From the Lab: Protein and amino acids requirements for athletes - PART 1; a scientific review

Thu 27 May 2010

The importance of protein to athletes has long been recognised, although to date there is much concerning protein nutrition for athletes that has yet to be resolved.

Nitrogen balance studies have been established as the most common method of determining protein requirements. Lemon (1991) suggested that from these studies, it was assumed that exercise of sufficient intensity and duration would increase protein demands. Therefore recommendations for protein intake were made based on the type of exercise completed.

More recently, Millward (2001) suggested a more complex definition of the protein requirement, based on the 'Adaptive Demand Model'. In this case the metabolic demands include both the obligatory requirements (i.e. the minimum amount required) and the adaptive needs. Therefore protein requirement is defined as the minimum protein intake that satisfies the metabolic demands and maintains body composition. Furthermore, the metabolic demands of athletes will depend on the training goals of the athlete. Variations in obligatory protein requirements through training may be for; increased energy demands, increased repair, stimulation of hypertrophy, synthesis of enzymes, for strength and power. Moreover, strength and endurance athletes will have different requirements.

Protein balance is determined by protein synthesis and degradation, with changes in either likely to disturb the protein balance, into anabolic or catabolic states. Clearly this balance will be dependant on a ratio between the protein consumption and exercise demands of the athlete. Net muscle protein synthesis is where protein synthesis exceeds degradation, often a goal of hypertrophy training.

Tarnopolsky et al (1992) investigated the effects of varied protein intakes in strength athletes (n=7), Vs. age matched sedentary controls (n=6). The subjects consumed 3, 13-day diets with an 8-day washout period between. They were; low protein (0.86g protein per kg BM per day), medium protein (1.4g), and high protein (2.4g). The subjects were instructed to continue activity levels throughout. The study measured nitrogen balance and leucine (amino acid) oxidation. They showed that there was no effect on these parameters with the diets in the sedentary controls. However the medium protein diet (1.4g) increased whole body protein synthesis in the strength trained subjects. Interestingly there was no further increase with the high (2.4g) protein diet, although there was an increased leucine oxidation with the high protein diet. This increased oxidation is thought to be due to increased utilisation of amino acids as fuel, and therefore regular, repeated exercise would lead to increased protein requirements.

It is common knowledge that muscle cross-sectional area increases following resistance training, which would imply a positive protein balance, i.e. protein accretion, following resistance training. This may therefore imply that there may be a role for protein feeding to promote protein synthesis, through hyperaminoacideamia.

A study by Biolo (1997), looked at the effects of hyperaminoacideamia (high levels of amino acids in the blood) in 6 untrained men, involving intravenous infusion of a balanced amino acid mixture following resistance training. The study showed that there was no difference in muscle protein breakdown from basal to post exercise, compared to with no feeding. BUT, amino acid infusion & resistance training stimulated significantly greater protein synthesis than amino acid infusion alone at rest. Therefore net protein balance becomes positive with feeding of amino acids (protein solutions) following exercise.

This study showed that protein degradation is increased after resistance training, suggesting that both catabolic and anabolic processes play key roles in the muscle recovery and adaptive regeneration from resistance training. Furthermore this effect is minimised with the consumption of amino acids after exercise via a prevention of protein degradation, an effect that may be augmented via combination with carbohydrates. In addition, the amino acids achieve this effect by increasing protein anabolism, through increased protein synthesis and decreased protein degradation.

This study utilised direct infusion of amino acids, which leads to the question of can orally administered amino acids be as effective at increasing protein anabolism? Tipton (1999) studied exactly this. The study showed that exercise alone increased protein synthesis and degradation in all trials. Net protein synthesis was (-) on the placebo trial. However there was a (+) net protein balance with amino acid ingestion. The magnitude of this change in protein synthesis was similar to that during infusion of amino acids, suggesting that oral ingestion of amino acids is an effective means of increasing net protein synthesis.

To in part summarise, making recommendations for protein intake based on g/day as was introduced by Lemon (1991) is probably too simplistic as it doesn’t account for varied athlete demands and protein kinetics. Exercise and nutrition provide a potent stimulus for net protein balance. Protein synthesis is increased while protein degradation is unchanged or minimised with the consumption of amino acids (protein solutions) post exercise.

Part Two; why timing is important!

References

Lemon P.W. (1991). Effect of exercise on protein requirements. Journal of Sports Sciences, 9, 53-70.

Millward D.J. (2001). Protein and amino acid requirments of adults: current controversies. Canadian Journal of Applied Physiology, 26, S130-S140.

Tarnopolsky et al. (1992). Evaluation of protein requirements for trained strength athletes. Journal of Applied Physiology, 73, 1986-1995.

Biolo et al. (1997). An abundant supply of amino acids enhances the metabolic effect of exercise on muscle protein. American Journal of Physiology, 273, E122-E129.

Tipton et al. (1999). Postexercise net protein synthesis in human muscle from orally administered amino acids. American Journal of Physiology, 276, E628-E634.

Tipton K.D, Wolfe R.R (2004). Protein and amino acids for athletes. Journal of sports Sciences, 22, 65-79.