Chapter 7 - Protein.docx

Chapter 7: Protein Proteins are organic molecules composed of a folded chain of amino acids. They are an energy-yielding nutrient, but their main roles relate to tissue growth and repair. We eat dietary protein primarily so we can make the various body proteins that are essential to the body’s struc...

Chapter 7: Protein Proteins are organic molecules composed of a folded chain of amino acids. They are an energy-yielding nutrient, but their main roles relate to tissue growth and repair. We eat dietary protein primarily so we can make the various body proteins that are essential to the body’s structure and function. Accordingly, protein deficiencies, while rare in North America, can challenge a person’s ability to grow and thrive. By the end of this chapter, you will be able to: Describe the structure of proteins and amino acids. Differentiate between essential and non-essential amino acids, and complete and incomplete proteins. Outline the steps of protein digestion and absorption. List the various roles of proteins and provide examples of each. Differentiate between the protein quality of animal vs. plant products. Describe the relationship between protein and health and disease. Proteins are comprised of long chains of amino acids (Figure 7.1). Each amino acid has a nitrogen-containing amine group on one side and a carboxylic acid group on the other side. There are 21 amino acids found in our foods and bodies; they are structurally identical except for their side chains. Some amino acids are essential from the diet, while others can be synthesized by the body from other amino acids (Box 7.1). Figure 7.1: Amino acid structure. A food is considered a complete protein if it has all nine essential amino acids. Animal products are complete proteins, while plant products are typically incomplete proteins. Essential amino acids missing from incomplete proteins are called limiting amino acids. Individuals who eat little or no animal products can ensure their diet includes all the essential amino acids by eating the few plant sources of protein that are complete, such as tofu, quinoa and buckwheat. Another option is eating a variety of plant products so the amino acids complement each other. This concept is called complementing proteins or mutual complementation (Figure 7.2). Good examples of this include eating rice with beans, pita with hummus or peanut butter on bread. Figure 7.2: Complementing incomplete proteins. To become part of a fully functional protein, amino acids must bind together in a specific order. Our DNA holds the instructions for the sequence of amino acids needed in order to build specific proteins (Box 7.2). To make these chains, the body draws on the amino acid pool, a reservoir of amino acids that are available for making protein. If the diet lacks a non-essential amino acid, the liver can make it from another one through transamination. However, if the diet lacks an essential amino acid, it is not available for the amino acid chain and protein synthesis stops. Accordingly, limiting amino acids can compromise protein synthesis. This situation is rare in the highly varied North American diet, even in individuals that do not consume animal proteins. A well-planned vegetarian or vegan diet with a rich array of protein sources rarely limits protein synthesis and body function. A polypeptide chain is not a functional protein unless it folds in a specific way to form its secondary, tertiary and sometimes quaternary structure. The first level of protein structure arises from adjacent amino acids bonding to each other. This occurs through a condensation reaction and leads to the formation of a peptide bond (Figure 7.3). These peptide bonds help build the polypeptide chain. The amino acid order in the polypeptide chain (i.e., which amino acids are located where) and the chain length determine how the protein folds to form its secondary,tertiary and potentially quaternary structures. This, in turn, determines the protein’s function. The primary structure is therefore critical to that protein’s role in the body. Figure 7.3: The primary structure of a protein. Figure 7.4: The secondary structure of a protein. The secondary structure of a protein is evidenced by the presence of alpha-helices and beta-pleated sheets (Figure 7.4). These folds occur when non-adjacent amino acids form hydrogen bonds with each other. Figure 7.5: The tertiary structure of a protein. Tertiary structure is formed after secondary structure and arises from interactions between amino acid side chains (Figure 7.5). There are several bonds that can occur between these side chains, including salt bridges and disulfide bonds. This allows the polypeptide to fold even further and may result in the formation of a fully functioning protein. Figure 7.6: The quaternary structure of a protein. Some proteins have a quaternary structure (Figure 7.6). In this case, several proteins with a tertiary structure bind together to form the final protein. The tertiary structure proteins that make up the quaternary structure are typically referred to as protein subunits. Most quaternary proteins are made up of eight or less protein subunits, each contributing a specific structure and func- tion to that protein. A protein is denatured when it loses its folded three-dimensional structure. This change in structure also alters the protein’s function. In our bodies, the digestion of protein begins with denaturation. The acid in our stomachs unfolds protein units, allowing enzymes to work on the primary structure of a protein, separating off amino acids for absorption. Protein denaturation is also important in food preparation and can lead to some desired properties. Heat, acid and agitation can all denature proteins. For instance, cracking an egg on a heated frying pan, adding lemon juice to ceviche or beating egg whites are all examples of the protein within the food changing shape or structure. Once this has occurred, it is impossible to go back because the protein has been unraveled. For instance, we cannot un-cook a steak or un-beat egg whites. The stages of protein digestion are outlined in Figure 7.7. The mechanical digestion of protein begins in the mouth as the teeth rip apart protein-containing structures from the rest of food. The mouth and saliva do not contain any protein-digesting enzymes, so chemical digestion does not begin until food reaches the stomach. As we learned in Chapter 3, the hydrochloric acid released into the stomach lumen unravels proteins and activates the enzyme pepsin. Pepsin helps break the bonds between specific amino acids, further breaking down the unraveled polypeptide chain. Figure 7.7: Protein digestion. The main site of protein digestion is the small intestine. Here, the proteases (protein-digesting enzymes) tryp- sin and chymotrypsin, which are secreted by the pancreas, act on the remaining polypeptide structures. Proteases act on different amino acid sites to further break polypeptides into single, double (dipeptides) or triple (tripeptides) amino acids. All three are absorbed through active transport into the centre of the small intestine villus. Conversely, the body cannot absorb polypeptides longer than three amino acids. Once the amino acids enter the villus space, they pass into the bloodstream via capillaries and then proceed to the liver. Amino acids that are not metabolized at the liver are released into the general blood circulation. Our tissues can then pick up amino acids from the blood and use them for various functions. Proteins have a diverse set of functions in the body. There are endless examples of protein’s importance to body structure – bones, muscles, skin as well as every or- gan in the body contain protein. A good example of protein’s structural role is the protein collagen (Figure 7.8). Collagen is a protein found in bone, skin and connective tissue; it is the most abundant protein in humans and other mammals. In bone, collagen organizes itself into a rigid matrix that calcium and phosphate then harden. In skin, collagen promotes firmness and strength. Collagen is also found in the connective tissue of ligaments and tendons. Another important structural protein is elastin. As its name implies, this protein adds elasticity and can allow a structure to be slightly de- formed and then resume its shape. The outside portion of our ears have both collagen and elastin. Collagen allows the outer ear to maintain its shape, while elastin permits its flexibility. Figure 7.8: Collagen is a key structural protein. We constantly need to move things around the body and certain proteins allow us to do this. For instance, our cells contain protein tracks that allow other proteins to “walk” along them, carrying things around the cell. In cell membranes, there are protein transporters that extend from one side of the mem- brane to the other. Some protein channels can function as pores allowing a range of items to freely move in and out of cells. Other transmembrane proteins have subunits that act as doors opening and closing the protein channel depending on certain conditions. In the blood, proteins can also be used to transport substances. For example, the blood protein hemoglobin picks up oxygen at our lungs and delivers it where it is needed (Figure 7.9). Figure 7.9: The blood protein hemoglobin allows red blood cells to transport oxygen. As we learned in Chapter 3, enzymes speed up the rate of cellular reactions by providing a site for reactions to occur. Most enzymes, like lipases, amylases and proteases are proteins. After a reaction has occurred, enzymes maintain their structure and can go on to catalyze other similar reactions. Two proteins are critical for moving us around our world. Myosin and actin are muscle proteins that interact to shorten our muscles so they can move our skeleton. During muscle contraction, millions of myosin proteins bind to actin chains and then kink their heads to shorten the overall muscle length (Figure 7.10). When many actin- and myosin-con- taining muscle cells shorten in a coordinated way, the body can produce complex movements like walking or picking something up from the ground. Figure 7.10: The proteins myosin and actin help produce body movements. Figure 7.11: Edema in the foot. Recall from Chapter 5 that proteins in the blood help maintain appropriate fluid levels in the extracellular space. The amino acids found in blood proteins can carry positive or negative charges. Water is attracted to these charges and moves from the extracellular space into the bloodstream due to the draw of these proteins. This ensures that water does not build up in the extracellular space, reducing the risk for edema (Figure 7.11). Certain proteins protect us from infectious agents such as bacteria and viruses. Collagen in skin promotes structural integrity and allows skin to act as a barrier, stopping infectious agents from entering and causing harm. If a virus or bacteria does reach our body fluids, antibodies are part of our second line of defence. Antibodies are Y-shaped pro- teins that stick to the outside of pathogens, labelling them for removal and restricting the harm they can cause the body (Figure 7.12). Figure 7.12: Antibodies are proteins that stick to infectious agents, promoting their removal. Amino acids can also be used to synthesize substances that are not classified as proteins. For instance, creatine, DNA, RNA and the neurotransmitters dopamine and serotonin can all be synthesized from amino acids. The energy-yielding function of protein is not its main function. Only 5–10% of the energy we use each day is derived from amino acids. This is partly because using amino acids for energy is a wasteful process. The first step in using amino acids for energy involves deamination. Deamination removes the nitrogen-containing amine group so that the remaining structure can be used to form pyruvate, acetyl CoA or citric acid cycle intermediates (Figure 7.13). Which reactant of cellular respiration is formed depends on the side chain of the original amino acid before it was deaminated. A product of deamination is ammonia, which is toxic to humans. Ammonia is thus converted into urea and excreted. The body prefers to be efficient, so it would rather use all parts of an amino acid to make necessary body proteins instead of excreting part of it in the urine. Accordingly, proteins are not a preferential fuel source. Figure 7.13: Amino acids can be used for energy. There is currently no upper limit for protein consumption because complications from excessive protein intake are uncommon in the general population. Protein deficiency, however, is linked with compromised growth and develop- ment and is a significant problem in parts of the world where poverty leads to undernutrition. Malnutrition is one of the main causes of mortality in children under the age of five (World Health Organization, 2019). Two of the main conditions associated with death as well as disability are marasmus and kwashiorkor. Marasmus is a wasting syndrome and occurs when energy from all sources, including protein, is inadequate (Fig- ure 7.14). In addition to very low body weight, symptoms include difficulty managing body temperature, anemia, dehy- dration and heart irregularities. Marasmus can occur at any age but is most common in children and infants. Kwashiorkor is a form of protein malnutrition that occurs in young children. It is believed to be associated with inadequate protein intake when energy needs are still being met. Kwashiorkor translates as, “The disease the first child gets when the second child is born.” The disease has this name because in impoverished families when the second child is born it will typically receive breast milk instead of the first child. The first child might then be switched to a diet high in corn, maize, rice or other staples. These diets are low in protein and essential amino acids, leading to protein deficiency. Kwashiorkor is typically evidenced by a distended, swollen abdomen and swollen legs and feet, but an otherwise slim ap- pearance. The distended belly is partly due to edema. The cause is believed to be decreased protein in the blood, but the exact mechanism by which kwashiorkor develops is unclear (Briend, 2014). While kwashiorkor is rare in North America, cases have been reported that were associated with children consuming fad diets and/or restricting milk intake due to perceived milk allergies (Liu et al., 2001). Figure 7.14: Protein deficiency can compromise growth and development. The primary dietary factor that promotes weight gain is a caloric intake that is consistently above the body’s needs. Carbohydrates, lipids and protein all contribute energy to the body. A high-protein diet that exceeds caloric needs can therefore promote fat gain. The converse is also true. However, diets that are higher in protein are sometimes recommended. This stems from the fact that a high-pro- tein diet may favour a caloric deficit and has been associated with increased weight loss. Indeed, a meta-analysis of randomized control trials found slightly higher weight loss and more retention of lean mass when individuals consumed a high-protein diet (Santesso et al., 2012). There are several proposed reasons for this association: Protein-rich foods tend to be more filling compared to foods higher in carbohydrates and fat (Halton & Hu, 2004; Rolls et al., 1988; Vandewater & Vickers, 1996). Therefore, eating protein-rich foods may promote a lower overall caloric intake because hunger is more satisfied, and the individual may consume less overall. A protein-rich diet may also help with energy balance since protein has a higher thermic effect (Halton & Hu, 2004), which increases total energy expenditure. More calories are required to digest and absorb protein as compared to carbohydrates and lipids, again promoting a caloric deficit. Lastly, amino acids may also play an important role in the metabolism of lipids and carbohydrates, fur- ther promoting an energy deficit. Taken together, there is enough evidence that a high-protein diet may favour a caloric deficit and reduction in weight. Therefore, if weight loss is someone’s goal, increasing protein intake to no more than 2.0 g/kg body weight and consuming protein primarily from high-quality whole food sources can promote its weight managing effects, while still promoting health. The main role of protein is to promote the growth and repair of tissues. Therefore, it is particularly important when the body is repairing from exercise and rebuilding tissue, specifically muscle (Figure 7.15). Muscle protein synthesis increases in the 24 hours following resistance training (Burd et al., 2011) and dietary protein can facilitate this process. Specifically, providing approximately 10 g of essential amino acids from protein in the first two hours after exercise en- courages muscle growth (Beelen et al., 2010; Phillips, 2012). In addition to the protein requirements following exercise, athletes have a higher protein requirement overall. The Dieticians of Canada, The Academy of Nutrition and Dietetics and the American College of Sports Medicine recommend that athletes increase their protein intake from 0.8 g/kg body weight to 1.2–2.0 g/kg body weight, depending on the intensity and load of training (Thomas et al., 2016). This amount of protein supports muscle and metabolic adaptation and repair. Protein intake from high-quality sources is recommend- ed. For instance, milk-based protein has been shown to increase muscle strength and improve body composition (Hart- man et al., 2007; Josse et al., 2010). Figure 7.15: Protein can promote muscle growth. About half of the volume of bone and a third of its mass is composed of protein (Heaney, 2007). Collagen is the main bone protein, providing the structural matrix that calcium and other minerals bind to and harden. While protein is essential for bone structure, a high-protein diet has been associated with calcium excretion (Kerstetter & Allen, 1994). Therefore, whether diets high in protein are positive or negative for bone health is an active area of research. A system- atic review of 16 randomized control trials and 20 prospective epidemiological studies concluded that, overall, higher protein intake did not negatively affect bone health. They also found moderate evidence of increased bone density in the lower spine at higher protein intakes (Shams-White et al., 2017). Dietary protein can impact the health of the kidneys, especially in those who have kidney disease. Filtration rates at the kidney increase following dietary intake of protein (King & Levey, 1993; Martin et al., 2005). This occurs because the kidneys must work harder to excrete the waste products produced when protein intake is higher. Recall that when amino acids are used for energy, the waste product urea is produced, which the kidneys must excrete. Accordingly, low protein diets have been recommended for those with kidney disease. A review of three large prospective epidemiological studies found that patients with reduced kidney function could decrease their risk of further kidney decline, while also avoiding potential negative effects of excessively low protein intake, by consuming 0.8 g/kg body weight of protein per day (Bilancio et al., 2019). This number aligns with the daily recommendation for protein in a healthy adult, but North American protein intake is typically much higher. Protein quality typically refers to two factors: how well a protein is digested and how the types and quantities of amino acids in that protein source match the body’s requirements. There are several ways of measuring protein quality. Regardless of the method used, animal products are typically higher in protein quality compared to plant products, espe- cially if they are unprocessed (Box 7.3). In the early 1990s, The Food and Agricultural Organization (FAO) of the United Nations recommended that the protein digestibility corrected amino acid score (PDCAAS) become the industry standard for assessing protein quality; it is still used today (FAO, n.d.). The PDCAAS compares the amino acid content of a food against a standard amino acid profile. The highest score that can be achieved is 1.0. Having a PDCAAS of 1.0 means that, following digestion, each unit of protein meets or exceeds the human requirement for essential amino acids. The PDCAAS was recommended because it links the amino acids available to the body to the actual requirements of humans, not just the total amount of protein found in that food (Schaafsma, 2000). In 2013, the FAO suggested that a new method be used: the digestible indispens- able amino acid score (DIAAS). This score measures how well amino acids are digested in the ileum and more closely estimates the amount of amino acids absorbed by the body. This method is also better at determining the protein quality of mixed meals, not just ones with single ingredients (Bailey & Stein, 2019). For now, until a sufficient database of DIAAS values is established, the PDCASS remain the preferred method. In Canada, a different system is often used for evaluating the protein quality of food. The protein efficiency ratio (PER) is calculated in a laboratory setting by determining how much weight an animal gains when consuming a specific amount of protein, divided by the amount of food it consumed. The Canadian Food Inspection uses the PER to make sure that the food industry is making appropriate claims about a product’s protein content and/or quality (Government of Canada, 2014). While the PER is the assessment value currently used in Canada, there is a push to change to one of the methods mentioned above. One of the main criticisms is that growth in rats – the test subjects typically used for deter- mining the PER – does not correlate strongly with the growth of humans (Schaafsma, 2012). Box 7.3: An Indigenous lens: Protein quality of game meats vs. processed meats. The traditional diets of Indigenous people were very rich in pro- tein. Hunting was important for both food and resources; mammals, game birds and fish were key dietary staples. With colonization came limits on where Indigenous people could live, hunt and obtain food. Processed meats started to replace traditional meats due to their affordability, taste and ability to be shipped long distances. This has led to a shift in dietary protein quality. To assess the change in protein quality, researchers compared the composition of the amino acid tryptophan in the wild meat found in a traditional Indigenous diet to the composition found in processed meats. Tryptophan is an essential amino acid that has several im- portant roles beyond tissue growth, including the creation of the mood-associated neurotransmitter serotonin and the sleep-asso- ciated hormone melatonin. It was found that in addition to having higher total protein content, wild meats also had significantly higher tryptophan levels compared to processed meats (Spiegelaar et al., 2019). This adds more support to the high nutritional quality of tra- ditional Indigenous diets. Protein needs can be met by consuming whole food sources of protein. Some people also supplement their diet with processed protein supplements (e.g., protein powders, shakes and bars) or extra whole food supplementation (e.g., animal products, milk and tofu) in order to meet their needs and/or promote specific goals. For instance, protein supple- ments are often used by people who engage in muscle-building exercise to gain muscle strength and size (Figure 7.16). A meta-analysis of 49 randomized control trials found that both strength and muscle size increased with protein supple- mentation, especially in younger people who trained regularly (Morton et al., 2018). Since this analysis included many other studies, there was a range of ways protein was supplemented – either with more processed versions (e.g., whey, casein and soy protein supplements) or with whole food supplementation (e.g. beef and yogurt). The study found that supplementation beyond 1.62 g/kg body weight per day did not produce further gains. Thus, protein supplementation may improve muscle-related gains, but only up to a certain amount of protein intake, and both whole and processed protein sources can provide this gain. However, processed sources of protein are typically low in phytochemicals and fibre and they can also be expensive. On the other hand, these processed protein supplements can provide an easy and quick way to supplement protein for those with a busy lifestyle. In addition to a mostly whole foods-based diet, protein supplementation from processed sources can fit into a healthy diet and lifestyle. Figure 7.16: Protein supplements. Branched-chain amino acids (BCAAs) are essential amino acids that have a branched side chain (Figure 7.17). They are key components of the amino acid pool, accounting for 35–40% of the dietary essential amino acids that are found in body protein (Fernando et al., 1995). Also, unlike most amino acids, BCAAs are metabolized in the muscle instead of the liver (Harper et al., 1984). This has sparked interest into the study of whether BCAAs have a uniquely im- portant role in minimizing exercise-associated muscle damage. Indeed, a meta-analysis of eight randomized control trials showed that compared to rest or passive recovery, BCAA supplementation modestly improves recovery after exhaustive resistance exercises. A different systematic review of 11 experiments found that BCAA supplementation was effective at reducing muscle damage that is moderate in nature. This study also found that the effects were particularly strong if the BCAA supplement was consumed prior to exercise (Fouré & Bendahan, 2017). Since these amino acids are essen- tial, consuming enough of them is critical for the repair of protein-based tissue such as muscle. However, BCAAs can be consumed through whole sources of protein, like chicken, salmon and yogurt. To date, there is no evidence to show an advantage to consuming BCAAs from supplement capsules or powders compared to whole foods. Figure 7.17: The three branched-chain amino acids. Compared to plants, animal sources of protein tend to have more total protein as well as a higher PDCAAS. They are also all complete sources of protein, having all the essential amino acids. In addition, they are higher in vitamin B12, vitamin D, iron, zinc and omega-3 fatty acids. However, they are also higher in saturated fat and absent in fibre and phytochemicals. Also, consuming processed animal products may lead to a higher health risk. Several large reviews have found a link between the consumption of processed red meats, such as bacon, deli ham and sausage, and an increase in cardiovascular and mortality risk (Kaluza et al., 2015; Micha et al., 2010; Rohrmann et al., 2013). Diets that include animal products also provide a significantly higher environmental impact compared to plant- based diets (Figure 7.18) (Pimentel & Pimentel, 2003). More land and water are needed to feed a population with animal products than with plant products. Further, animals, especially larger ones, emit methane gas through their burps and flatulence. This contributes to net carbon emissions that produce a greenhouse warming effect on the planet. Converse- ly, vegan diets result in the lowest net carbon emissions (Chai et al., 2019). Figure 7.18: Consuming animal products has a significant environmental impact. Plant sources of protein, such as beans, nuts and lentils, offer less total protein and are more likely to be in- complete proteins. However, a well-planned plant-based diet may provide the same protein-related benefits as animal protein if a proper combination of sources is used (Hoffman & Falvo, 2004). Also, a few plant proteins, such as soy, tofu, quinoa and buckwheat, are complete proteins. However, while they have all the essential amino acids, they are more likely to be lower in them. For example, soy is a complete protein that is lower in the essential amino acid methionine (Friedman & Brandon, 2001). Still, diets that favour plants over animals can be nutritionally complete if they are properly planned and account for potential areas where deficiencies may occur. Vegetarians are individuals who avoid some or all animal-related foods. People choose to become vegetari- ans for a variety of reasons. For instance, Buddhism and Seventh Day Adventism both limit the consumption of animal products as part of their religious practices. Nowadays, vegetarianism is becoming more and more popular for a range of non-religious reasons including animal welfare, environmentalism and health (Fox & Ward, 2008). Table 7.1 outlines the key terms associated with the plant and animal content of a person’s diet. Note that this table covers the nutrition-re- lated definitions of these terms. Some of these terms, like veganism, can sometimes also apply to more than just nutri- tion. For instance, the Vegan Society defines veganism as a lifestyle that excludes all types of cruelty and exploitation of animals for food, clothing or other purposes (The Vegan Society, n.d.). Table 7.1: Plant- and animal-based diets. Vegetarian and vegan diets may have several health-related strengths. These diets are higher in fibre and phy- tochemicals and lower in saturated fats. In addition, they may reduce disease risk. A meta-analysis of 86 cross-sectional epidemiological studies found that vegetarians are more likely to have lower body mass indexes, total cholesterol and LDL cholesterol (Dinu et al., 2017). In addition, 10 prospective epidemiological studies included in the analysis showed a reduced risk of ischemic heart disease, cancer and cardiovascular-related deaths. A different meta-analysis of seven pro- spective epidemiological studies similarly found a reduced risk in these cardiovascular outcomes with a vegetarian diet (Glenn et al., 2019). However, no clear link has been established between vegetarianism and a decreased risk of all-cause mortality (Appleby & Key, 2016; Dinu et al., 2017; Glenn et al., 2019). Caution is needed when considering these results because all vegetarian and vegan diet health research comes from epidemiological studies. We cannot ethically or logistically ask a certain group of people to become vegetarian and vegan for a long period of time in a controlled fashion and then compare this to another group whose diet is also con- trolled. Thus, randomized control trials, the gold-standard for establishing cause and effect, cannot be used to study hu- man vegetarian diets and health. While an association has been found between vegetarian and vegan diets and several important health outcomes, recall from Chapter 1 that association does not equal causation. In other words, we cannot say that the absence of certain diseases is caused by eating a vegetarian diet. With that in mind, given the nutritional quality of a plant-based diet, the improved health parameters such as BMI and LDL in vegetarians, and the decreased risk of cardiovascular and cancer incidence found from epidemiological studies, there is an evidence base to support the beneficial effects of a vegetarian diet. While well-planned vegetarian and vegan diets can be nutritionally adequate, there are certain considerations that need to be accounted for. Vegetarian diets tend to be lower in vitamin B12, vitamin D, omega-3 fatty acids, calcium, iron and zinc (Craig, 2010). Searching out plant foods that are higher in these nutrients may prevent deficiency. The one exception is vitamin B12, which is absent in almost all plant products, with the exception of nutritional yeast. Fortified foods and supplements can help make up for these inadequacies and various vegan- and vegetarian-friendly options can be easily found in a health food store. Proteins are made up of long, folded amino acid chains. The specific sequence and length of amino acids is de- termined by our genes. The structure of a protein dictates its function. Proteins have many important functions related to growth and repair of the body. For instance, proteins promote body structure and are also critical to key physiological tasks such as enzymatic activity, transport, movement and defence from infection. While proteins can also be used for energy, this is a minor role. Protein can be consumed from both plant and animal sources. Animal sources tend to have a higher protein quality as they have more total protein and have all the essential amino acids. Plant sources typically lack at least one amino acid but are high in fibre and phytochemicals. There are potential health and environmental benefits of consuming a plant-based diet, and protein needs can be met by consuming complete plant proteins or through com- plementing protein sources. Proteins are composed of a folded polypeptide chain. Polypeptide chains are made up of amino acids. Nine of the 21 amino acids are essential, meaning they cannot be synthesized by the body and are required from the diet. DNA has the instructions for putting amino acids in the correct order to make specific proteins. Once this occurs, a polypeptide chain must fold to become a fully functional protein. Animal products are complete sources of protein, meaning they have all nine essential amino ac- ids. Plant products are typically incomplete, meaning they lack one or more of the essential amino acids. Proteins have a wide range of roles in the body. Overall, they are key to the growth and repair of tissues. While amino acids can be used for energy needs, this process is wasteful and not a pre- ferred role. Depending on the method used, protein quality typically refers to one or more of the following considerations: (1) How much total protein a food contains; (2) How well amino acids in that food are absorbed; and (3) Whether those amino acids match the body’s needs. Protein malnutrition impacts growth and development. It is rare in Canada. Diets higher in protein may have beneficial effects on body weight and muscle growth. Vegetarians and vegans in Canada can meet their protein needs by consuming a variety of plant sources of protein. These diets may also be associated with several health benefits. However, they are more likely to lead to micronutrient deficiencies. This can be mitigated with proper planning and/or supplementation. Consume most protein from whole sources. A wide range of plant proteins including nuts, beans and legumes as well as unprocessed animal products such as fish and lean meats can satisfy protein needs while promoting health. Based on body weight, the protein recommendation is 0.8 g/kg body weight per day for those 18 years of age and older. Children and infants have higher protein requirements to support their growth and development. Athletes have higher protein needs and may benefit from increasing intake to 1.2 g/kg body weight per day. In some cases, needs may be even higher. Pregnant women have higher protein needs. The DRI is 1.1 g/kg body weight. Lactating women have higher protein needs. The DRI is 1.3 g/kg body weight. Appleby, P. N., & Key, T. J. (2016). The long-term health of vegetarians and vegans. Proceedings of the Nutrition Society, 75(3), 287–293. https://doi.org/10.1017/S0029665115004334 Bailey, H. 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