Protein, Part 3: Individual Amino Acid Requirements

by Lyle McDonald, CSCS
Author of The Ketogenic Diet

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Articles in the series:

Part 1: Technical Definitions and Background
Part 2: Protein Metabolism and Requirements
Part 3: Individual Amino Acid Requirements
Part 4: Amino Acid Kinetics and Adaptations

Introduction

In Part 2 of this series, the topic of protein quality was discussed, including examination of the major methods currently used to rate protein quality. For a variety of reasons, most of the currently available methods to rate proteins are deficient, especially when they are applied to an active population. Continuing with that discussion, this part of this article series will address the topic of individual amino acid (AA) requirements, which are arguably more important in terms of determining how well or how poorly a given protein will sustain tissue. Finally, Part 4 will develop a simplified model of AA metabolism in the body and examine the adaptations which occur to high- and low-energy and protein intakes.

Section 6: Determining AA requirements

Most methods of rating protein for adults are based on the assumption that the majority of incoming AAs are being used primarily for maintenance of existing tissue, not growth of new tissue. Obviously this assumption is incorrect for bodybuilders who are trying to synthesize new tissue, making most measures of protein quality (which are aimed at determining maintenance protein requirements) fairly irrelevant. This raises several important questions.

First and foremost, are there any differences in the AA profile required by bodybuilders than by sedentary individuals? A second, and related question, is whether it is sufficient for bodybuilders to simply consume more of the same AA profile which is required by sedentary individuals or should the additional protein have a different AA profile?

Unfortunately, both of these questions require examination of the changes in AA metabolism that occur during weight training, and very few studies are available. While numerous studies have examined the effects of endurance training and other forms of stress such as surgical trauma on AA metabolism, these models are inappropriate to apply directly to bodybuilding.

A comment regarding methodology

The determinant of human AA requirements has been the subject of considerable research for several decades. And while improvements have been made in the determination of those requirements, there are numerous methodological problems associated with determining true AA requirements in human subjects (1). Numerous assumptions regarding the model used, the radioactive tracer used, etc. have to be made to do research in this area.

As well, since the body can show adaptations to both low and high protein intakes, it is difficult to determine true AA requirements. In the words of one researcher "In my view, definition of adult IAA [indispensable AA] requirements for protein quality scoring is not currently possible or likely to be useful." (2)

Compared to glucose and fat metabolism, where researchers only have to follow one fuel source, AA metabolism is infinitely more complex as 20 AAs, which can be metabolized and interconverted, must be traced into tissues which show drastically different utilization (as well as rates of protein breakdown and synthesis). What this means is that current technology (based on the use of radioactive tracer) can only provide small bits of information regarding the topic of AA requirements. In this vein, advances in research technology may leave everything discussed in this article completely incorrect.

Growing vs. non-growing subjects and protein requirements

As stated above, the average adult human or animal does not grow appreciably and bodily tissues are more or less set in place (although there is constant breakdown and synthesis of these tissues). This means that most of the incoming protein is being used for maintenance of current tissue, not for the synthesis of new tissues. Even in children, who are growing fairly rapidly, the growth component of total protein requirements is 15% or less (3) with the remainder of protein requirements being used for maintenance. In adult bodybuilders, we might expect this percentage to be even lower due to the generally slow rate of muscle growth (see below).

This difference in metabolism between children and adults appears when one examines protein requirements (and especially indispensable AA requirements) in infants versus adults. Thus, it seems appropriate to compare protein and AA requirements in a variety of sub-groups to see if any patterns develop.

Table 1 compares protein and AA requirements in a variety of age groups based on current FAO/WHO recommendations (4). It should be noted that the FAO/WHO recommendations for AAs has been criticized and recent research shows that the adult values for indispensable AA requirements may be as much as three times greater than predicted (5-9). Finally, protein requirements which have been suggested for athletes are presented for comparison purposes (10).

Table 1: Protein and indispensable AA needs at different ages

1. FAO/WHO/UNU. Energy and protein requirements. Report of a joint FAO/WHO/UNU expert consultation. WHO Tech Report Ser 1985; 724.

2. Suggested values for indispensable AAs in Young, V. 1987 McCollum lecture. Kinetics of human amino acid metabolism: nutritional implications and some lessons. Am J Clin Nutr (1987) 46: 709-725.

3. Lemon P. Is increased dietary protein necessary or beneficial for individuals with a physically active lifestyle? Nutrition Reviews (1996) 54: S169-S175.

It is an interesting coincidence that total protein requirements (in terms of g/kg) for infants, the group who is growing the most rapidly, are similar to those which have been determined for strength athletes (see Part 1 of this article). One might expect that indispensable AA requirements are also relatively higher in adult bodybuilders compared to sedentary individuals. If so, this would further support the notion that protein ratings, which have been developed around adults who are maintaining their body protein stores should not be applied to bodybuilders at all (see Part 2 for more details).

It should be noted that bodybuilders are not growing at nearly the rate of an infant, and the new tissues being synthesized in the infant are far different from those being synthesized in the adult bodybuilder. Therefore, any quantitative extrapolation from this section is inadvisable. In all likelihood, the AA profile required by a bodybuilder synthesizing mainly muscle protein is far different than the amino acid requirement for an infant synthesizing a variety of tissues (muscle, organ, brain etc).

However, as mentioned above, researchers have suggested that adult AA requirements may be three times higher than current values (5-9), which puts them at roughly the same level of requirement as children aged 2 years (Table 1). Table 2 compares the AA requirements for children 2 years old to the amounts of AAs found in some commonly eaten proteins.

Table 2: Comparison of AA requirements to amounts found in common proteins

All values are mg AA/g protein

Comparing the values for indispensable AAs for a variety of high-quality proteins to the AA requirements of children aged 2 years, we see that all of the proteins examined contained greater quantities of Aas than required. Additionally, the differences in AA content between whey protein and other high-quality proteins is not nearly as great as implied by advertisements. With the exception of valine, lysine and threonine, milk protein is quite comparable to whey, and significantly cheaper. Interestingly, and quite against the commonly held belief, soy protein isolate is sufficient to meet indispensable AA requirements.

Based on the above table, there is no reason to believe that one high-quality protein will show any benefit over another high-quality protein as they all contain indispensable AAs in excess of what is required. This brings us back to the question raised in the introduction to this article: do bodybuilders require a different profile of AAs than sedentary individuals, or simply the consumption of more AAs (and protein) as a whole?

Before moving on to the next topic, there is a claim sometimes made regarding proteins that Table 2 can address. It is sometimes argued that a given protein is superior to another because it has higher quantities of a given indispensable AA (or some combination of indispensable AAs). While this might be true if we were comparing a high-quality protein to a lower-quality protein (defined here as one that did not meet or exceed requirements), the simple fact is that all of the high-quality proteins commonly eaten by bodybuilders contain sufficient indispensable AAs to cover requirements (this of course assumes that the stress of training does not affect the profile needed by bodybuilders). As we shall see in Part 4 of this series, the excess AAs not needed are simply disposed of anyway. So whether whey has a higher proportion of indispensable AAs than milk is rather irrelevant, since both are in excess of what is required.

Exercise and AA requirements

As discussed in Part 1 of this series, recent studies have established that both strength and endurance training increase protein requirements, although through different mechanisms.

It is well established that long-duration endurance exercise increases the oxidation (burning) of AAs, especially as glycogen is depleted (see Part 1 for references). Human skeletal muscle is only capable of oxidizing 6 AAs during exercise: the branch chain amino acids (BCAA :leucine, isoleucine, valine), asparagine, aspartate and glutamate (14). One would expect that exercising muscle would release these AAs (and others) in proportion to their concentration in skeletal muscle but this is not the case and none of the above AAs is released in proportion to their concentration in the muscle.

Rather, the main AAs released from muscle during endurance exercise (as well as during times of stress such as starvation and perhaps dieting) are alanine and glutamine, and they are released in much higher concentrations than they are present in the muscle in the first place. This indicates that these two AAs are being synthesized within the muscle, most likely from metabolism of the 6 AAs listed above (14). In fact, the synthesis rate of glutamine in the body has been estimated at 20-80 grams/day (14).

During weight training, protein makes an almost non-existent contribution to energy production. Additionally, the amount of protein needed to cover daily synthesis in non-drug using bodybuilders is tiny, roughly 30 mg/kg per day (15). For a 100 kg (220 lbs.) lifter, this means that an additional 3 grams of protein/day above maintenance requirements are all that is needed to cover the synthesis of new tissue. Even steroid users exhibiting maximal rate of muscle growth may only need 180 mg/kg (approximately 18 grams of protein extra for an 100 kg lifter) (15). This however does not explain the significantly higher protein requirements which have been found necessary to maintain nitrogen balance in strength trainers (10).

In all likelihood, the majority of increased protein requirements seen with strength training (10) are needed to cover the breakdown of tissue which occurs during training. Unfortunately, the exact amount (or AA profile) of tissue broken down has not yet been quantified. Despite it's shortcomings, the nitrogen balance data for strength trainers (reviewed in 10) is the best data we have, simply indicating that a greater amount of protein is needed by weight trainers. However, we might be able to make some educated guesses regarding specific AAs based on what is known about AA metabolism.

Since glycogen depletion is known to activate the enzyme involved in oxidizing BCAA (16), it seems plausible that the glycogen depletion seen with weight training might increase BCAA oxidation. Considering that all the dietary proteins listed in table 2 contain BCAA far above the proposed requirements, it seems unlikely that the consumption of extra BCAA would have a large impact on growth.

Ensuring optimal muscle glycogen stores and providing glucose during a workout (such as with the consumption of a dilute carb drink) should prevent any oxidation of BCAA during exercise (16). Additional BCAA (either from whey protein or supplements) might show a benefit during a diet (when carbohydrate intake is decreased) and a recent study involving wrestlers noted greater fat loss when additional BCAA was given (17).

As glutamine is involved in acid-base balance (18), the increases in lactic acid with weight training might increase glutamine synthesis (and hence depletion of the above AAs) in muscle and increase requirements. In contrast to this idea, a recent study examined blood concentrations of glutamine in different athletes and found that powerlifters showed the lowest blood glutamine levels, suggesting a difference in glutamine metabolism compared to endurance athletes (19). Another recent study found no change in glutamine levels with heavy eccentric weight training (20).

A different approach

Some protein researchers have suggested that human AA requirements be based on the mixed tissue AA profile in the body (3). That is, since maintenance of existing tissue is the goal of dietary protein intake in most individuals, it makes sense that the AA profile needed would match that of the tissues in the body.

This idea might be extended to suggest that the optimal AA profile for the additional protein needed by bodybuilders is the same as that found in muscle in the first place. That is, while the AA requirements determined by the FAO/WHO, etc. are based on whole-body AA requirements, bodybuilders are ultimately interested in providing the AAs necessary for growth of skeletal muscle. Arguably the protein with the closest AA profile to human muscle is animal muscle (i.e. meat) and this type of argument has been used as 'proof' that meat protein will build the most muscle. Additionally, the occasional protein powder has been developed based on the AA profile of human muscle.

The problem: digestion and metabolism revisited

Although Part 1 of this series discussed digestion of protein in great detail, we need to examine what happens after AAs are released from the gut and into the portal vein (the vein which takes all ingested nutrients, except for fatty acids, into the liver). As amino acids move into the portal vein, their first stop is in the liver. This is referred to as the hepatic 'first pass'.

With the exception of fat and cholesterol, the metabolism of most nutrients occurs initially in the liver. With the exception of the BCAAs, which are metabolized primarily in muscle, the degrading enzymes for all other AAs are found in highest concentrations in the liver (21) and up to 58% of all AAs ingested may be oxidized in the liver upon first pass (22). Feeding has long been known to stimulate AA oxidation in liver, especially when AA in excess of requirements is consumed (21). AA oxidation will be discussed in greater detail in Part 4.

Even when AAs are infused, the majority (70-75% of total) of AAs are absorbed by the splanchnic bed (liver, etc) with the remainder (25-30%) being absorbed by the muscle (23). In contrast, muscle absorbs 65-70% of the total BCAA infused (23). Other studies have shown that the increase in plasma AA levels matches the profile of indispensable AAs in the protein ingested but not the profile of dispensable AAs (24).

The take home message of this section is this: the AA profile of the protein ingested only marginally determines the profile of AAs which will be 'seen' (i.e. absorbed) by muscle. Rather, AAs consumed in excess will be burned off (via the stimulation of the oxidizing enzymes in the liver) while those which are needed will be released into the bloodstream for use by various tissues. Ultimately, this points to the conclusion drawn in Part 2 of this article: as long as bodybuilders and other athletes obtain sufficient amounts of both protein AND indispensable AAs, there is little reason to believe that any one protein will have a greater impact on growth than any other. Considering that all high-quality proteins contain indispensable AAs in excess of requirements, and considering the generally excessive protein intake seen in bodybuilders to begin with, any effect of different proteins on gains is that much more unlikely.

Summary

Although arguments are commonly made regarding the superiority of one protein over another in terms of supporting mass gains, we have seen that most high quality proteins more than fulfill the requirements for indispensable AAs, even if we assume requirements to be three times higher than are currently recommended. Although there is some evidence that specific AAs, such as the BCAAs or glutamine, might be needed in higher quantities, the amounts required have not yet been quantified. At this time, it seems unlikely that one high-quality protein will show significantly different results in terms of mass gained over another, especially considering the high protein and caloric intakes seen in bodybuilders. One would expect there to be a greater difference in protein during a diet. In this situation, extra intake of BCAAs might have a benefit in sparing muscle loss.

Coming up

In the final part of this series, a simplified model of AA metabolism will be developed so that the adaptations to high- and low-protein and energy intakes (e.g. protein cycling) can be discussed.

References

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2. Millward, J. Can we define indispensable AA requirements and assess protein quality in adults? J Nutr (1994) 1509s-1516s.

3. Young, VR and El-Khoury, AE. Can amino acid requirements for nutritional maintenance in adult humans be approximate from the amino acid composition of mixed body proteins? Proc Natl Acad Sci, USA (1995) 92:300-304.

4. FAO/WHO/UNU. Energy and protein requiements. Report of a joint FAO/WHO/UNU expert consultation. WHO Tech Report Ser 1985; 724.

5. Marchini JS et. al. Requirements for indispensable amino acids in adult humans: longer-term amino acid kinetic study with support for the adequacy of the Massachusetts Institute of Technology amino acid requirement protein. Am J Clin Nutr (1993) 58: 670-683.

6. Meguid, MM et. al. Leucine kinetics at graded leucine intakes in young men. Am J Clin Nutr (1986) 43: 770-780.

7. Meguid, MM et. al. Valine kinetics at graded valine kinetics in young men. Am J Clin Nutr (1986) 43: 781-786.

8. Meredith, CN et. al. Lysine kinetics at graded lysine intakes in young men. Am J Clin Nutr (1986) 43: 787-794.

9. Zhao, Xi-he et. al. Threonine kinetics at graded threonine intakes in young men. Am J Clin Nutr (1986) 43: 795-802.

10. Lemon P. Is increased dietary protein necessary or beneficial for individuals with a physically active lifestyle? Nutrition Reviews (1996) 54: S169-S175.

11. National Research Council. Recommended Dietary Allowances, 10th ed. National Academy Press, 1989.

12. Boza, JJ et. al. Nutritional value and antigenicity of two milk protein hydrosylates in rats and guinea pigs. J Nutr (1994) 124:1978-1986.

13. Young, VR. Soy protein in relation to human protein and amino acid nutrition. J Am Diet Assoc (1991) 91: 828-835.

14. Wagenmakers, AJ. Protein and amino acid metabolism in human muscle. Skeletal Muscle Metabolism in Exercise and Diabetes. ed. Richter et. al. Plenum Press: New York, 1998.

15. Millward, DJ et. al. Physical activity, protein metabolism and protein requirements. Proc Nutr Soc (1994) 53: 223-240.

16. Wagenmakers, AJ et. al. Carbohydrate supplementation, glycogen depletion, and amino acid metabolism during exercise. Am J Physiol (1991) 260: E833-E890.

17. Mourier, A et. al. Combined effect of caloric restriction and branched-chain amino acid supplementation on body composition and exercise performance in elite wrestlers. Int J Sports Med (1997) 18: 47-55.

18. Walsh, NP et. al. The effects of high-intensity intermittent exercise on the plasma concentrations of glutamine and organic acids. Eur J Appl Physiol (1998) 77:434-8

19. Hiscock, N and MacKinnon LT. A comparison of plasma glutamine concentration in athletes from different sports. Med Sci Sports Exerc (1998) 30: 1693-1696.

20. Gleeson, M et. al. The effect of severe eccentric exercise-induced muscle damage on plasma elastase, glutamine and zinc concentrations. Eur J Appl Physiol (1998) 77:543-6

21. Benevenga, NJ et. al. Role of protein synthesis in amino acid catabolism. J Nutr (1993) 123:332-336.

22. Jungas, RL et. al. Quantitative analysis of amino acid oxidation and related gluconeogenesis in humans. Physiological Reviews (1992) 72: 419-448.

23. Gelfand, RA et. al. Removal of infused amino acids by splanchnic and leg tisues in humans. Am J Physiol (1986) 250: E407-E413.

24. Ashley, DV et. al. Plasma amino acid responses in humans to evening meals of differeing nutritional composition. Am J Clin Nutr (1982) 36: 143-153.