Taking a carbohydrate supplement after exercise helps to slow
breakdown of muscle protein, report researchers, clarifying the
role of complex sugars in sports nutrition.
But the improvement in protein balance was small compared to the
effects seen after intake of amino acids or amino acids with
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
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
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.