Introduction
Proteins are an essential component for the normal functioning of the body, where they fulfill numerous roles: metabolism, structure, and/or transport. Within the skeletal muscle, they form the myofibrils responsible for muscle contraction.
In the context of sports, athletes' ability to produce adequate movement is crucial to their performance, thus the development and/or maintenance of appropriate skeletal muscle mass is paramount. Consequently, daily protein intake must be adjusted according to athletes' discipline and performance level, such as muscle mass gain in preparation for competition or maintenance during "off" periods throughout the season.
In order to maintain normal body function and normal muscle mass, the WHO recommends a minimum daily protein intake of 0.8 to 1.0 g/kg(body weight)/day for all individuals. However, several recent studies have shown the emergence of resistance to the anabolic stimulus of protein intake in elderly individuals. Indeed, Moore et al. demonstrated that to provoke identical muscle protein synthesis, an elderly man had to ingest 0.40 g/kg(body weight) of protein during a meal compared to 0.24 g/kg(body weight) for a young man. These results led several teams to continue this research, particularly in women, showing that the recommended minimum intake in elderly individuals was underestimated by 30 to 50%. These results subsequently led several authors to recommend a daily protein intake of 1.2 g/kg(body weight)/day, well-distributed throughout the day with a minimum of 3g of leucine per intake, for individuals over 60 years old in order to maintain normal muscle mass.
This example observed in elderly individuals clearly demonstrates the need to adapt protein intake to the population based on its characteristics, particularly its sports practice.
Protein Intake in Athletes
As mentioned earlier, it is necessary to adjust an athlete's protein intake based on two factors: their sports practice and their goals, both long-term and/or during the season, which may include muscle mass gain or maintenance. In both cases, the objective is to provide a sufficient quantity of amino acids to create new myofibrils and induce hypertrophy, or to prevent myofibrils from being degraded to provide the necessary amino acids for the body's functioning.
Since 2017, the International Society of Sports Nutrition has recommended a daily protein intake of 1.4 to 2.0 g/kg(body weight)/day to increase muscle mass or to maintain it at a minimum. Furthermore, in the specific case of athletes trained in strength, such as weightlifters or rugby players, it recommends an intake of 2.3 to 3.1 g/kg(body weight)/day to maintain their muscle mass during hypocaloric phases.
Although adapted to athletes, these recommendations do not distinguish between the daily protein intake appropriate for maintaining muscle mass and that appropriate for gaining muscle mass. In a meta-analysis conducted in 2018, combining 49 studies, Morton et al. demonstrated that muscle mass gains associated with protein intake increased between 0.9 and 1.6 g/kg(body weight)/day before reaching a plateau from 1.6 g/kg(body weight)/day onwards. In other words, an athlete with an intake between 0.9 and 1.6 g/kg(body weight)/day will experience increasingly significant muscle mass gains as their protein intake increases, whereas from 1.6 g/kg(body weight)/day to 2.2 g/kg(body weight)/day, muscle mass gains will be the same for the same resistance training.
Thus, the authors of this study recommend a daily protein intake between 1.6 and 2.2 g/kg(body weight)/day to maximize muscle mass gains induced by resistance training. Therefore, in the context of sports requiring significant resistance training (e.g., weightlifting), it is interesting to recommend this protein intake in order to maximize athletes' muscle mass.
Traditionally, increased protein intake is often present in sports requiring high levels of resistance training and less so in endurance sports. Indeed, performance in endurance sports relies mainly on the ability of carbohydrates and lipids to provide energy during exercise, so these two macronutrients are the most monitored in these athletes. However, it appears that approximately 20% of energy intake during endurance exercise comes from proteins, thus significantly increasing athletes' daily protein needs. Therefore, a daily intake between 1.4 and 2.0 g/kg(body weight)/day is also recommended to prevent muscle proteins from being consumed to compensate and thus maintain athletes' muscle mass trained in endurance.
Given that up to 20% of energy provided during exercise comes from proteins, it is legitimate to question whether an appropriate protein intake during exercise would improve performance, through a higher energy intake by proteins. Several studies have been conducted to answer this question, and protein intake during exercise does not increase performance, but it seems that an intake of 0.25g/kg(body weight)/hour reduces muscle damage induced by exercise.
In conclusion, athletes require a higher daily protein intake than the WHO recommendations, with doses of:
1.6 to 2.2 g/kg(body weight)/day to increase muscle mass 1.4 to 2.0 g/kg(body weight)/day to maintain muscle mass Increased protein intakes and kidney function
A barrier to increasing daily protein intake in athletes would be a progressive degradation of kidney filtration function. Indeed, a temporary increase in protein intake causes an increase in renal filtration rate, which would gradually damage the kidneys. This has been particularly observed in individuals already presenting moderate renal insufficiency or comorbidities favoring the appearance of renal insufficiency, such as hypertension.
In a study conducted in resistance-trained men, Antonio et al. showed that a daily intake higher than 3 g/kg(body weight)/day for one year did not alter creatinuria, indicating an absence of renal damage. However, cases of renal insufficiency have been previously observed in bodybuilders, suggesting a deleterious effect of high daily protein intake on renal function. However, this result remains to be tempered since the daily protein intake of these bodybuilders was between 4 and 30 g/kg(body weight)/day, which is 10 times the intake used in Antonio et al.'s publication! Furthermore, many of the reported cases are associated with creatine supplementation 22 times higher than the recommended maximum intake, as well as the use of anabolic steroids and diuretics at the time of competition. These three factors can also severely damage the kidneys, making it extremely difficult to conclude that protein intake alone is responsible for the renal damage observed in these bodybuilders.
In conclusion, current recommendations for an adapted protein intake in athletes do not lead to renal problems while allowing benefits on body composition. The observed cases of renal insufficiency seem to be related to the combination of several factors:
A daily protein intake associated with creatine supplementation at doses much higher (between 10 and 22 times) than the recommended maximum intakes, use of anabolic steroids, for hypertrophy purposes, and diuretics, for dehydration purposes, both of which are considered doping products. Conclusion
Given their sports practice, athletes require a higher daily protein intake than the recommended daily intakes for the general population in order to 1) maintain their skeletal muscle mass, with an intake of 1.4-2.0 g/kg(body weight)/day, or 2) gain skeletal muscle mass, with an intake of 1.6-2.2 g/kg(body weight)/day. In healthy subjects, these intakes above the recommendations do not seem to pose risks to kidney function, although attention should be paid to the presence of comorbidities in the case of amateur athletes.