Daily protein requirements and sports
It is now accepted by athletes, coaches and athletic trainers that proper diet is one of the cornerstones for achieving better athletic performance. Despite this widely spread assumption, many, even at the highest levels, still believe that an high protein intake is fundamental in the athlete’s diet. This opinion is not new and is deeply rooted in the imaginary of many people almost as if, eating meat, even of big and strong animals, we were able to gain their strength and vitality too.
The function of proteins as energy-supplier for working muscle was hypothesized for the first time by von Liebig in ‘800 and it is because of his studies if, even today, animal proteins, and therefore meats, are often believed having great importance in the energy balance in the athlete’s diet, despite nearly two centuries in which biochemistry and sports medicine have made enormous progress.
Really, by the end of ‘800 von Pettenkofer and Voit and, at the beginning of ‘900, Christensen and Hansen retrenched their importance for energy purposes, also for the muscle engaged in sport performance, instead bringing out the prominent role played by carbohydrates and lipids.
Of course we shouldn’t think that proteins are not useful for the athlete or sedentary people. The question we need to answer is how many proteins a competitive athlete, engaged in intense and daily workouts, often two daily sessions (for 3-6 hours), 7/7, for more than 10 months a year, needs per day. We can immediately say that, compared to the general population, and with the exception of some sports, (see below) the recommended amount of protein is greater.
Metabolic fate of proteins at rest and during exercise
In a healthy adult subject engaged in a non-competitive physical activity, the daily protein requirements is about 0.85 g/kg desirable body weight, as shown by WHO.
Proteins turnover in healthy adults, about 3-4 g/kg body weight/day (or 210-280 g for a 70 kg adult), is slower for the muscle than the other tissues and decreasing with age, and is related to the amount of amino acids in the diet and protein catabolism.
At rest the anabolic process, especially of synthesis, uses about 75% amino acids while the remaining 25% undergoes oxidative process, that will lead to CO2 and urea release (for the removal of ammonia).
During physical activity, as result of the decreased availability of sugars, i.e. muscle glycogen and blood glucose used for energy purposes, as well as the intervention of cortisol, the percentage of amino acids destinated to anabolic processes is reduced while it increases that of amino acids diverted to catabolic processes, that is, it occurs an increase in the destruction of tissue proteins.
At the end of physical activity, for about two hours, anabolic processes remain low whereupon it occurs their sharp increase that brings them to values higher than basal ones, so, training induces an increase in protein synthesis even in the absence of an increase in proteins intake.
What determines the daily protein requirements?
There are many factors to be taken into account in the calculation of the daily protein requirements.
- The age of the subject (if, for example, he/she is in the age of development).
- Gender: female athletes may require higher levels as their energy intake is lower.
- An adequate carbohydrate intake reduces their consumption.
During physical activity, glucogenic amino acids may be used as energy source directly in the muscle, after their conversion to glucose in the liver through gluconeogenesis.
An adequate carbohydrate intake before and during prolonged exercise lowers the use of body proteins.
- The amount of carbohydrates stored in muscles and liver (glycogen) (see above).
- The energy intake of the diet.
A reduced energy intake increases protein requirements; conversely, the higher energy intake, the lower the amount of protein required to achieve nitrogen balance; usually there is a nitrogen retention of 1-2 mg per kcal introduced.
If the athlete is engaged in very hard competition/workouts, or if he requires an increase in muscle masses (e.g. strength sports) nitrogen balance must be positive; a negative balance indicates a loss of muscle mass.
The nitrogen balance is calculated as difference between the nitrogen taken with proteins (equal to: g. proteins/6.25) and the lost one (equal to: urinary urea in 24 hours, in g., x0.56]; in formula:
Nb (nitrogen balance) = (g. protein/6.25) – [urinary urea in 24 hours, in g., x0.56)]
- The type of competition/workouts that the athlete is doing, either resistance or endurance, as well as the duration and intensity of the exercise itself.
Resistance training leads to an increase in protein turnover in muscle, stimulating protein synthesis to a greater extent than protein degradation; both processes are influenced by the recovery between a training and the next one as well as by the degree of training (more training less loss).
In the resistance and endurance performances the optimal protein requirements in younger people as for those who train less time are estimated at 1.3 to 1.5 g protein/kg body weight, while in adult athletes who train more time is slightly lower, about 1-1.2 g/Kg of body weight.
In subjects engaged in a hard physical activity, proteins are used not only for plastic purposes, which are incremented, but also for energy purposes being able to satisfy in some cases up to 10-15% of the total energy demand.
Indeed, intense aerobic performances, longer than 60 minutes, obtain about 3-5% of the consumed energy by the oxidation of protein substrates; if we add to this the proteins required for the repair of damaged tissue protein structures, it results a daily protein demand about 1.2 to 1.4 g/kg body weight.
If the effort is intense and longer than 90 minutes (as it may occur in road cycling, running, swimming, or cross-country skiing), also in relation to the amount of available glycogen in muscle and liver (see above), the amount of proteins used for energy purposes can get to satisfy, in the latter stages of a prolonged endurance exercise, 15% of the energy needs of the athlete.
- The physical condition.
- When needed, the desired weight.
Athletes attempting to lose weight or maintain a low weight may need more proteins.
From the above, protein requirements don’t exceed 1.5 g/kg body weight, also for an adult athlete engaged in intense and protracted workouts, while if you consider the amount of protein used for energy purposes, you do not go over 15% of the daily energy needs.
So, it’s clear that diets which supply higher amounts (sometimes much higher) of proteins aren’t of any use, stimulate the loss of calcium in bones and overload of work liver and kidney. Moreover, excess proteins don’t accumulate but are used to fat synthesis.
How to meet the increased protein requirements of athletes
A diet that provides 12 to 15% of its calories from protein will be quite sufficient to satisfy the needs of almost all of the athletes, also those engaged in exhausting workouts.
In fact, with the exception of some sports whose energy expenditure is low, close to that of sedentary subject (for example: shooting, or artistic and rhythmic gymnastics), athletes need a high amount of calories and, for some sports such as road cycling, swimming or cross-country skiing, it may be double/triple than that of a sedentary subject.
The increase in food intake is accompanied by a parallel increase in protein intake, because only a few foods such as honey, maltodextrin, fructose, sugar and vegetable oils are protein-free, or nearly protein-free.
Calculation of protein requirements of athletes
If you consider an energy demand of 3500 kcal/die, with a protein intake equal to 15% of total daily calories, you have:
3500 x 0.15 = 525 Kcal
As 1 gram of protein contains 4 calories, you obtain:
525/4 = 131 g of proteins
Dividing the number found by the highest protein requirements seen above (1.5 g/kg body weight/day), you obtain:
131/1.5 = 87 kg
that is, the energy needs of a 87 kg athlete engaged in intense workouts are satisfied.
Repeating the same calculations for a caloric intake of 5000 , you obtain 187 g of protein; dividing it by 1.5 the result is 125 kg, that is, the energy needs of a 125 kg athlete are satisfied.
These protein intakes can be met by a Mediterranean-type diet, without protein or amino acids supplements.
Giampietro M. L’alimentazione per l’esercizio fisico e lo sport. Il Pensiero Scientifico Editore. Prima edizione 2005
Protein and amino acid requirements in human nutrition. Report of a joint FAO/WHO/UNU expert consultation. 2002 (WHO technical report series ; no. 935) [PDF]