Protein Requirements In Humans

Protein Requirements In Humans
by Dr. Gary Farr on 29 March 2002

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By Michael G. Kurilla
September 13, 1996

Here's an article discussing the data regarding protein requirements in humans. I've also touched on the some of the arguments put forth by low protein proponents.

One frequent criticism of a Zone style diet and other low carb diets in general is the notion that these diet are "high-protein." In addition, there is a great deal of confusion about actual protein requirements in humans as well as potential adverse effects from excessive protein ingestion. This article will serve to review the existing data on protein requirements as well as discuss some of the potential concerns which are in general (except for specific populations) unwarranted for most people.

There are many groups who feel that recommended protein intakes are overdone. This situation likely stems from several independent factors that all serve to come down in favor of low protein intakes. We can discuss some of these issues first and then discuss the actual data on protein requirements.

Protein appears to be receiving a bad rap for several reasons. First, animal products are an excellent source of good balanced protein. Since plant based foods have lower protein and since the mix of amino acids in many plants is not balanced according to dietary needs of humans, many vegetarians have tried to argue that protein requirements are not as high as indicated by the RDA. Second, given the emphasis in nutrition over the last 15 years has been at lowering fat intakes and saturated fat intakes, sources of these items have been targeted for reduction in diets. Since, many of the "bad" fat foods are also excellent sources of balanced protein, part of the public health message has been to convince people that they don't need as much protein as they have been led to believe. Third, individuals with certain forms of renal disease can slow progression with a low protein diet. Fourth, high protein intakes have been associated with a greater degree of calcium excretion and reduced bone formation in young women suggesting that low protein diets may prevent osteoporosis. And fifth, low protein diets appear to extend lifespan.

The vegetarian argument can be dealt with easily. The assumption is made that the current RDA actually doubles the measured protein requirement in order to provide enough leeway for variability in diets. This is simply erroneous. Where this doubling notion arises from is the fact that someone fed a diet devoid of protein will result in a loss of protein from the body of around 30 grams per day. However, this is a gross simplification and makes many assumptions that consuming 30 grams of protein a day is sufficient to maintain protein homeostasis. The fallacy lies in ignoring the body's compensatory adaptations that permit protein to be conserved when in short supply. This is not to say that people cannot survive on lower protein intake, but rather that over time the body will adapt by slowly lowering lean body mass.

The second point is equally fallacious, since it ignores the use of low fat and/or low saturated fat sources of animal protein. There are clearly other ways to reduce saturated fat intake without resorting to restricting protein intake, but for this and some of the other reasons discussed here, public health official tend to adopt a narrow viewpoint.

The renal disease issue first surfaced in animal studies and does apply to humans (in the setting of particular disorders); however, it is quite another leap to conclude that excess protein actually initiates or contributes to creation of renal disease, rather than merely exacerbating it. There is little if any evidence in humans that high protein intakes are associated with the promotion of renal disease.
The data on osteoporosis are also rather clear even if largely misinterpreted. While there is a slight negative correlation of bone density formation with protein intake, and a weak positive correlation with calcium intake, there is a strong correlation with a high calcium to protein ratio. Thus, for a given level of protein intake, a minimum level of calcium intake in required. More protein means more calcium.

Finally, the calorie restriction, low protein diets extending lifespan have only been demonstrated in small animals with short lifespans living under well controlled laboratory conditions. What is not clear is how applicable these results are to humans free living in a normal environment.

Now on to the actual data. Much of the following discussion will focus on a recent review (Nutr Rev. 54:S169-S175, 1996) by Peter Lemon of Kent State University. Lemon is a respected researcher in the protein requirement field for over 15 years. I'll quote occasionally so that statements I am making are not regarded as my interpretation, but actually reflect what is written.

To begin, quoting Lemon, "...the current recommended dietary allowance (RDA) for protein (NRC, 1989) does not include an additional allowance for those who regularly engage in physical exercise."

Resistance Training

Lemon divides physical activity into two broad categories, heavy resistance exercise and endurance exercise. For resistance exercise, Lemon cites several studies that have approached this subject from different aspects. Fern (1991) compared body mass gains using 3.3 versus 1.3 grams of protein per kilogram of bodyweight per day. After 4 weeks, the 3.3 group had larger gains. Meredith et al. (1992) found the addition of 23 grams of protein per day enhanced muscle gains. Two studies are also cited that held individuals at the RDA for protein and found negative nitrogen balance after engaging in strength training (Lemon et al., 1992, Tarnopolsky et al., 1992). The Tarnopolsky study also used protein intakes of 1.4 and 2.4 grams per kilogram per day with no difference in protein synthesis between the 1.4 and 2.4.

Lemon summarizes this work as follows, "These studies indicate that the RDA (which was determined using subjects who were essentially sedentary) is insufficient for individuals who are involved in a heavy resistance training program." Lemon goes on to show linear regression analysis to arrive at a protein intake of 1.7 - 1.8 grams of protein per kilogram of bodyweight per day to maintain nitrogen balance while working out with weights.

Endurance Exercise

For endurance exercise, the issue of protein requirement is a little different. While muscle growth is not likely to occur, increased protein needs do occur for two other reasons. The first is applicable to strength training as well which is that physical activity increases protein turnover and requires a greater daily intake to sustain itself. The second aspect for endurance is that protein is burned for fuel along with fats and carbs. Both exercise intensity (Babij et al., 1983) and exercise duration (Haralambie and Berg, 1976) influence protein utilization. For exercise intensity, the protein contribution is roughly linear. For duration, the utilization of protein becomes substantial after about 60 minutes.

One complicating factor is that certain amino acids are utilized to a greater extent than other. Branched chain amino acids (BCAAs) for example, feed into the TCA cycle directly for energy production. Thus, with 2 hours of moderate intensity exercise (55% VO2max), 86% of the daily requirement for leucine (a BCAA) can be burned (Evans et al., 1983). With protein, loss of one amino acid can render any other excess protein nonusable for general protein synthesis. Combining the results of several studies (Gontzea, 1974; Tarnopolsky et al., 1988; Brouns et al., 1989; Friedman and Lemon, 1989; Meredith et al., 1989) results in the recommendation of 1.2 - 1.4 grams of protein per kilogram of bodyweight per day for endurance athletes.

One final note for those pursuing a low carb diet with exercise can be found in an article, Lemon and Mullin (1980). Previous nitrogen balance studies have relied on measuring nitrogen excretion by adding up nitrogen in urine and stool. One major source for error in physically active people is that nitrogen is also lost through sweat. Clearly, sweating occurs more readily in physically active individuals. In this study, sweat urea nitrogen loss was measured in carb loaded verses carb depleted individuals during exercise at 61% VO2max. At rest, nitrogen losses in sweat were almost undetectable. In the carb depleted individuals, the sweat urea nitrogen loss was more than doubled that of carb loaded individuals performing the same activity. This suggests that a reduced carb diet would utilize more protein during endurance activities.

Summing Up

Lemon recommends 1.2 - 1.4 grams per kilogram per day for endurance work and 1.7 - 1.8 grams per kilogram per day for strength. So the question arises, how do these values mesh with the advice of carb reduction diets? Sears uses a sliding scale of 0.5 - 1.0 grams of protein per pound of lean body mass per day. One difference is that Sears is using lean body mass, while Lemon and other researchers use total mass. Sears' specification of lean body mass appears sound for two reasons; first, physiologically, lean body mass should be the major source of metabolism with fatty tissue contributing very little; second, most of the studies cited here were done in relatively lean individuals so that kilogram of bodyweight versus kilogram of lean body mass are going to be in close agreement.

Activity Level

Grams Protein
per Kilogram of
Bodyweight

Grams Protein
per Pound of
Bodyweight

Grams Protein
per Pound of
Lean Body Mass
(assuming 20% bodyfat)

Sedentary

0.8

0.36

0.45

Endurance

1.2 - 1.4

0.55 - 0.64

0.69 - 0.80

Strength

1.7 - 1.8

0.77 - 0.82

0.96 - 1.03

The first column lists the activity level. The second column lists the Lemon values of grams of protein per kilogram of bodyweight per day. The third column converts this value to per pound of bodyweight. The fourth column converts column three to lean body weight by assuming a typical body fat percentage of 20%. What is clear is that the activity factors of Sears, 0.5 - 1.0, are well in line with research study results. In fact, given the increased protein utilization by carb depleted individuals, one could reasonably argue for increasing protein intakes above Sears' guidelines, but there is little data available on the in between carb loaded versus carb depleted to make specific recommendations.

A final point to be noted is that many criticisms of Zone style eating are based on the erroneous assumption that 30% of calories from protein is too much protein. This is clearly misguided thinking. Protein needs of the body are determined by absolute amounts, not percentages. The only way to determine if 30% is too much or too little is to know the total caloric intake. For example, a diet consisting of only 25 grams of protein per day is 100% protein, but is clearly deficient in terms of protein needs; while a diet of 5000 calories with 10% protein is still consuming 125 grams of protein per day which is about double the current RDA recommendations. The percentage of protein is meaningless in terms of protein needs.
References
References

Babij, P, SM Matthews, and MJ Rennie. (1983). Changes in blood ammonia, lactate and amino acids in relation to workload during bicycle ergometer exercise in man. Eur J Appl Physiol 50:405-411.

Brouns, F, WHM Saris, E Beckers, et al. (1989). Metabolic changes induced by sustained exhaustive cycling and diet manipulation. Int J Sports Med 10:S49-S62.

Evans, WJ, EC Fisher, RA Hoerr, and VR Young. (1983). Protein metabolism and endurance exercise. Phys Sportsmed 11:63-72.

Fern, EB, RN Bielinski, and Y Schultz. (1991). Effects of exaggerated amino acid and protein supply in man. Experentia 47:168-172.

Friedman, JE and PWR Lemon. (1989). Effects of chronic endurance exercise on the retention of dietary protein. Int J Spt Med 10:118-123.

Gontzea, I, P Sutzecu, and S Dumitrache. (1974). The influence of muscular activity on the nitrogen balance and on the need of man for protein. Nutr Rep Int 10:35-43.

Haralambie, G, and A Berg. (1976). Serum urea and amino nitrogen changes with exercise duration. Eur J Appl Physiol. 36:39-48.

Lemon, PWR. (1992). Effect of exercise on protein requirements. In Williamsc, JT Devlin (eds), Food, nutrition and sports peformance. E&FN Spon, London, pp65-86.

Lemon, PWR. (1996). Is increased dietary protein necessary or beneficial for individuals with a physically active lifestyle? Nutr Rev 54:S169-S175.

Lemon, PWR and JP Mullin. (1980). Effect of initial muscle glycogen levels on protein catabolism during exercise. J appl. Physiol 48:624-629.

Meredith, CN, MJ Zackin, WR Frontera, and WJ Evans. (1989). Dietary protein requirements and protein metabolism in endurance-trained men. J Appl Physiol 66:2850-2856.

Meredith, CN, WR Frontera, KP O'Reilly, and WJ Evans. (1992). Body composition in elderly men: effect of dietary modification during strength training. J Am Ger Soc 40:155-162.

NRC (1989) Recommended dietary allowances, Vol 10, National Academy Press, Washington, pp 52-77.

Tarnopolsky, MA, SA Atkinson, and JD MacDougall. (1992). Evaluation of protein requirements for trained strength athletes. J Appl Physiol. 73:1986-1995.

Tarnopolsky, MA, JD MacDougall, and SA Atkinson. (1988). Influence of protein intake and training on nitrogen balance and lean body mass. J Appl Physiol64:187-193

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