Proteins are organic compounds made up of carbon, hydrogen, oxygen and nitrogen. It is the presence of the nitrogen which sets protein apart from other nutrients. Since we have no other source of nitrogen (being unable to absorb it from the air, like plants can), one of the most important roles of protein is to bring nitrogen into the body.
One of the primary uses of protein in the body is to synthesize structural proteins such as muscle, skin and hair. Protein is also used to synthesize peptide hormones such as growth hormone (GH), insulin-like growth factor 1 (IGF-1), insulin and glucagon. Additionally, protein is used to synthesize transport proteins, such as albumin, which is used to transport other substances through the bloodstream (1).
Proteins are made up of sub-units called amino acids (AAs). There are 20 AAs which occur in food, although more are present in the body (1). Two examples of non-dietary AAs are hydroxyproline, which is a by-product of the breakdown of connective tissue, and 3-methylhistidine, which is a by-product of the breakdown of muscle tissue.
Individual AAs are called peptides. When two AA's are bonded together, it is called a di-peptide. Three AA's bound together are called a tri-peptide, and four or more are called an oligo- or poly-peptide.
Indispensable and dispensable amino acids
The 20 dietary AAs are typically subdivided into two categories. In the past, these categories were termed essential and inessential (or non-essential). Essential AAs could not be synthesized in the body, and had to come from the diet, while inessential AAs could be made within the body.
Since all amino acids are essential, in that they are required for life, the categories have been renamed. The more accurate/modern terminology is dispensable AA (which can be made within the body) and indispensable AA (which must come from the diet).
However even this nomenclature is incomplete and is not sufficient to cover all possibilities. Depending on the metabolic state of the body, certain dispensable amino acids may become indispensable (2). For example, glutamine, which is normally considered a dispensable amino acid, may be required in such high quantities under certain conditions, that enough can not be made in the body, and it becomes indispensable (3). One of these situations is following trauma or surgery, where glutamine requirements go up significantly. In this case, glutamine is termed a conditionally indispensable AA.
Another example is cysteine, which reduces the requirements for methionine, and tyrosine, which reduces the requirements for tryptophan. Under situations where insufficient methionine or tryptophan are being consumed, cysteine and tyrosine would become indispensable (1). There are several other sub-categories which can also be delineated but they can be ignored for the purpose of this article (2).
A list of dispensable and indispensable amino acids appears in table 1.
Table 1: Dispensable and indispensable amino acids
Dispensable Indispensable
alanine lysine
glutamic acid isoleucine (c)
aspartic acid leucine (c)
glycine valine (c)
serine histidine threonine
proline methionine
glutamine(a) phenylalanine
asparagine
cysteine (b) tryptophan
tyrosine (b)
a. Conditionally indispensable under times of severe bodily stress
b. Conditionally indispensable if methionine and phenylalanine are not available
c. Leucine, isoleucine, and valine are called branch chain amino acids (BCAAs) because of their structure. They will be discussed in greater detail in part 3 of this series.
Complete and incomplete proteins
In the past, dietary proteins were classified as complete, meaning that all indispensable AAs were present ; or incomplete, meaning that one or more of the indispensable AAs was absent. However, with few exceptions (e.g. gelatin) every dietary protein contains all of the AAs in varying amounts. This means that the concept of 'complete' and 'incomplete' proteins is incorrect.
Since all proteins are complete, it is more accurate to refer to the limiting AA of a given protein, which is the indispensable AA occurring in the lowest quantity relative to what is required. One can also determine the second limiting AA, which is the indispensable AA occurring in the second least quantity, etc. The limiting AA will affect how well a given protein can be used by the body.
As an example, grains are typically very low in lysine but high in methionine while legumes are low in methionine but high in lysine. This complementarity between vegetable proteins led to the premise of combining grains and legumes together, to obtain a 'complete' protein (1). In general, limiting AAs should be a non-issue unless an individual is consuming all of their protein from a single source, and only if that source is a poor quality protein to begin with. That is, since proteins differ in their limiting AA, someone eating a variety of protein sources should fulfill their AA requirements with little difficulty. However, consuming large amounts of a low quality protein, one could consume adequate AAs for health and functioning. This would simply be an inefficient way of doing things because an excess of non-limiting AAs would be consumed simply to provide enough of the limiting AA.
Section 2: Protein digestion
Overview of protein digestion
Proteins are broken down in the stomach into smaller chains of AAs via various enzymes such as hydrochloric acid, trypsinogen and pepsinogen which break the bonds between AAs (1). In essence, these enzymes can be though of as scissors which cut large proteins into smaller chunks, which are then further digested. The specifics of breakdown are unimportant for this discussion and interested readers are referred to any biochemistry or nutrition textbook for details (1).
The breakdown of whole protein ultimately produces peptide chains which vary in length. This includes single AAs (peptides), chains of two AAs (di-peptides), and chains of three AAs (tri-peptides). Less than 5% of ingested protein is lost daily in the feces (1).
AAs are absorbed through the intestine via specific transporters. Depending on the transporter type, a certain AA will be absorbed and transported across the intestinal wall into the bloodstream. Additionally, many AA transporters carry more than one amino acid (1). This means that individuals who take large amounts of a single amino acid may overload a given transporter, and impair transport of a different amino acid which is carried by that same transporter. That is, like most nutrients, excess intake of one amino acid can potentially lead to deficiencies of another, due to competition for the same transport mechanism (1).
In addition to the single AA transporters, there are also transporters to carry di- and tri-peptides into the bloodstream (4). Chains longer than 4 AAs cannot be transported across the intestinal wall directly, and must be broken down to smaller chains prior to absorption (4).
The chemistry of protein digestion and transport have implications for a number of supplements. The first is the rash of 'oral peptide hormones' (such as GH or IGF-1) which are being marketed. Since peptide hormones are far longer than 4 AA in length, there is simply no way that oral ingestion can get any of the active hormone into the bloodstream. This assumes that such a product was real in the first place, which it is probably not since GH and IGF-1 are very expensive and are only available by prescription.
The protein digesting enzymes will break down orally ingested peptide hormones into smaller amino acid chains which will be treated just like any other dietary protein. Put differently, there's a reason that GH, IGF-1 and insulin have to be injected: because they are broken down in the stomach to peptide chains, losing any value they might have as hormones.
The same holds for glandular supplements. For those who weren't bodybuilding back in the 80's, glandulars were dried extracts of various glands, which were supposed to improve the function of that same gland in the person who took it. So a thyroid glandular would consist of ground up thyroid gland (which is made of protein like most other tissues in the body) and ingestion was supposed to improve your thyroid function. A testicular (or orchic) glandular was ground up testicular tissue and was supposed to improve testicular function. As a large protein, any glandular will be simply broken down into smaller amino acid chains and treated like any other form of protein, making the claims for glandulars absurd.
Whole proteins, hydrosylates, and free form AAs....oh my
The three major sources of dietary protein available to individuals are whole proteins (food), partially digested proteins called hydrosylates (most protein powders) and free form AA's (formulations containing single peptides). Arguments can be made for and against all three types of protein.
An extremely important point that should be understood is that once AA's enter the bloodstream, they are indistinguishable from one another unless they have been radioactively labeled for research purposes. In fact, it is impossible to distinguish AAs from dietary protein from the AA's present in the body already (5). Therefore, the amino acids derived from an egg will be treated no differently than from an amino acid capsule in terms of it's physiological effects on the body. In this sense, there is no difference between whole, partially digested and free form proteins because they all eventually end up in the bloodstream and are treated the same.
However, there will be differences in the speed of absorption between the various sources of protein. Whole proteins will take longer to digest and be released into the bloodstream than a protein hydrosylate since the hydrosylate has already been partially broken down. This is the idea behind consuming a protein hydrosylate immediately after training: to try and get AAs to the recovering muscles as quickly as possible.
A possible advantage of free-form AAs is that one can tailor the formulation to contain specific quantities of each AA. However this assumes that one knows what the optimal amounts of each AA are in the first place. Additionally, di- and tri- peptides (generated from the breakdown of whole proteins or hydrosylates) show slightly better and faster uptake than free form AA; most likely due to the presence of transporters for di- and tri-peptides in the body (1,4). When the decreased efficiency of uptake of free-form AA is combined with their generally higher cost per gram of protein, free-form AA mixtures should be considered an ineffective form of protein supplement. However, there are some specific AAs, such as glutamine or the BCAAs, which may possibly be of benefit taken by themselves, but these will be discussed in part 3.
Fast vs. slow dietary proteins: the next big thing or much ado about nothing?
A recent paper (6) has kicked off an entirely new category/fad of protein nutrition and marketing, that of fast versus slow dietary proteins. This idea is conceptually similar to the glycemic index (GI) concept which is applied to carbohydrates, and represents the speed at which they digest and affect blood glucose and insulin levels.
In this study, healthy subjects with a normal protein intake (16% of total calories) were fasted for 10 hours and then given either 30 grams of whey protein or 30 grams of casein (milk) protein. The primary finding of the study was that whey protein caused blood leucine levels (which are used as a marker of a variety of metabolic processes in the body) to increase rapidly, hitting a peak in 1 hour. However leucine levels decreased equally rapidly returning to normal by 4 hours. In contrast, casein caused a much slower rise in blood leucine levels, reaching a lower peak around 1 hour, but maintained that level for almost 7 hours (see figure 1).
Additionally, the researchers found that whey protein stimulated protein synthesis (which refers to the building of larger proteins from individual AAs) with no effect on protein breakdown (which refers to the breakdown of larger proteins to individual AAs), while the casein inhibited protein breakdown without affecting protein synthesis. Another observation was that whey protein increased leucine oxidation (burning) slightly more than the casein (31% vs. 24%), probably due to the faster rate of entry. Finally, leucine balance (determined as the amount ingested versus the amount stored) was higher for casein than whey. These observations lend themselves to multiple interpretations.
On the one hand, the effects on protein synthesis and breakdown are interesting and it appears that whey is an 'anabolic' protein while casein is 'anti-catabolic' protein, at least over a span of 7 hours. However, body leucine stores are also important for a variety of reasons (discussed in part 2 of this article) and it could be argued that casein is superior in that it led to net leucine retention by the body. I'm sure that how the study is interpreted will depend on whether the person who is interpreting it wants to sell whey, casein or a blend of the two.
This one study has already generated an article in the prestigious journal Nature (7) as well as in several bodybuilding magazines, with authors suggesting that whey and casein can be used to elicit differential physiological effects and spur growth. Protein supplements containing mixes of the so-called fast and slow proteins have also appeared on the market, with the idea being that one can get increased protein synthesis AND decreased protein breakdown, as well as keeping blood AA levels more stable.
To say that too much has been read into this single study would be putting it mildly. There are a number of issues which have been completely ignored by those reporting on this article which are discussed here. The first and perhaps most important point is that the subjects were fasted for 10 hours prior to being given the protein supplements. Protein synthesis and breakdown rates are very different after a 10 hour fast compared to the rates in the middle of the day after food has been eaten. After an overnight fast, muscle protein synthesis rates may be 50% lower than after food has been consumed (8). This means that any effects from a protein meal would be expected to be much higher first thing in the morning, versus a similar comparison done at a different time of the day.
Additionally, it is well known that the mixing of nutrients (i.e. carbohydrates and protein or carbohydrates, protein and fat) changes the absorption rate of nutrients into the bloodstream (1). Similarly, the presence of undigested food from a previous meal also affects digestion rate. All this study tells us is what will happen when whey or casein is taken by itself, after a 10 hour fast, on an empty stomach. To draw meaningful conclusions from this study to what might happen with the consumption of whey or casein with dietary fat (i.e. flax oil in the blender drink) or carbohydrates, or to other times of the day is impossible.
A final unanswered question is where the protein synthesized during the whey trial was stored (7). That is, the methodology of the study only told the researchers that protein was being synthesized and stored, not where it was going. This is, in fact, a major problem with most human protein research: it is generally difficult to know where stored protein is going unless a biopsy is taken. Since the goal of bodybuilders is to influence muscle protein synthesis, and not just increase whole body protein synthesis, it is important to know where the ingested protein is going. This topic will be discussed in greater detail in part 2 of this series.
Sufficed to say that it is just as reasonable to assume that it was being stored as liver protein as it is to assume it was being stored as muscle protein. Of course, saying that whey will increase liver protein synthesis won't sell a lot of supplements. In part 2 of this series, the issues of protein synthesis and storage after a meal will be addressed in more detail, to help answer this question.
As a final comment, most serious bodybuilders eat a protein containing meal every 2-3 hours as it is. Since blood leucine didn't drop until the 4 hour mark in the whey trial, is it going to make a huge difference whether a bodybuilder consumes whey or milk protein if they are eating every 3 hours? And if casein keeps blood leucine levels up for 7 hours, and whole proteins take even longer to fully digest, is it truly necessary to ingest protein every 3 hours during the day?
Some have suggested consuming a mix of whey and casein right before bedtime to get a sustained release of AA's into the bloodstream and there may be some validity to this. Of course, any whole protein, combined with some carbs, fat and fiber would accomplish just as much.
A final question that this study raises has to do with the post-workout protein feeding. Consider that even the whey protein took 1 hour to raise blood leucine levels to it's peak. If the idea is to provide AAs to a recovering muscle immediately after training, it might make more sense to consume protein an hour or two before the workout, so that AAs are hitting the bloodstream as the workout is ending.
Section 3: Protein requirements
There has been a long running debate over the protein requirements for athletes. On the one hand are dietitians and nutritionists, who continually argue that the Recommended Dietary Allowance (RDA) for protein is sufficient. On the other are the strength athletes and bodybuilders who have traditionally consumed a high protein diet in an effort to gain muscle mass more quickly. To address this controversy, we first have to look at the methodology used to determine protein requirements, and then look at basal requirements for protein. Then we can examine the effects of exercise on protein requirements.
Protein turnover and nitrogen balance
Every day your body is constantly breaking down some proteins, and synthesizing others. This is referred to as protein turnover and will be discussed in much greater detail in part 2 of this series (8). Under normal dietary conditions, an average person may turn over 300 grams of protein in a 24 hour period but obviously the body doesn't require 300 grams of dietary protein per day. This is because most of the protein broken down is reused for protein synthesis.
However, no reactions in the body work with 100% efficiency and protein turnover is no different. In the process of protein turnover, some AAs will be oxidized, and nitrogen will be lost in the form of urea, creatinine, and other substances. Under normal protein intakes, as little as 4% of the total protein turnover may be lost (9). This can be altered with a high or low protein intake, a topic which is discussed in part 3 of this series (9).
Nitrogen is lost primarily in the urine, but some is lost in the feces, sweat, through the skin, finger nails, hair and other bodily excretions. Since it is tedious and difficult to measure all routes of nitrogen loss from the body (do you really want to measure fecal nitrogen losses) estimates are frequently used for fecal, skin, sweat, hair and nail losses (1).
Nitrogen balance compares the amount of nitrogen (from dietary protein) coming into the body versus what is being lost. If an individual is consuming more nitrogen than they are losing, they are in positive nitrogen balance, and are storing nitrogen in the body. If an individual is consuming the same amount of nitrogen as they are losing, they are simply in nitrogen balance, and are neither storing nor losing body nitrogen. If an individual is losing more nitrogen than they are consuming, they are in negative nitrogen balance and are losing body protein.
Since the breakdown of AAs are the main source of nitrogen loss, the excretion of nitrogen gives an indicator of AA breakdown. However nitrogen excretion does not tell you which amino acids are being broken down, or where they are coming from, so it is a crude measure at best.
Also, nitrogen balance depends heavily on total calorie intake (1). An individual who is fasting will lose more nitrogen than someone who is eating carbohydrates but is on a zero protein diet. Additionally, calories from dietary fat will not improve nitrogen balance as well as calories from dietary carbohydrate (10). Finally, individuals who consume very high protein intakes will also excrete more nitrogen, a topic discussed in part 2 of this series (9).
Obligatory protein requirements
The obligatory requirement for protein is defined as the amount necessary to equal what is being lost on a daily basis, so that a person remains in nitrogen balance. This is determined by measuring nitrogen excretion while an individual is on a protein-free diet. Since dietary nitrogen intake is zero, all excreted nitrogen is coming from the breakdown of body proteins. As mentioned above, this value assumes adequate dietary calories and a normal proportion of dietary carbohydrates.
The obligatory protein requirement has been estimated at 50-60 mg/kg/day (9,11). Thus a 100 kg (200 pound) person will lose 5-6 grams of nitrogen per day. Since protein is roughly 16% nitrogen, the loss of 5-6 grams of nitrogen is equivalent to the loss of 33 grams of protein per day.
To correct for digestibility issues, as well as individual variability, a 'safety factor' has been added to the above value (11). This is the basis of the RDA for protein which comes out to 0.8 g/kg/day or 0.36 g/lb/day, which should be sufficient protein for 95% of the population to maintain their current protein stores (11).
For the average male, this works out to roughly 55 grams of protein per day, for the average female 44 grams of protein per day, although this assumes that high quality protein and sufficient energy is being consumed (1). It should be noted that most Americans habitually consume two or three times this much protein on a daily basis, due to their high reliance on animal proteins (11).
In addition to protein requirements, the body has requirements for individual indispensable AAs. There is currently a great deal of debate over the AA requirements for humans at varying times of life (see for example 12,13). Since this requirement for indispensable AAs is tied intimately in with the issue of protein quality, a detailed discussion will have to wait until part 2.
Athletes and protein
Contrary to what many dietitians believe, the RDA was never meant to provide protein requirements for active individuals. In fact, the RDA handbook, which is the official guidelines handed down from the government regarding dietary needs, states "No added allowance is made here for the usual stresses encountered in daily living, which can give rise to transient increases in urinary nitrogen output. It is assumed that the subjects of experiments forming the basis for the requirement estimates are usually exposed to the same stresses as the population generally." (11, pg. 71). As any bodybuilder knows, intense training falls outside the definition of 'usual stresses encountered in daily living'. Additionally, numerous studies have shown that exercise increases the need for protein (for example 14,15).
Both aerobic exercise and strength training increase protein requirements although they do so for different reasons. During aerobic exercise, AAs can be used for energy production (especially the branch-chain AA's, leucine, isoleucine, and valine) and may provide up to 10% of the total energy produced during long-duration activity (16). This occurs to a greater degree if glycogen is depleted (17) which is why excessive cardio may be even more catabolic on a low-carbohydrate diet.
While AAs do not contribute significantly to energy production during weight training, there is still net breakdown of AAs, as well as increased requirements for new protein synthesis.
Ultimately, the cause of the increased protein requirements is less important than the observation that protein requirements most definitely increase. Data reviewed by Dr. Peter Lemon indicates that endurance athletes may need 1.2-1.4 g/kg of protein (0.54-0.63 g/lb) and strength athletes 1.6-1.8 g/kg (0.72-0.8 g/lb) per pound to maintain a positive nitrogen balance (meaning that protein is being stored in the body) (16).
Although there is some limited research suggesting that even higher protein intakes may increase the rate of lean body mass gain (5,18), this has not been found in all studies. As will be discussed in part 2 of this article, excess protein tends to be oxidized (burned off).
This article is the second in a series of articles discussion the details of protein metabolism in bodybuilders. Part 1 addressed some basic concepts and definitions regarding protein, digestion of different forms of protein as well as total protein requirements for athletes. Part 2 of this article series builds on Part 1 with a discussion of protein digestibility and the beginning of a discussion of protein quality, which will be continued in detail in Part 3.
Section 4: Protein digestibility
An important aspect of protein metabolism is how well or how poorly a given protein is digested by the human body. Claims are sometimes made that protein powders (especially predigested or hydrolyzed proteins) are digested more efficiently than whole food proteins. Claims are also occasionally made that vegetable source proteins are more digestible than animal-based.
Protein digestibility is measured by seeing how much nitrogen is excreted in the feces compared to the amount of nitrogen which is ingested. A correction is made for the amount of nitrogen which is normally lost in the feces. Therefore, digestibility research examines how much more nitrogen is lost over normal levels when a given protein is fed.
If an individual were fed 5 grams of nitrogen (approximately 30 grams of protein) and 1 gram of nitrogen was excreted in the feces, this would represent a digestibility of 80% (4 grams retained divided by 5 grams consumed). Table 1 shows the digestibility for some common proteins.
Table 1: Digestibility of common proteins foods
Food source Protein digestibility (%)
Egg 97
Milk and cheese 97
Mixed US diet 96
Peanut butter 95
Meat and fish 94
Whole wheat 86
Oatmeal 86
Soybeans 78
Rice 76
Source: National Research Council. Recommended Dietary Allowances, 10th ed. National Academy Press, 1989.
With the exception of some plant-based proteins, we see that most commonly eaten proteins (e.g. those eaten by bodybuilders) have extremely high digestibilities. Assuming that someone has a normally functioning digestive tract, there is no reason to think that a protein powder will digest significantly better or have a greater impact on growth than a whole food protein (2). Even if a given protein powder did achieve a true digestion of 100% (unlikely, since no process in the body is 100% efficient), this would only mean a 3% difference compared to egg or milk protein. In practical terms, for every 100 grams of protein eaten, there would be a 3 gram difference in intake. While this might be of importance at low levels of protein intake, at the kinds of protein intakes seen in bodybuilders, slight differences in digestibility are unlikely to have a great impact on growth.
Therefore the primary difference between a protein powder and a whole food protein will not be in how well one is digested relative to the other, but in how quickly they will digest. As discussed in Part 1 of this article, the predigested nature of many protein powders will speed digestion and release into the bloodstream.
Section 5: Protein quality
Protein quality is a topic of major debate, both in the research world, as well as in the realm of protein supplements. Arguments have been made that one protein is of higher quality than another, or that protein powders are superior to whole food protein. Since this is an area of such debate, the issue of protein quality will be discussed in some detail.
Protein quality refers, in a general sense, to how well or poorly a given protein will be used by the body. More specifically, it refers to how well the indispensable amino acid (AA) profile of a protein matches the requirements of the body (3). However, this should not suggest that the content of dispensable AAs in a protein is irrelevant to protein quality as the body. As we will see below, the problem of AA requirements is not an easy one to solve, making arguments about protein quality even more problematic.
Methods of measuring protein quality
There are a variety of methods available to measure protein quality. To a great degree, how a protein's quality is rated depends on what method is used. This is part of what allows protein sellers to argue the superiority of one protein over another. For example, measured by one method, egg protein may be the highest quality protein, but by another method, casein may be the highest. Additionally, and perhaps more importantly, the quality of a protein is directly related to the physiological needs of the subject being studied (3).
The protein that is optimal for a bodybuilder in a mass phase may not be the same as the protein that is optimal while dieting or for an endurance athlete. Diet and activity can affect how AAs are used in the body. For example, long-duration endurance activity tends to oxidize high quantities of the branch-chain amino acids (BCAAs) (4), suggesting that endurance athletes might have a higher BCAA requirement than non-endurance athletes. In all likelihood, there is no single protein that can be rated as the highest quality for all situations.
So the first question which has to be answered is which method of rating proteins is ideal for humans. The short answer is that none of them are ideal, since all make assumptions, or are based on models that may or may not be accurate. The second question, which no one has addressed, is whether the AA requirements of a sedentary individual is the same or different than that of a hard training bodybuilder.
Although there are numerous different methods to compare proteins, only a few are used frequently enough in popular literature to require discussion. They are: chemical score, biological value, protein efficiency ratio, and protein digestibility corrected amino acid score.
Chemical score (5)
Chemical score is method of rating proteins based on it's chemical composition (more specifically it's indispensable AA levels). To determine chemical score, a protein is picked as a reference and other proteins are rated relative to that reference protein. This is conceptually similar to giving white bread a value of 100 on the glycemic index scale and rating other carbohydrates relative to that.
Typically, egg protein has been used as the reference protein, but this assumes that the amino acid profile of egg is the ideal for humans. Recently, other amino acid patterns have been suggested to replace egg based on increasing information about the AA requirements of humans. In 1985, a joint committee on protein requirements suggested an idealized reference pattern (6), but this pattern has been criticized by many researchers as being too low (for example 7,8).
Since chemical score is a relative, and not an absolute scale, it is possible to have values greater than 100. If 5 grams of the reference protein contains 800 mg of a certain amino acid, and 5 grams of the test protein contains 1000 mg of that same amino acid, the second protein would be rated as 125% for that amino acid.
The indispensable amino acid present in the lowest quantity (relative to what is required) is defined as the first limiting amino acid (see Part 1 for a discussion of limiting amino acids). The second lowest indispensable AA relative to requirements would be the second limiting amino acid, etc. In general, the limiting amino acid will determine how well or how poorly a given protein is used by the body. This suggests that supplementation of the limiting amino acid (i.e. taking additional methionine with a protein which is limited by methionine) or combining proteins with different limiting amino acids might increase the quality of the protein.
Chemical score can also be used to compare a given protein to the amounts required by the subject in question. This is somewhat more useful in that it takes into account the needs of the individual, assuming they are known. That is, if a given protein provided 100 mg/kg of a certain AA, and 150 mg/kg of that AA were required by an individual, the chemical score would be 0.67 for that amino acid (meaning that the protein in question provided only 67% of the amount required by that person).
While chemical score is useful for rating proteins based on their composition, it has one major drawback: it has little to do with how a food protein will be used in the body (human or otherwise) since it does not take into account digestibility. For this reason, chemical score is rarely the only measure of protein quality used to rate a protein.
Biological value (BV) (5)
Biological value (BV) is probably one of the most commonly used measures of a protein's quality. The BV of a protein is given as the amount of nitrogen retained in the body divided by the amount of nitrogen absorbed from that protein. Therefore, digestibility of that protein is taken into account. Thus:
A BV of 100 would indicate complete utilization of a given dietary protein, in that 100% of the protein ingested was stored in the body with none lost.
To measure BV, subjects are typically fed a zero protein diet so that baseline losses of nitrogen can be measured (i.e. the amount of nitrogen that is lost normally). Then the test protein is fed at varying levels (generally 0.6, 0.5, 0.4 and 0.3 g/kg are fed) and a nitrogen balance study is done (9). Some studies use longer periods of starvation and this is an important consideration in evaluating the data.
For example, the study often cited by advertisers to demonstrate the 'superiority' of whey protein hydrosylate measured nitrogen balance in rats after three days of starvation, which corresponds to a longer period in humans (10). In this study, whey protein hydrosylate led to better nitrogen retention and growth than the other proteins studied. What is not mentioned is that starvation affects how well the body will store incoming protein, leading to falsely elevated BV measures. This study has little bearing to an individual with a habitual high-protein intake. A full discussion of the effects of low protein eating (i.e. protein cycling) will appear in Part 3 of this article series.
Although nitrogen balance methodology has it's problems (see Part 1 of this article series), it is a rough indicator of how well or poorly a given protein supports the body's needs. If a given amount of protein (more accurately, a given amount of nitrogen) places an individual in nitrogen balance (or positive nitrogen balance) it can be assumed that the protein in question is of sufficient quality to support maintenance of body protein stores.
The biggest drawback of the nitrogen balance method is that it gives no information regarding specific amino acid metabolism (and deficiencies) or the specific tissues which are being affected (e.g. muscle vs. liver), only an indication of what is occurring on the whole body level (9). Depending on the individual amino acid requirements of a given tissue, it is possible that a protein might optimally support protein synthesis in one organ, such as the liver, while not optimally supporting synthesis in another tissue, such as muscle. The issue of whole body versus specific tissue protein metabolism will be discussed in Part 3 of this series.
Despite what is sometimes claimed, it is impossible to have a BV greater than 100. Additionally, there is no indication that the percentage sign was ever dropped from the BV measure. For example, it's been suggested that whey protein has a BV of 157, but this would imply that 1.57 grams of nitrogen were stored for every 1 gram of nitrogen consumed. Since it is thermodynamically impossible for the body to store more nitrogen than was ingested, a BV of 157 is equally impossible. Protein advertisements claiming BV higher than 100 should be looked upon with suspect.
One aspect of measuring BV that can cause problems in interpretation of results is that the BV of a protein is affected by a number of factors. The first of these is caloric intake. A very high caloric intake will improve nitrogen balance at any given protein intake and vice versa. This means that an individual consuming a lot of calories (e.g. a bodybuilder on a mass-gaining diet) will show improved nitrogen retention and 'apparent' BV will go up (i.e. more nitrogen will be retained compared to the amount eaten). By the same token, if calories are decreased (e.g. during a diet), BV will go down. A secondary factor which affects BV is activity. Exercise, especially weight training, increases nitrogen retention which will give a protein a higher apparent BV.
A third factor, and one that is typically ignored in popular literature is that the BV of a protein is related to the amount of protein given (9). BV is measured at levels below the maintenance level. As protein intake goes up, the BV of that protein goes down. For example, milk protein shows a BV near 100 at intakes of 0.2 g/kg. As protein intake increases to roughly maintenance levels, 0.5 g/kg, BV drops to 70 or so (9).
To quote from Pellett and Young, "....protein is utilized more effectively at suboptimal levels than at levels in the near-maintenance range of intake. Accordingly biological measures of protein quality conducted at suboptimal levels in either experimental animals or human subjects may overestimate protein value at maintenance levels." (9) Therefore, while BV may be important for rating proteins where intake is below requirements, BV has little bearing on individuals with protein intakes far above requirements. Table 2 presents the BV of some common proteins.
Table 2: BV of some common proteins
Protein BV
whey 100?
egg 100
milk 93
rice 86
casein, fish and beef 75
corn 72
peanut flour 56
wheat gluten 44
Source: Normal and Therapeutic Nutrition, 17th ed. Corinne H. Robinson, Marilyn R. Lawler, Wanda L. Chenoweth, and Anne E. Garwick. Macmillan Publishing Company, 1986.
Considering the high protein intakes of most strength athletes (2.0 g/kg or higher) it is hard to see how BV will play a meaningful role in rating proteins in this population. In all likelihood, any decent quality protein will be as good as any other at these types of protein intakes. Additionally, even if proteins such as whey have slightly higher BV ratings than protein sources like casein (milk) or egg, such a small difference is unlikely to affect mass gains in the long run.
Protein efficiency ratio (PER) (5)
PER is sometimes used to rate proteins and represents the amount of weight gained (in grams) relative the amount of protein consumed (in grams). For example, a PER of 2.5 would mean that 2.5 grams of weight was gained for every gram of protein ingested. Since it is impossible to measure weight gain in grams in humans, PER is generally measured in young, growing animals placed on a diet which is 10% protein by weight. This begs the question of whether young animals, who are growing, provide a good model for adult humans. While the Food and Drug Administration has suggested the use of PER with casein as a reference model for labeling protein foods (12), the use of PER to estimate human protein requirements has been criticized by some authors (13).
While the use of PER to rate proteins for humans is debatable, it should be noted that a recent animal study found that combinations of animal (30% of total) and plant based proteins (70% of total) had a higher PER value than the animal or vegetable proteins eaten alone (14). This may have to do with the proteins 'combining' to decrease the impact of the limiting AA. Individuals who wish to decrease their intake of animal-based proteins may be able to achieve higher PER values with a combination of animal and plant based proteins than someone eating only animal based proteins.
Protein digestibility corrected amino acid score (PDCAAS)
PDCAAS is the newest method of protein quality to be developed. It has also been suggested as the ideal scale to rate proteins for their ability to meet human requirements (15). Similar to chemical score, it rates protein foods relative to a given reference protein. In this case, the AA profile used is that one determined to be ideal for children two to five years old as its reference protein for adults (15). This obviously raises the immediate question of how much relevance this AA profile has to adult bodybuilders.
PDCAAS goes beyond chemical score, however, by factoring in the digestibility of a given protein, giving the AA profile more relevance to human needs. Interestingly, using the PDCAAS method, along with the proposed AA reference patter, proteins which were previously rated at poor quality, such as soy, have obtained higher quality ratings (16). This is more in line with studies showing that certain purified soy proteins, such as Supro (tm) which is found in Twinlab Vege-fuel, can maintain adults in nitrogen balance (3,16). Once again, whether the use of PDCAAS to rate proteins for adult bodybuilders is debatable since the physiology of weight training may affect requirements for certain amino acids (i.e. glutamine, BCAAs).
Summary of protein quality
Although a variety of methods of measuring protein quality have been proposed, none are perfect in rating proteins for human use. While some methods of rating protein are based on how well (or poorly) an animal grows (or the nitrogen balance which is attained), these methods provide no information on specific amino acid requirements or protein synthesis at a given tissue. Rather, only data regarding growth in the whole body are obtained.
Another strategy to rate proteins is to compare the AA profile in food protein to some reference protein. Previously, food proteins such as egg or milk were used as a reference but there has been a recent move toward the use of an idealized reference pattern of AAs to rate proteins. This assumes that the true requirements for a given AA are known, which is discussed in section 6.
Ultimately, all the methods of rating protein quality described above are insufficient for rating proteins for bodybuilders. They are used primarily to determine minimum requirements to either support optimal growth in children (which differs physiologically from the growth seen in bodybuilders since much of the tissue synthesized is organ, and not muscle tissue) or maintenance in adults. None are meant (or should be) used to determine the quality of various proteins for an adult bodybuilders interested in gaining muscle tissue (who is maintaining other bodily tissues).
In part 3 of this series, we will further examine the issue of protein quality by looking at AA requirements. Various dietary proteins will be examined within this context. Part 3 will also include the development of a basic kinetic model of AA flow within the body including the various fates that amino acids may have. This model will be used to examine the adaptations which occur to both high and low protein intakes so that dietary strategies such as protein cycling can be examined.
References
1. National Research Council. Recommended Dietary Allowances, 10th ed. National Academy Press, 1989.
2. Moriarty, KJ et. al. Relative nutritional value of whole protein, hydrolysed protein and free amino acids in man. Gut (1985) 26: 694-699.
3. Young, VR. Soy protein in relation to human protein and amino acid nutrition. J Am Diet Assoc (1991) 91: 828-835.
4. Wagenmakers, AJM. Muscle amion acid metabolism at rest and during exercise: role in human physiology and metabolism. Exercise and Sports Science Reviews (1998) 26: 287-314.
5. Advanced Nutrition and Human Metabolism, 2nd ed. James L. Groff, Sareen S. Gropper, Sara M. Hunt. West Publishing Company, 1995.
6. Energy and protein requirements. Report of a joint FAO/WHO/UN expert consultation. WHO technical report series 724. Geneva: World Health Organizations, 1985.
7. Young, VR and El-Khoury, AE. Can amino acid requirements for nutritional maintenance in adult humans be approximated from the amino acid composition of body mixed proteins? Proc Natl Acad Sci USA (1995) 92: 300-304.
8. Millward, DJ. Metabolic demands for amion acids and the human dietary requirement: Millward and Rivers (1988) revisited. J Nutr (1998) 128: 2563S-2576S.
9. Pellett, PL and Young, VR. Nutritional evaluation of protein foods. United Nations University, 1980.
10. Poullain, MG et. al. Effect of whey proteins, their oligopeptide hydrosylates and free amino acid mixtures on growth and nitrogen retention in fed and starved rats. J Parenteral and Enteral Nutrition (1989) 13: 382-386.
11. Normal and Therapeutic Nutrition, 17th ed. Corinne H. Robinson, Marilyn R. Lawler, Wanda L. Chenoweth, and Anne E. Garwick. Macmillan Publishing Company, 1986.
12. Henley, EC. Food and Drug Administration's proposed labeling rules for protein. J Am Diet Assoc (1992) 92: 293-294, 296.
13. Young, VR. and Pellett, PL. Protein evaulation, amino acid scoring and the Food and Drug Administrations's proposed food labeling regulations. J Nutr (1991) 121:145-150.
14. Hernandez, M et. al. The protein efficiency ratio of 30:70 mixtures of animal:vegetabls protein are similar or higher than those of the animal foods alone. J Nutr (1996) 126: 574-581.
15. Food and Agriculture Organization and World Health Organization (1990) protein quality evaluation. Report of a joint FAO/WHO expert consultation. Food and agriculture organization of the United Nations, Rome, Italy.
16. Young, VR and Pellett, PL. Plant proteins in relation to human protein and amino acid requirements. Am J Clin Nutr (1994) 59 (suppl): 1203S-1212S.
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