaccharides. Monosaccharide means that "mono" means one unit of sugar; "saccharide" means sugar. So we can see here, we just wrote this out: C6 H-12 O-6. This is glucose, and glucose is one of three monosaccharides that are found in the human diet. The other two are fructose and galactose. Now, the three monosaccharides combined with three disaccharides—so again, monosaccharide meaning one unit of sugar, disaccharide meaning two units of sugar—and together the mono- and disaccharides combine to form a class of carbohydrates known as simple sugars. And as you can see, the three disaccharides are simply combinations of two units of one or two of the three monosaccharides linked to each other. So here's an example of maltose, which is simply two glucose units linked to each other. This would be sucrose, which is by far the most abundant disaccharide found in the human diet; it is the main component of table sugar, and sucrose is a unit of glucose linked to a unit of fructose. The third disaccharide that kind of rounds out this class of carbohydrates is lactose, and lactose is a unit of glucose bonded to a unit of galactose and is found in the human diet primarily in dairy products and milk. You'll hear lactose referred to quite often as milk sugar.

Now, our second major class of carbohydrates are our polysaccharides, and as the name suggests—as the chemical structure of all of these molecules that you see depicted in this class would suggest—where a mono- or disaccharide or simple sugar is one to two units of sugar linked to each other, a polysaccharide are molecules that are typically three or more glucose units linked to each other. And we see three examples of polysaccharides depicted here, and the first that I'll draw your attention to is this compound known as maltodextrin. Now, maltodextrin is found primarily in the human diet; it is a—as a synthetic polysaccharide or polymer of glucose—and this compound doesn't occur naturally, but it's found in food primarily as an additive in order to thicken or fill foods, and it also gets used quite a lot as a preservative. But one of the interesting facts about maltodextrin is that when it is synthesized—again, this is a man-made compound—the synthesis process—process by which maltodextrin is made—causes these units of glucose that maltodextrin is made up of to become partially hydrolyzed or partially broken down. So while maltodextrin is technically a polysaccharide, because it's already partially broken down when the body digests it, it releases these individual glucose units very quickly and causes a rapid spike in blood glucose that oftentimes exceeds that of—that of the rise in blood glucose that would be encountered when eating a simple sugar like sucrose. And this is why maltodextrin is thought to be a source of what you may have heard referred to as hidden sugars. So while it's not a simple sugar, it gets processed very quickly like a simple sugar because of the man-made synthesis and the partial hydrolysis that occurs during that man-made synthesis.

Now, our most abundant form of polysaccharide in the human diet are definitely our starches, and you'll hear these commonly referred to as complex carbohydrates. And the term complex carbohydrate certainly refers to the chemical structures in which starches are found in the human diet, and there are two primary chemical structures in which we see starches show up in the diet. And the first one that you're looking at here is amylose. Now, amylose is a long helical strand of glucose units linked to each other in sort of this road arrangement; it's kind of like a straight line. The second form of starch that is found in the human diet is amylopectin, and if you look at amylopectin compared to amylose, you'll notice that rather than having glucose units arranged in one long helical straight row, amylopectin's chemical structure can be signified by several shorter branches of glucose units linked to each other in this sort of branched formation. And because of this branched formation compared to this long straight helical formation, amylopectin is actually digested and broken down more rapidly than amylose is. This branched arrangement, again, allows for shorter rows of glucose to be arranged, and it increases the amount of surface area that is available for all of these chemical bonds linking these glucose units together to be broken down. Additionally, this branched structure is more preferential for storing carbohydrate, and that's why if you look at the chemical structure of our primary storage form of carbohydrate—muscle or liver glycogen—you would see that it also follows a similar branched structure to what is found in starches that contain a lot of amylopectin.

Now, our final type of polysaccharide that is found in the human diet is fiber, and fiber is a special class of polysaccharide mainly because fiber contains chemical bonds—mainly from the plant polysaccharides that it is made from—the primary one that you see referred to most often is cellulose—and these plant polysaccharides like cellulose actually can't be broken down and digested by human enzymes; therefore, they don't play a role in energy metabolism. However, as we probably already know, fiber plays several other very important roles in the body and is a very necessary component of a healthy diet.

So what are the primary functions of dietary fiber? First, let's look at the two main classes of dietary fiber. The first is soluble fiber, and soluble fiber is fiber that dissolves well in water, as where insoluble fiber, in contrast, does not dissolve well in water. Now, both soluble and insoluble fiber are present in plant foods, and we can see some examples of some good food sources of both soluble and insoluble fiber. Now, again, most all plant foods have both soluble and insoluble fiber; however, some are better sources or have a more abundant source of soluble fiber, as where some are more abundant in insoluble fiber. Now, functionally, both soluble and insoluble fiber are important and play different roles in how they impact metabolic and digestive health. So first, let's take a look at soluble fiber. Now, the primary functions that soluble fiber plays are to slow down the absorption of nutrients after they are digested, and we'll talk a little more about this when we get into our specific unit on digestion and absorption and the gastrointestinal system. But because fiber—and soluble fiber is not digestible by human enzymes—and that it dissolves well in water, when it is digested, it is—it forms kind of a gelatinous sort of paste, and this allows it to slow down the rate of absorption in the intestine. And as a result, it helps us lower the rate of absorption into the blood of things like low-density lipoproteins, or bad cholesterol, which we know are associated with cardiovascular and metabolic disease. So soluble fiber helps regulate blood lipid levels in this fashion, and then, because it also slows down the rate of absorption of carbohydrates, it also helps to normalize blood glucose. So it slows down the rate at which those sugars found in either our mono- or di- or our polysaccharides that we eat in the diet—it helps reduce the rate at which those sugars are released into the bloodstream.

Now, insoluble fiber has its effects mainly in the colon, and because insoluble fiber does not dissolve well in water, it actually takes those water molecules and it attracts them, and it stores them. And storing these water molecules is important because what happens is, ultimately, storing those water molecules helps fiber improve or soften the stool or fecal matter, and this is why insoluble fiber is often used to treat constipation because it softens the bowels, and ultimately, as a result of this, it decreases the amount of time that fecal matter spends as it passes through the intestines. So this is why insoluble fiber is important for what's often referred to as bowel regularity.

So let's take a look at where in our diets we find each of our main classes of carbohydrates, and this is pretty intuitive, but we think about where the sources of simple sugars are in our diet, we can usually associate those with things that have a very sweet taste: things like fruit juices, cereals with sugar added to them, baked goods with sugars added to them, and then, of course, our sweetening agents such as our different types of sugar, syrup, and honey, as well as fruits, are going to be high in these simple sugars. Now, if we compare that to starches, starches are typically found in more dense foods. You remember the chemical structure of—of starches that we just talked about; you're gonna find them in these more densely packed foods, and the primary sources of starches in the diet are typically potatoes, pastas, rice, and, of course, our breads are very high in starches. Then, when it comes to fiber, we've sort of referenced this already, but fiber is primarily going to be found in our whole-grain foods as well as our legumes like beans and peas, and then, of course, our fruits and vegetables are going to be rich sources of both soluble and insoluble fiber.

Now, when we begin to think about the effects of carbohydrate intake on health, we have to consider how present these different types of carbohydrate are in the typical Western diet. And as you can see here, unfortunately, about half of carbohydrate intake in the Western diet comes from our simple sugars, as where fiber intake is very, very low on average, and we can see on the pie chart depicted here that the vast majority of carbohydrate comes from things like sodas, candies, cakes, fruit drinks, and desserts, as where very little comes from our sources of more complex carbohydrates and fiber. Now, this dietary pattern is unfortunate because it's been linked to poor health outcomes such as obesity, diabetes, and cardiovascular disease.

So how do these patterns influence the risk for these types of disease? Well, the primary mechanism is through the effects of simple sugars and low fiber intake on the body's sensitivity to the hormone insulin. Now, we'll remember that insulin is the primary hormone responsible for clearing sugar from the blood. So if you decrease the sensitivity to this hormone, you're going to impair the body's ability to metabolize glucose and carbohydrates as they're broken down into glucose, and as a result of this, there's going to be a greater proportion of glucose that stays in the bloodstream opposed to being taken up by the target cells in which it's supposed to be metabolized. Um, so what is having this elevated blood sugar do? Well, it does a lot of things, but some primary factors are that having this increased systemic circulation of glucose chronically over time produces states of inflammation, which can affect the structure and function of different organs, of our arteries, and the ways in which blood and nutrients are delivered to their target cells. It can also provide a signal to the body, because of the presence of elevated sugars, that there's plenty of energy around and that we should start storing some of that energy, and we know that the primary storage mechanism for extra energy in the human body is to store things as adipose tissue, or fat. So decreasing the body's sensitivity to insulin through these dietary patterns that are typical in the Western diet really impacts the risk for these poor metabolic and cardiovascular disease outcomes through its effects on the hormone insulin and the body's sensitivity to the release of that hormone.

However, we can't discuss the relationship between carbohydrate intake and health without talking about how exercise and/or physical activity might affect this relationship, and given that this is a sport nutrition class, let's take a look at the dietary habits of many athletes. And we know that depending upon where they are in their training and what their sport is and what their goals are, this varies, but some athletes—or many athletes—consume greater than sixty percent—sixty to seventy percent—of their total calories come from carbohydrates. A high-carbohydrate diet is a classically observed pattern in performance nutrition for various sports. But if that's the case, then why do we see that athletes would see Michael Phelps here with all his gold medals on, who famously ate upwards of 10,000 calories—very likely that the majority of them were coming from carbohydrates and simple sugars—during his most famous Olympic runs in which he was winning many competitions on a given day? So why do individuals like this not fit the profile of elevated risk for cardiovascular metabolic disease, obesity—things that have been related to carbohydrate and simple sugar intake in the general population? And if we think about why that is, there could be many factors at play here. Certainly, it's possible that athletes eat more complex carbohydrates and fewer simple carbohydrates. However, we know that most sports drinks, which are the most popular choice for fueling calories during competition, are very high in things like simple sugars. So what mechanisms and factors might be helping athletes avoid this elevated risk for disease when they're consuming high amounts of carbohydrates? And one of the answers is again related to insulin sensitivity. And if we take a look at the study on the right—or the graphic from this study on the right—this is taken from a study in which the researchers investigated the relationship between participating in habitual physical activity and the body's sensitivity to insulin. So if we take a look at what they did in this study, they measured the individual's habitual physical activity—this is a cross-sectional study—and they grouped individuals into those who do not participate in any moderate to vigorous physical activity, individuals who participated in some moderate-intensity physical activity, and individuals who participated in some vigorous-intensity physical activity. Now, what they did was they had measured their physical activity, and then they also looked at their sensitivity to insulin, so that's what we can see here on the y-axis, and again, a higher number here would be considered better sensitivity to insulin. So for every unit of insulin that is being released, there's going to be a greater clearance of glucose from the blood. And across all the conditions, you see that men and women—men are depicted in the white bars, and women are depicted in the black bars—and what we can see primarily as we go from no moderate activity to some habitual moderate-intensity activity to some vigorous-intensity physical activity is that, first of all, there is a slight benefit—a small improvement—in insulin sensitivity in individuals who participate in some moderate activity versus individuals who participate in no moderate or vigorous activity. However, the biggest difference—the most significant difference—is that in both men and women, the individuals who participated in some vigorous-intensity physical activity had significantly better sensitivity to insulin. And I want to point out that the individuals in this study were not athletes; these were individuals who reported or were measured to have participated in some vigorous-intensity activity, but we know that athletes are participating in much more than just some vigorous-intensity physical activity. So when we start to think about how this affects the relationship between carbohydrate intake and health in these populations, it's very likely that due to participating in very large amounts or volumes of vigorous activity that these behaviors help athletes avoid some of the potentially negative health outcomes that can be a result of increased carbohydrate and simple sugar intake.

Now, along with carbohydrates, fats are one of the primary sources of energy for the working muscle during exercise and physical activity, and therefore they're a very important component of any sport nutrition course. And like carbohydrate, we will spend one entire lecture or unit on fat and fat metabolism as well. Let's talk about some research in the area of fat intake and sport nutrition; however, other than just serving as energy for the working muscle, fat serves several other necessary functions in the body, and we'll discuss what a few of those are. So one thing that fat does when it's stored in the form of adipose tissue is that it serves as cushioning, and this is important in order to protect a lot of our vital organs as we go about our day and move around and change position and potentially absorb impacts. It's very important that the integrity of these vital organs are maintained, and fat helps to prevent them—for lack of a better term—of crashing into each other and causing damage. An important metabolic function of fats is that they are related to both the intake and transport of a key class of micronutrients, and these are our fat-soluble vitamins: A, K, E, and D. So the intake of these—this class of vitamins—is highly related to its solubility in fat, and then therefore, because of that solubility, these vitamins are found pretty often in foods that have certain sources of fat. And as a result, the transport of those vitamins, because they are fat-soluble and not water-soluble, depends upon some level of or some presence of fat. Another important way that fat works in the body is that it serves as a constituent of our cell membranes, and we know that most all cell types have lipid…

Bilayers that help regulate the entry and exit of things into and out of the cell, and as the name would suggest, those lipid bilayers are largely made of fats or fatty acids. Another important function of fats is that they are used to form bile. Bile is important, and we're going to talk more about bile and how it works when we talk about how fats are digested and absorbed. But its primary function is to help coat fats and emulsify fats into droplets that can be easier, or more easily, absorbed out of the intestine into the bloodstream. So bile is important for metabolizing fat, but fat is also a component of bile and needed to form bile. We'll talk more in detail about that again when we talk about our unit on digestion and absorption.

Another important function of fats is that they are used to form classes of hormones known as steroid hormones. Several important hormones fall under this classification; somewhat key ones that we might be interested in might be testosterone, which is a very potent anabolic hormone; also cortisol, which is used to help break down carbohydrates—it's a catabolic agent—but it also is related to physical and emotional responses to stress. But in any case, these hormones, as the name kind of suggests—a steroid hormone—well, the raw material for these steroid hormones is a lipid, and that particularly is cholesterol. You can see in the name here—colesterol—sterol kind of implies that these steroid hormones have a heavy cholesterol component. So fats and sources of cholesterol are important to maintain the production and regulation of these steroid hormones. And then, of course, most famously and importantly, we know that fat typically improves the taste of a lot of foods, and it also improves the appearance; it's responsible for giving things a more creamy texture, for giving them a more pleasing visual and taste-related properties.

So let's talk about our different classes of fats now. First, let's just make a little statement about the chemical structure of fats in relation to carbohydrates. We can see that the same three elements comprise fats that comprise carbohydrates: we have carbon, we have hydrogen, and we have oxygen. However, the key difference is that fats, compared to carbohydrates, have very little oxygen relative to carbon and hydrogen in their chemical makeup.

Now, the simplest form of lipids are our fatty acids. Fatty acids exist in many different forms in human nutrition, and they are categorized in a couple of different ways. But let's first talk about just the general structure of all fatty acids. So what we'll look at here is we see on one end we have what is known as a carboxylic acid group; on the other end, this end is known as the methyl group; and in between this carboxylic acid and methyl groups are a series of carbons—hydrocarbons—and the number, as you can see here, these are all the number of carbon that comprised this chain. These chains—the number of carbons—is actually one of the first properties that is used to classify fatty acids. And there are three distinct classifications of fatty acids that refer to the number of carbons that they have in this hydrocarbon chain. The most abundant fatty acids, and all of the fatty acids that are depicted here on this graphic, would fall into this category: long-chain fatty acids. Long-chain fatty acids are fatty acids whose carbon chain has a length of 12 carbons or longer. We also have medium-chain fatty acids, and those fatty acids have carbon chains that are eight to ten carbons in length. And then we have our short-chain fatty acids, which are fatty acids that have carbon chains of six carbons or fewer.

Now, the second way in which fatty acids are categorized is via the presence or lack of these double bonds that you see located here in the bottom three examples, as where this first example of stearic acid doesn't contain any of these double bonds. So this is another way that we categorize fatty acids: the presence or lack of these double bonds. And you may be familiar with some of these terms, but a saturated fatty acid—so this would be an example of a saturated fatty acid—stearic acid has zero double bonds, and that's reflected here in this numbering system that is often used to refer to fatty acids. Stearic acid is an 18-carbon long-chain fatty acid, and it's also saturated because it has zero double bonds. Now, unsaturated fatty acids all have one or more double bonds. So you can see here in the case of oleic acid, or alpha-linoleic acid rather, to go to the other end of the spectrum here, alpha-linoleic acid is an 18-carbon long-chain fatty acid; it has three double bonds—we can count them right here: 1, 2, 3—and then the last piece of information here in parentheses indicates that the first double bond starts on the third carbon.

Now, most dietary fat, especially that which comes from animal-derived products, exists in the form of triglycerides or triacylglycerol. And if we take a look at how that name is structured, we can tell what the chemical structure is of these molecules. And what we have here are, as you can see, three fatty acid chains, and these could be long chains, these could be medium chains, these could be short chains—again, most will be long-chain fatty acids because that's the primary form that we see in the human diet—and these chains, there are three of them, and they're linked to this three-carbon glycerol backbone. So in order for the fatty acids to be used from this triglyceride, they need to be freed from this glycerol backbone, which is a process that we'll talk about in our specific unit on fat metabolism.

Now, the most complex chemical molecules that lipids exist in in the human body are a class of lipids known as lipoproteins. A lipoprotein—various sources are made of various sources of lipids—primarily cholesterol, phospholipids, and triglycerides packaged together with these apolipoproteins, which are specific proteins that combine with fats to make these lipoprotein compounds. And there are various types of lipoproteins that are found in the human body; they have different relationships with health and serve different functions. And the way that these are usually classified—the one that you'll be most likely familiar with—is by their density. So we have high-density lipoproteins, low-density lipoproteins, very-low-density lipoproteins, and so on and so forth. And then they also are sometimes variable in the amounts of cholesterol and triglycerides that comprise these apolipoprotein structures. And we'll take a look at a couple of examples of lipoproteins that are able to be used and oxidized for their fatty acids and used in energy metabolism on the next slide.

Now let's take a look at which sources of lipids can be used and oxidized for energy metabolism. And there are only certain forms of lipids that can actually serve as fuel for exercise and muscle metabolism. And the first are our fatty acids. Now, these fatty acids—the carbon chains specifically in these fatty acids—can be broken down and chunked off into two-carbon units, which are needed to run the Krebs cycle. They'll form acetyl-CoA—acetyl coenzyme A—which is a necessary component of the Krebs cycle; we'll talk about in more depth, and you've already learned about in exercise physiology—and those acetyl-CoA units are essential so that we can make hydrogen carriers for the electron transport chain via the Krebs cycle. So these fatty acids, however, normally, as we've already stated, are the most abundant form of them that we find in human beings are stored in the form of triglycerides. Now, triglycerides come from the diet, but they're also the primary storage form of fat, and we store triglycerides in our adipose tissue. And certainly, given that a goal of exercise oftentimes is to lose adipose tissue, it's a good thing that one of the primary fuel sources for exercise is cleaving off these fatty acids from their triglyceride form in adipose tissue and using them in energy metabolism. Those triglycerides are also found intramuscularly. So as you can kind of see here, these darker circles here in each of these pictures are muscle fibers, and this sort of white matter in between the muscle fibers are all triglycerides. So there are triglycerides that are found in between muscle fibers, or intramuscular triglycerides. Now, these are important fuel sources for energy metabolism, and we'll also talk about how intricate muscular triglyceride contents and location changes as a positive adaptation to endurance training.

So we have fatty acids that can be oxidized from two sorts of forms of stored carbohydrates in the form of triglycerides—those in adipose tissue and intramuscularly—and then another important set of compounds that we referenced on the previous slide are two specific types of lipoproteins: chylomicrons and very-low-density lipoproteins. And the significance of these two lipoproteins is that they both transport triglycerides through the plasma so that those triglycerides can be oxidized for their fatty acids by the working tissues that are going to use them for energy metabolism. So these lipoproteins are not directly oxidized as a fuel source, but the triglyceride content of these two lipoprotein compounds are used for energy metabolism when they're broken down for their individual fatty acids. So while VLDL and chylomicrons aren't directly oxidized, they're very important in transporting triglycerides to the sites where they need to be oxidized.

In terms of the relationship between dietary fat intake and health, what are the things that we know is that certain types of fatty acids have been specifically linked to cardiovascular disease. And the first, and probably most well-known and oldest relationship that's been observed in studies of nutritional epidemiology, are the effects of saturated fat intake—or fats that have no double bonds in their carbon chains—versus unsaturated fat intake, which have one or more double bonds in their carbon chains. And again, across a multitude of epidemiological studies, it has been consistently observed that the intake of saturated fatty acids increases these circulating levels of low-density lipoprotein cholesterol, which, if you've taken pathophysiology or if you've ever had a blood lipid panel, you will be aware that low-density lipoprotein cholesterol is—is termed bad cholesterol because it increases the risk for cardiovascular disease and atherosclerosis. So the mechanisms by which saturated fats increase low-density lipoprotein cholesterol are not terribly well established, but what is primarily thought to be one of the mechanisms, or primary mechanisms, is that their lack of double bond structure allows them to be more densely packed. So more of these carbons are hydrogenated, as you can see, because there are no double bonds occurring, and as a result of that, it results in a—in a more densely packed, less fluid structure that is also less mobile. So it has a harder time kind of getting through membranes and getting to places where it needs to be; therefore, it stays out in circulation for longer periods of time and potentially increases the production and concentrations of low-density lipoprotein cholesterol, or bad cholesterol. So again, saturated fatty acids have been consistently linked to risk for cardiovascular disease, but that's not to say that all unsaturated fatty acids are blameless.

If we take a look at specific types of unsaturated fatty acids, most unsaturated fatty acids have double bonds that are arranged in what is known as a cis structure. And this structure simply means that there are two hydrogens that are aligned on the same side of this double bond. Now, trans fatty acids—a specific type of unsaturated fatty acids that are made primarily in the process of making hydrogenated oils, such as corn oils—that hydrogenation—hydrogenation process that hydrogenated corn oils and other hydrogenated oils undergo causes some changes in the structure of these double bonds where they take on this transformation. And the trans double bond structure is signified by having two hydrogens aligned on opposite sides of the double bond. And what this does is, because of these bond—excuse me, trans bond structures—these trans—trans bond structures in the double bonds make trans fatty acids behave more like saturated fatty acids; it allows them to be more densely packed and therefore less fluid and less mobile. So we see that in taking higher levels of trans fatty acids, much like in taking higher levels of saturated fatty acids, has a similar relationship in terms of elevating low-density lipoprotein cholesterol levels as well as increasing cardiovascular disease risk.

Let's take a look at our third energy-yielding nutrient, which is protein. Now, while protein is an energy-yielding nutrient, it does not contribute as much to energy metabolism in terms of the relative proportion or percentage of energy that it provides as carbohydrates and fat. Nonetheless, it does play a small role, and we do break down and oxidize some protein for energy, and we'll talk more about that in our subsequent unit on protein and amino acid metabolism. But what we know that protein is extremely important for is that it provides the structure to most all of our cell types. So it's an extremely important structural nutrient, and as a result, it comprises the majority of some pretty important tissues: muscle, hair, skin, and bone, just to name a few. But protein isn't just the building blocks for—for tissues; it also plays a role, or is—is a primary component of many metabolic mediators. So it is both a precursor for things like enzymes, which help catalyze and regulate all of our metabolic reactions, as well as the building block of an entire class of hormones known as peptide or polypeptide hormones. And an important example of this we've already discussed would be the hormone insulin, which is a polypeptide hormone that is necessary for carbohydrate metabolism and also plays a whole host of other roles in the human body. So protein, while it doesn't directly contribute as much energy to metabolism as carbohydrates and fat do, it is very important as a precursor and regulator of metabolic function.

If we take a look at the basic chemical structure of a protein, we see that proteins are helical structures that are a series of what are known as amino acids, and those amino acids are bound together by peptide bonds. And there are 20 different amino acids that are found in human nutrition. But what determines a protein's completeness, or when you hear individuals referring to getting complete proteins in their diets, they're referring to proteins that contain all nine of what are known as the essential amino acids. Now, what determines whether or not an amino acid is essential is quite simply whether or not it can be naturally synthesized by the human body. So there are 11 amino acids which we make naturally, and these are considered dispensable or non-essential amino acids. However, the caveat to that is, while they are termed essential, they still need to be included in the diet in order to maintain adequate supplies for healthy function. So they're not to be completely ignored. But in contrast, the nine essential amino acids are not produced or synthesized naturally by the body; therefore, our only option to get adequate supplies of these essential amino acids are to try to eat proteins or complete proteins that have all nine of those essential amino acids in their chemical structure.

Now, another term to be familiar with that is related to the amino acid content—specifically the essential amino acid content of a protein—is protein quality. And protein quality is simply a measure of how well that protein contributes to daily essential amino acid requirements. So if we take a look at the protein quality score of some various sources of protein, we see that several sources of protein—some examples being whey protein, egg white protein, milk protein—all of the examples within this box, they all have a protein quality score of 1.0. And what that means is that, after digestion, per unit of protein that these sources provide, they provide 100 percent or more of the essential amino acids that are required. Compare that to something like whole wheat, which has protein in it, but whole wheat has a protein quality score of 0.54. So after digestion, per unit of protein, whole wheat would only provide about 54 percent of the essential amino acids required for healthy function.

Now, our last macronutrient is water. And of course, water is not an energy-yielding macronutrient, but hydration and levels of water are extremely important for a whole host of physiological functions. And this is a really important issue in exercise and sport nutrition, especially when exercising in certain environmental conditions that challenge fluid levels. And these provide significant challenges when fluid levels are low or individuals are performing in dehydrated states because nutrients are transported via water. So water is a heavy component of blood and plasma and of cytosol and the mediums in which all of these chemical reactions that drive metabolism occur. We also know that water is extremely important in terms of helping regulate body temperature. One of the ways in which we dissipate heat or thermoregulate is through perspiration—by moving fluid to the skin so that we can move the heat from central locations in the body to the outside of the body, and that we can begin to level off core temperature. We know that water is necessary for lubrication, cleansing, and cushioning; a good example of this is the fluid that lubricates and cleanses—cleanses the eyeballs. We also know that water participates in very important biochemical reactions, namely, as we've talked about already, the process of breaking down our energy currency of the cell—adenosine triphosphate, or ATP. We know that there is hydrolysis that occurs in order to break down that energy-yielding compound. And again, we also know that water is an important medium in which biological reactions occur. And finally, water is a heavy component of urine and, as a result, and also fecal matter, and as a result is necessary in terms of removing waste products and toxins from the body.

Now, if we look at the function of micronutrients—or nutrients that are found in much smaller relative proportions to our macronutrients in the human diet—we'll see that primarily micronutrients serve regulatory functions in the process of metabolism. And that really depends on which micronutrients we're talking about. Again, micronutrients are going to be our vitamins, minerals, and trace elements, but I'll give you a couple of examples to start you off before we get into the specifics on micronutrients later in the course. So here are a couple of examples of how a vitamin and a mineral help regulate oxidative metabolism. Vitamin B5, for example, can be converted into acetyl coenzyme A, which is a primary component of the Krebs cycle. Another example would be to a regulator of the delivery of oxygen, and the mineral iron is a primary component of the carrier protein hemoglobin, which is found in red blood cells and serves to carry oxygen to its target cells to fuel metabolism. Another example that is related to hydration status is the effect of electrolytes, which are a specific class of minerals that are osmotically active—i.e., they pull water with them—and how they regulate the absorption of water in the kidney. So one of the goals in the kidney is to maintain fluid in the plasma, and in order to do that, water has to move from the tubule of the lumen across that membrane and into the bloodstream. And in order to do that, that diffusion is dependent upon the concentration of electrolytes on either side of these membranes. So in order to maintain the pull of water in—in the appropriate direction, it's important to maintain healthy levels of these electrolytes, which can be challenging again when you're losing a lot of fluid and losing a lot of osmotically active particles.

Now, looking at vitamins in the human diet, there are 13 different vitamins, and they all play important roles in most metabolic processes in the body and therefore must be consumed in the diet, with two particular exceptions. And those exceptions are vitamin D—and vitamin D can be obtained from sunlight—and vitamin K, which is synthesized naturally from bacteria in the intestines. But the other vitamins, in order to prevent a vitamin deficiency, need to be consumed in the diet. So this wraps up our first discussion on the topic of nutrients. We will pick up in our next online lecture discussing the roles of our three macronutrients—carbohydrate, fat, and protein—as fuel sources for exercise and muscle metabolism.