But the improvement in protein balance was small compared to the effects seen after intake of amino acids or amino acids with carbohydrates.
This study is the first to compare net muscle protein balance (protein synthesis minus breakdown) after carbohydrate ingestion with control after exercise.
The body's net muscle protein balance (i.e. the difference between muscle protein synthesis and protein breakdown) generally remains negative in the recovery period after resistance exercise as the muscle's protein is breaking down complex chemical compounds to simpler ones. However, it has been demonstrated that infusion or ingestion of amino acids after resistance exercise stimulates muscle protein synthesis.
As little as 6 grams of essential amino acids (EAA) alone effectively stimulates net protein synthesis after a strenuous resistance exercise session. But the body's response to the 6 grams of EAA does not appear to differ when 35 grams of carbohydrates are added. This reflects the uncertainty of the independent effects of carbohydrates on muscle protein metabolism after resistance exercise.
Additionally, it is unclear how carbohydrate intake causes changes of net protein balance between synthesis and breakdown and how it relates to changes in plasma insulin concentration. Increase in insulin concentration causes a fall in plasma amino acid concentrations, and this reduced amino acid availability could potentially counteract a direct effect of insulin on synthesis.
A past study found that the normal postexercise increase in muscle protein breakdown was slowed by insulin, thus improving net muscle protein balance. However, whereas local infusion of insulin may effectively isolate the effect of insulin per se, the response may differ from when insulin release is stimulated by ingestion of carbohydrates.
Researchers from the University of Texas recruited 16 recreationally active and healthy subjects. They all performed a resistance exercise (10 sets of eight repetitions of leg presses at 80 per cent of one repetition maximum) before they rested in bed for four hours. Half received a drink consisting of 100 grams of carbohydrates one hour after exercise; the other half received a noncaloric placebo drink.
Leg amino acid metabolism was determined by infusion of 2H5- or 13C6-labeled phenylalanine, sampling from femoral artery and vein, and muscle biopsies from vastus lateralis, the lateral head of quadriceps muscle of anterior (extensor) compartment of thigh.
Net muscle protein balance between synthesis and breakdown did not change in the placebo group but improved in the carbohydrate group during the second and third hour after the drink. This was due primarily to a progressive decrease in muscle protein breakdown, report the researchers in this month's issue of the Journal of Applied Physiology.
The findings demonstrate that carbohydrate intake alone can improve net protein balance between synthesis and breakdown, they said, but improvement was small compared with previous findings after intake of amino acids or amino acids and carbohydrates.
The researchers conclude that intake of carbohydrates alone after resistance exercise will modestly improve the anabolic effect of exercise. However, amino acid intake is necessary for a maximal response, one desired by most participating in resistance exercise programmes, they said.