With over 50 years of research, we are still debating over how much protein you actually need for given types of exercise (i.e. weight training vs. endurance training etc.). On the conservative side of protein intakes we tend to have the mainstream nutritionists who maintain the idea that athletes don’t need to consume protein in excess of the recommendations outlined for non-athletes. On the other side of the argument are the athletes and bodybuilders themselves who have for several decades maintained the idea that they need to consume greater amounts of protein than the average person.
As research exists for both sides of the argument, scientists tend to have mixed views on how much protein a person actually needs, whether they are sedentary, in training or a professional athlete.
In this article I will address the popular, yet often misunderstood, topic of protein requirements. I will examine the research from both sides of the argument, as well as what I’ve seen work in practise, and present my own recommendations.
The basics of protein metabolism
The amount of muscle tissue in the human body remains fairly stable over time. However these tissues are undergoing a continuous process of breakdown and resynthesis; these processes are referred to as protein turnover.
The amount of muscle mass a person has depends on the long-term relationship between muscle protein breakdown and synthesis. For example, if muscleprotein synthesis exceeds breakdown, there will be an increase in the amount of that protein.
Protein turnover is mediated by severalfactors including hormones (testosterone, growth hormone, thyroid, insulin, glucagon & cortisol), caloric intake, amino acid/protein availability and training. The largest factors that influence skeletal muscle metabolism are eating and training.
Eating a meal
Theprimary factors influencing protein breakdown and synthesis following the consumption of a meal are the concentrations of insulin andamino acids in the blood. With regards to protein synthesis, the essential amino acid (EAA) content of a meal plays a significant role in promoting synthesis; insulin plays a smaller role. In a more direct role, insulin increases amino acid transport into skeletal muscle. Assuming sufficient amino acids are available, only small elevations in insulin are required to maximally stimulate protein synthesis. Regarding protein breakdown, consuming a meal would appear to decrease protein breakdown by increased amino acid availability as well as the presence of insulin.
The take home message here is that insulin combined with increased amino acid availability, results in net protein gain. This may lead one to assume that the simple act of eating a load of protein will lead to gains in muscle mass. However, this isn’t the case due to a process called diurnal cycling, whereby net protein synthesis following a meal is matched by an increased protein breakdown when food is not being consumed. So the more protein someone eats and the more they store in the day, the more they break down at night. This process is thought to provide amino acids from the ingested food more evenly over a 24-hour period.
The effects of training
In untrained people diurnal cycling tends to keep the body at a stable amount of muscle mass. However, when exercise is introduced, it basically “forces” the body to store more protein (assuming sufficient protein and overall caloric intake that is).
The type of training dictates how dietary protein is used to synthesise new proteins in the body. Following weight training both protein synthesis and breakdown are increased, though breakdown is stimulated to a greater degree, meaning that the body is likely to be in a catabolic state following weight training. The provision of protein/calories around the workout will shift the body into an anabolic state “forcing the body to store protein at a higher level (increase muscle mass).
In contrast, endurance exercise results in a different effect on skeletal muscle. Instead of increasing the size of contractile proteins, it mainly stimulates the synthesis of mitochondrial enzymes within the muscle, contributing to an increase in energy production (oxidative capacity) during such activity.
The recommended daily intake of protein
Challenging the RDA
According to thelatest dietary reference intake (DRI) in 2005, the amount of protein that all adults should consume on a daily basis (RDA) is 0.8 grams per kilogram of bodyweight (0.8g/kg). Based on nitrogen balance data, this intake is estimated to cover the basic needs of 97-98% of individuals. The authors of this report state that this intake is suitable for both sedentary individuals as well as endurance or strength athletes: “In view of the lack of compelling evidence to the contrary, no additional dietary protein is suggested for healthy adults undertaking resistance or endurance exercise.”
In contrast, the most recent scientific literature would recommend intakes from 1.2-2.2g/kg for strength and endurance trained athletes. Even for untrained individuals the RDA would seem insufficient for some individuals. For example, a 14 week study by Campbell and colleagues observed that subjects (55-77 years) consuming the RDA for protein (0.8g/kg) experienced significant muscle loss in the mid-thigh area, despite consuming calories to maintain bodyweight. Since this study was published in 2001, the authors of the latest DRI report had sufficient time to amend these recommendations based upon the most recent literature. Either sincere ignorance or laziness may account for this. In keeping with the latter, this may also explain why the RDA for protein hasn’t changed for several decades.
While the RDA may meet the need for the majority of sedentary individuals in energy balance, this amount is simply inadequate for strength and endurance athletes as well as for the elderly or dieting individuals, regardless of training status. Given the lower digestibility and inferior quality of many grain and legume (i.e. beans and nuts) proteins, vegetarians and vegans may need to consume protein in the excess of the RDA to compensate.
Protein requirements for strength and endurance athletes
An early review article by Lemon in 1991 recommended protein intakes of 1.2-1.4 g/kg for endurance athletes and 1.4-1.7 g/kg for strength/power athletes; for athletes competing in sports where a mix of these training methods is warranted (i.e. MMA, Rugby etc.), the upper limit recommended for strength athletes has been suggested to cover protein needs.
These increased requirements for endurance and strength athletes occur for different reasons. During intense aerobic activity (i.e. running or cycling) amino acids, namely the branched chain amino acids (BCAAs), have been shown to account for roughly 5-10% of total energy cost of the activity; this percentage may increase further when muscle glycogen is depleted. In contrast, amino acids provide little energy during strength training. Instead, the increased need for protein comes from the repair of damaged muscle tissue as well as the synthesis of new muscle proteins.
Lemon’s conclusions have since been criticised by Millward, mainly because of the inherent limitations associated with nitrogen balance (the technique used to deduce protein needs). Indeed, the errors associated with estimating nitrogen losses may accumulate and lead to an overestimation of true protein requirements. Additionally, nitrogen balance doesn’t consider the dynamic processes of protein turnover, rather, the amount of nitrogen going into the body (food and drink) minus nitrogen losses form the body (i.e. urine, faeces, sweat etc.).
Millward has also cited earlierwork suggesting that endurance training may actually lower protein requirements compared with untrained individuals, due to an adaptive downregulation increasing the re-use of amino acids. Indeed, in a widely cited study, it was demonstrated that nitrogen balance was negative at the start of an exercise training programme, but returned to equilibrium after a period of accommodation. This occurred despite a constant daily protein intake. Following the period of accommodation, no increase in protein intake was necessary to maintain a stable nitrogen balance.
However, while more recent research has suggested that this may be the case for individuals performing low intensity endurance exercise, endurance athletes (and the more serious recreational athletes) who perform many hours of high intensity exercise per week, would seem to benefit from higher intakes than their untrained counterparts. Furthermore, for the evidence suggesting an increased efficiency of amino acid re-use as protein intake decreases, nitrogen balance may be attained with a compromise in some physiologically relevant processes such as enzyme upregulation or capailliarisation following endurance training. It would therefore seem that while training may improve protein retention (thus achieving a neutral nitrogen balance), this might hinder potential performance enhancing adaptations associated with endurance exercise.
For athletes wishing to gain lean body mass, it has been shown that whole body and muscle protein synthesis rates are greater with increasing protein intakes. In one study, experienced weightlifters were fed either 1.2 or 2.1 g/kg of protein per day for six weeks. In the group that ingested the higher amount, gains in lean tissue mass increased significantly, whereas there was no change in the 1.2 g/kg group.
These results clearly refute the notion that trained individuals don't need more protein than the average person. However, these results still give no idea what an optimal intake might be in this case; just that 2.1 is better than 1.2 g/kg of protein.
Despite the above study, not all evidence supports the idea of increasing amounts of ingested protein leading to greater muscle mass gains. A study by Tarnopolsky and colleagues examined nitrogen balance, body composition and urea excretion during a habitual 10-day period followed by an altered protein intake for 10 days. However, by virtue of slow rates of muscle mass gains, with an altered protein intake period of 10 days, it is unsurprising that there were no changes in lean body mass seen among individuals in this investigation. A further study by the same lead researcher indicated that a protein intake of 2.4 g/kg was not superior to that of 1.4 g/kg in terms of lean body mass gain, among trained strength athletes. However, given the short (a few weeks) study period, it is possible that muscle gain was occurring in the higher protein group, and that the change was too small to be measured with existing body composition techniques.
From a separate standpoint, it has also been argued that the debate over protein requirements is pointless in the first place since the majority of both strength and endurance athletes habitually consume amounts in excess of the recommendations outlined by Lemon. While this is generally true, some athletes, particularly female athletes, have been shown to consume inadequate amounts of protein.
Why all the conflicting viewpoints?
Since the original review article on protein requirements by Lemon in 1991, much data has since been published regarding the interaction of protein and exercise. However, despite these advances, more questions remain unanswered than have been solved. Limitations in research methodology certainly confound results (i.e. nitrogen balance). Participant selection, their adaptation to the protein intakes during their respective studies, training status, nutritional status and exercise intensity further contribute to the discrepant findings, which ultimately to lead to different interpretations. As mentioned previously, short study periods are also far from realistic in order to observe true lean body mass changes with differing protein intakes.
Perhaps the most logical interpretation of the research findings thus far comes from the authors Tipton & Wolfe. Probably their most important point is that athletes and coaches are not interested in the academic debates surrounding protein requirements, rather, they are interested in the specific amount of protein that will optimise athletic performance. This is where the research has often fallen short; instead of looking at performance endpoints following the ingestion of different amounts of protein, studies have tended to simply examine protein “requirements”. That is, the daily amount of protein that supports net protein balance, measured via nitrogen balance or examining leucine kinetics. Indeed, a protein intake that stimulates optimal physiological adaptation (i.e. capillary and enzyme upregulation, muscle hypertrophy and total oxidative capacity of the muscle) as well as immune surveillance, may be in excess of the current recommendations outlined for strength and endurance athletes. It is therefore important to distinguish the difference between a protein “need” of a given athlete, and the amount of protein that will stimulate maximal “adaptations” leading to optimal performance.
A further point the authors make surrounding protein requirements is that the definition of a protein requirement is context specific. For example, the amount of protein that an strength power lifter is to consume to maintain lean body mass will be different to that of a bodybuilder trying to gain lean body mass or that of a bodybuilder aiming to lose body fat while maintaining their muscle mass. The list goes on…
This article also highlights the fact that a daily protein intake of 2.5-3 g/kg for strength and power athletes is not harmful and would appear more than is necessary for protein synthesis, any excess would simply be oxidised. As a slight tangent, it is worth noting that some authors feel that increased oxidation may contribute to the anabolic drive of the body. Given the scant and limited duration data on performance/bodycomposition endpoints, erring on the side of too much rather than too little dietary protein may be the best approach for strength and/or hypertrophy. Perhaps the single greatest risk of ingesting too much protein is the displacement of other macronutrients, namely carbohydrate, which might hinder high intensity exercise. However, since intakes of ~6400 kcal have been reported in strength athletes, an intake of 2.5 g/kg would only account for 14% of their daily intake.
Since endurance athletes are typically less interested in gains in lean body mass, or body weight for that matter, the upper limit for protein consumption would be lower than that for strength/ power athletes. Tarnopolsky’s research group hasdemonstrated that protein intakes above 1.7 g/kg result in the excess being oxidised. Therefore, there appears to be no reason to suggest protein intakes above 2 g/kg for endurance athletes. Similar to the strength/power athletes, this intake is unlikely to impede on carbohydrate or fat intakes. For example, if an endurance athlete consumes 3000 kcal per day, an intake of 2 g/kg would only account for 18% of total intake. Endurance athletes typically consume more total energy that this anyway. As mentioned earlier, athletes competing in team sports (i.e. soccer, rugby, basketball etc.), whose training consists of both endurance and strength work, the lower intake recommended for strength athletes (2.5 g/kg) would be a good starting point. However if hypertrophy is required, they maybe should consider the upper limit of 3 g/kg.
To clarify, based on these data, I feel that daily protein intakes of 2.5-3 g/kg appear optimal for strength/power athletes as well as many team sport players, or any athlete that combines both strength/hypertrophy and endurance training. For drug fuelled athletes looking to maximize muscle gains, more protein than 3 g/kg may be required (intakes in excess of 4-5 g/kg have been reported anecdotally). However, this is beyond the scope of this article. For endurance athletes, where muscle hypertrophy is generally undesired, daily protein intakes of 1.7- 2 g/kg have been suggested to maximise the adaptive response to such training.
Protein requirements when dieting
Given the significant protein sparing effect of energy intake, when athletes need to make weight, reduce unnecessary fat or when physique athletes need to prepare for a show or photo shoot, increasing protein above the recommendations outlined here are imperative to avoid muscle loss. As calories are reduced, the body retains less protein and more is used to supply energy (via gluconeogenesis); therefore, the body needs more.
It has been demonstrated that in overweight, non-trained individuals, protein intakes almost double that of the RDA (~1.5 g/kg) are required to maintain lean body mass losses during calorie restriction. Bodybuilders have successfully used this practice over the last 40 or so years by increasing habitual protein intakes by roughly 150% in the pursuit of rapid fat loss while maintaining muscle. However, for most performance athletes, such a drastic calorie reduction isn’t optimal. Instead, a more modest fat reduction is optimal for several reasons (perhaps, most importantly, as carbohydrate intake will suffer, thus hindering training performance).
Given the relatively high protein intakes recommended above, there would be little need to increase protein too much over the outlined recommendations. Without any scientific data to go by, the lower end of the protein intake should be replaced by the upper amount suggested. For endurance athletes, this would be an intake of 2 g/kg of protein per day. If fat loss is achieved in a preferable manner (i.e. gradually), these intakes will minimally impede of carbohydrate intake. Thus, the athlete is able to maintain performance to a degree, while consuming enough protein to cover all needs and minimising muscle loss. For physique athletes aiming to reach the limits of leanness (i.e. 3-5% body fat for men and 9-12% for women), then increasing protein intakes in excess of these recommendations, may limit muscle loss to a greater extent.
Summary and application
In summary, well it does appear that the RDA of 0.8g/kg of protein may be enough for most untrained individuals (assuming they are in nitrogen balance), this simply isn’t enough for some untrained populations (i.e. older or dieting individuals).
There is plenty of research highlighting that the RDA is indeed insufficient for trained individuals, with original protein intake recommendations of 1.2-1.4 g/kg for endurance athletes and 1.4-1.7 g/kg for strength/power athletes. However, given that research exists supporting both the consumption of more and less than these original recommendations, the debate over protein requirements is likely to continue for decades.
More importantly, coaches and athletes are less interested in these academic debates. They simply wish to know what intake will optimise performance. However, until more valid performance endpoint research is conducted, these questions will remain unanswered. Furthermore, given the methodological difficulties in carrying out such research, this won’t be any time soon. With that being said, there does appear to be sufficient research to draw out reasonable protein intake recommendations for athletes. This, combined with my personal observations in the field, I am able to outline some specific recommendations depending on your type of training and body composition goals (see table below).
Given that protein intakes at the higher ends of my recommendations are unlikely to have negative impacts on health and performance, the best approach may be to err on the side of consuming too much, as opposed to too little. The positive effects on body composition (i.e. fat loss and muscle gain) of higher protein diets compared to calorie-matched lower protein diets, would also support this notion.
Outlined below are the intakes that I ultimately recommend. These intakes should ensure optimal adaptations as well as allowing room for the consumption of other important macronutrients. For regular gym-goers, the lower end of the recommendations will suffice.
Although protein quantity would appear to be more important than quality, protein intake is certainly not the whole picture with regards to optimising body composition or performance. To finish off nicely, I’ll leave you with a final quote by Tipton and Wolfe:
“Current literature suggests that it may be too simplistic to rely on recommendations of a particular amount of protein per day. Acute studies suggest that for any given amount of protein, the metabolic response is dependent on other factors, including the timing of ingestion in relation to exercise and/or other nutrients, the composition of ingested amino acids and the type of protein”