Science of Sport: How much protein do athletes need – and how safe are high-protein diets?

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Protein is not just an essential nutrient, but the largest component in the body after water, typically representing about 15% of body weight. Most of this protein mass is found in skeletal muscle, which explains the importance of protein to athletes. However, proteins also play an important role in the following:

• Transport and storage of other nutrients;
• Catalysing biochemical reactions;
• Control of growth and differentiation;
• Immune protection;
• Providing our bodies with structural integrity.

Although the basic biochemistry and functional roles of protein in the body have long been understood, there’s still a huge amount of mythology and confusion surrounding protein nutrition, especially where athletes are concerned. This is partly because of general misconceptions about basic protein metabolism and partly because new research continues to throw up surprises about exactly what constitutes optimum protein nutrition!

Figure 1, below, provides a brief overview of protein metabolism. The protein we eat is made up of around 20 amino acid ‘building blocks’. The process of digestion breaks down dietary protein into its constituent amino acid building blocks, which can then be absorbed into the body and reassembled to make various kinds of human protein, such as muscle, connective tissue, immune proteins, and so on.

Figure 1: overview of protein metabolism

However, it is important to understand that protein metabolism is in a constant state of flux; although muscle and other tissues contain a large amount of stored protein, this protein is not ‘locked away’. When dietary amino acids are insufficient, tissue protein can rapidly be broken down back to amino acid building blocks, which are then used to replenish the ‘amino acid pool’, a reservoir of amino acids that can be drawn upon to support such vital functions as energy production or immune function. This explains why muscle mass is often lost during times of stress, disease and heavy training loads, or poor nutrition.

Conversely, when dietary amino acids are in plentiful supply and other demands for protein are low, tissue protein synthesis can become the dominant process. The overall control of protein turnover – ie whether the body is in a state of anabolism (building up) or catabolism (breaking down), also known as positive or negative nitrogen balance – is governed by hormonal factors, caloric intake and availability of amino acids, particularly of the nine ‘essential’ amino acids that cannot be synthesised in the body and therefore have to be obtained from the diet.

Maintaining optimum protein status

An athlete has to move his or her body to perform, and this requires the muscles to generate force to accelerate body mass. As a rule of thumb, the greater an athlete’s power-to-weight ratio, the faster he or she can move, and (to a lesser extent) the longer he or she will be able to maintain any given speed of movement. Since all force and movement is generated by muscles, most power athletes benefit from maximising muscle mass and strength, while minimising the amount of superfluous body mass – ie fat.

And while out-and-out muscle strength is less important for endurance athletes, maintaining sufficient muscle mass is critically important, not least because high training volumes are known to increase the rate of protein oxidation from the amino acid pool, potentially leading to delayed recovery, a loss of muscle mass and consequent loss of power, and increased injury risk.

Given that athletic training is known to increase the demands on the amino acid pool, many athletes, particularly bodybuilders and strength athletes, adopt high-protein diets to maintain a positive nitrogen balance, or at least prevent catabolism and loss of muscle tissue. However, even today there remains much debate about how much protein athletes really need to optimise and maintain performance.

Protein v carbohydrate

There are other questions too. For example, should any extra protein be ingested at the expense of carbohydrate, the body’s preferred fuel for high-intensity training? And what about the possible health implications of high-protein diets, about which health professionals often express concerns?

Until recently the protein requirements of athletes were thought to be similar to those of sedentary people, and athletes were advised that they need only consume the recommended daily amount (RDA) of protein (currently set at 0.8- 1.0g of protein per kg of body weight per day) to maintain proper nitrogen balance. For a 70kg athlete, this would equate to 56-70g per day.

However, research over the past decade has indicated that athletes engaged in intense training actually need to ingest about 1.5-2 times the RDA in order to maintain a positive protein balance^(1-5). This equates to 105-140g of protein per day for a 70kg athlete, which is equivalent to three to four medium-sized chicken breasts or 13-20oz of canned tuna per day! There is also evidence that training at altitude imposes an even higher demand for protein – perhaps as much as 2.2g per kg per day⁽⁶⁾.

Unfortunately, these more recent findings on protein needs have not yet become widely accepted by some of the powers that be. For example, the UK’s Food Standards Agency website (in its section on sports nutrition) simply states that protein is important in the diet, especially ‘if you’re trying to build muscle’. It goes on to advise: ‘Try not to eat more protein than you need because your body won’t use it to build muscle. Instead it converts excess protein to fat, which is then stored, so try to make sure your protein intake is just right for your needs.’ However, it never actually states what those needs are.

Meanwhile, the EU’s Scientific Committee on Food recently acknowledged that the increased training loads and energy expenditure of athletes can increase protein requirements, and now recommends that their protein intake should comprise around 10-11% of total energy intake⁽⁷⁾. For our mythical 70kg athlete, burning 3,000, 4,000 or even 5,000kcal per day (quite easily achieved with two-plus hours of vigorous training at or above 75% VO₂max per day), this equates to just over 75, 100 or 125g of protein per day respectively.

Although foods like breads, cereals and legumes contain significant amounts of protein, which can add to that contributed by high-protein foods, such as meat, fish, milk and eggs, larger athletes, or those engaged in high volumes of training, may struggle to include the increased amounts of protein now recommended in a ‘normal’ diet; indeed, a number of nutritional surveys have indicated that protein insufficiency may be a problem for certain groups of athletes, including runners, cyclists, swimmers, triathletes, gymnasts, skaters and wrestlers⁽⁸⁾.

Forty years ago, it was protein that dominated the thoughts of power athletes and bodybuilders. Employing the simple logic that muscles are made of protein, and that to build muscle you need lots of protein, steak-and-egg diets were the order of the day! But as the importance of carbohydrates in supplying energy and driving the insulin system (the most anabolic hormone in the body) became clearer, the emphasis gradually shifted.

This shift in emphasis was encouraged by an appreciation of the health benefits of dietary fibre present in unrefined carbohydrates, and also by research suggesting that very high protein intakes simply resulted in increased protein oxidation, imposing an additional load on the liver and kidneys. A scientific consensus began to form around the notion that diets containing substantially more than 1.0g of protein per kg per day were not only wasteful but potentially harmful, increasing the risk of kidney and liver problems, cardiac disease and even loss of bone density.

However, the recent meteoric rise in popularity of high-protein diets, such as Zone and Atkins, for slimmers has ignited a fierce debate about the safety and efficacy of high-protein diets, which is also relevant for athletes who routinely consume high-protein diets. In 2001, the American Heart Association’s nutrition committee published a statement on dietary protein intakes, claiming that: ‘Individuals who follow these [high-protein] diets are at risk for potential cardiac, renal, bone and liver abnormalities overall’⁽⁹⁾.

If you examine the scientific literature, it is hard to see how this consensus, linking high protein intakes to increased health risks, has become so widespread. In a recent meta-review of the literature, Finnish scientists searched for any evidence supporting the hypothesis that high protein diets, containing two to three times the current RDA for protein, increase the risk of a number of health conditions – and drew a big fat blank⁽¹⁰⁾. They concluded that:

• There is no evidence to suggest that (in the absence of overt disease) renal function is impaired by high protein diets;
• Far from reducing bone mineral density, high-protein diets may actually increase it;
• Such diets are associated with lower not higher blood pressures.

These conclusions have also been confirmed by other researchers; healthy athletes should not, therefore, be dissuaded from increasing their protein intake to up to three times the RDA level if they so wish.

High-protein diets and hydration

There’s a fairly linear relationship between protein intake and urea production, which means that high protein diets increase the amount of urea the kidneys have to excrete. With this elevated production of urea comes an increase in the obligatory water requirement of the kidneys to do their job, and that in turn has raised the question of whether athletes (whose fluids needs are already increased) on high-protein diets are at increased risk of dehydration.

To answer this question, scientists at the University of Connecticut compared the hydration levels of athletes consuming low (0.8g per kg per day), medium (1.8g) and high (3.6g) protein diets, each containing the same number of calories⁽¹¹⁾. Analysis of the results showed that, while there were significant increases in urine and plasma urea on the high-protein diet, the effects of increasing dietary protein on fluid status was minimal.

Moreover, to date there have been no studies conclusively demonstrating that increased protein intake leads to a loss in total body water. However, the researchers pointed out that the subjects in their study probably consumed enough water to meet any increased requirement, which explains – at least in part – why their hydration status was not compromised. They also concluded that more research is needed. In the meantime, however, it seems prudent to recommend that all athletes on high-protein diets should drink plenty of extra fluid, especially in warm conditions.

For many athletes, power-to-weight ratio is more important than outright power for optimum performance, and this explains why reducing excess body fat is often beneficial. New evidence is now emerging that high-protein diets might actually help in this process. Although research indicates that, providing the same number of calories are eaten, the relative proportions of protein and carbohydrate in the diet do not affect the amount or composition of weight loss in a reduced calorie regime^(12-14), these ratios do affect appetite, with subjects tending to be more hungry on higher carbohydrate intakes and less hungry on higher protein intakes.

More generally, scientists now believe that diet composition strongly affects ad lib energy intake, with both laboratory and free-living studies highlighting protein as a more satiating macronutrient than carbohydrate or fat⁽¹⁵⁾. This theory is supported by studies indicating that subjects consuming high-protein (more than 20% protein by energy) diets consume less overall than those on low-protein diets^(16,17). The exact mechanisms are as yet unclear, but probably involve hormonal and chemical changes in regions of the brain known to be associated in hunger and appetite control.

Protein and weight loss

In one of the studies mentioned above⁽¹⁷⁾, 13 obese men were split into two groups and fed lowcalorie diets. One group received a high-protein diet (45% protein, 25% carbohydrate and 30% fat) and the other a high-carbohydrate diet (12% protein, 58% carbs and 30% fat). Not only was weight loss greater in the high-protein group but basal metabolism decreased less than in the highcarb group, suggesting that the high-protein diet was able to offset the loss in lean body mass (basal metabolism being a function of lean body mass) that normally occurs while dieting.

No studies of this type have been carried out on athletes, but it seems likely that high-protein diets have something to offer athletes seeking a reduction in body fat while conserving muscle tissue. While high-protein/low-carbohydrate diets of the type described above would not contain sufficient carbohydrate to permit normal training, our mythical 70kg athlete, consuming a 25% protein diet on a mildly calorie-restricted diet of 2,500kcals per day, would be consuming around 600kcal of protein, or 150g, a day. This is well within the ‘safety zone’ of two to three times the RDA (0.8-1.0g per kg per day) yet with a sufficiently high protein content to exert an increased satiation effect.

Moreover, the athlete would still be able to consume up to 50% carbohydrates (1,250kcal per day, sufficient for moderate training volumes), while consuming enough calories (25%) from fat to meet essential fat requirements. However, athletes need to remember, given the importance of carbohydrate for energy requirements, that even this regime would contain insufficient carbohydrate for higher-volume training and competition phases!

In summary, there is good evidence that athletes need a plentiful supply of protein in their diets and that, contrary to previous recommendations, they do need substantially more protein than their sedentary counterparts – at least 50% and possibly up to 120% more. For a 70kg athlete, this can mean up to 150g of pure protein per day.

However, the role of carbohydrates in supplying energy for fuel and recovery remain as important as ever, which means the diet must contain high-quality, low-fat sources of protein in order to enable adequate carbohydrate intake without an overall excess of calories. Simply assuming that because you eat more food than the average person you’ll be consuming adequate protein is not good enough!

There is no evidence that routinely exceeding the recommended protein intake has any additional benefits on nitrogen balance, unless this extra protein is consumed as a protein/ carbohydrate drink before, during or after training – something we’ll tackle in the next article (see below). However, there is evidence that even higher protein intakes may help suppress appetite, control hunger and reduce lean tissue loss during restricted calorie routines, which may be useful for athletes needing to reduce or maintain body weight, although such diets are not really compatible with high-volume training routines.

Finally, despite what you may have read elsewhere, healthy athletes can rest assured that high protein diets containing up to three times the current RDA for protein are perfectly safe, although it is important to remain well hydrated on such diets.

Andrew Hamilton

References

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5. Sportscience 1999. Available: www.sportsci.org;3(1)
6. Butterfield G (1991). Amino acids and high protein diets. In Lamb D, Williams M (editors), Perspectives in exercise science and sports medicine, vol 4; Ergogenics, enhancement of performance in exercise and sport (pages 87-122). Indianapolis, Indiana: Brown & Benchmark
7. EU Scientific Committee on Food, 2004, Working Document – 20 April. Available: www.food.gov.uk
8. Sports Nutrition Review Journal 2004; 1(1):1-44
9. Circulation 2001; 104:1869-74
10. Sports Nutrition Review Journal 2004; 1(1):45-51
11. Presentation by WF Martin at Experimental Biology meeting, April 2002 New Orleans, USA
12. Am J Clin Nutr 1996; 63, 174-178
13. Diabet. Care 2002; 25, 652-657
14. N Engl. J. Med 2003; 348, 2074- 2081
15. Eur J Clin Nutr 1996; 50, 418-430
16. Int J Obes Relat Metab Disord. 1999; 23, 528-536
17. Int J Obes Relat Metab Disord. 1999; 23(11), 1202-6

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