approached 1–1.5 g · kg–1body weight. These
results imply that when carbohydrate supple-
ments are provided at 2-h intervals in amounts
below 1 g · kg–1body weight, the rate of muscle
glycogen resynthesis will be submaximal. The
reduced rate of resynthesis is probably due to the
inability of a small carbohydrate supplement to
adequately increase and maintain blood glucose
and insulin levels for a 2-h interval, as smaller
supplements taken more frequently have been
found to be adequate (Doyle et al. 1993).
The reason for similar glycogen resynthesis
rates when carbohydrate supplements exceed
1g·kg–1body weight was not immediately clear.
Estimates of gastric emptying rates, based on the
research of Hunt et al. (1985), suggest that carbo-
hydrate available to the muscle was far in excess
of the amount actually converted to glycogen.
This would indicate that under conditions of
high carbohydrate supplementation, the rate-
limiting step in glycogen resynthesis is either
glucose transport or the processing of glucose
through the glycogen synthetic pathway. To test
this hypothesis, Reed et al. (1989) continuously
infused glycogen-depleted subjects with 3 g
glucose · kg–1body weight during the first 3.75 h
of a 4-h exercise recovery period. The rate of
muscle glycogen resynthesis during infusion
was then compared with that which occurred
when a liquid supplement containing 1.5 g
glucose · kg–1body weight was consumed imme-
diately after and 2 h after exercise. During infu-
sion, blood glucose increased to 10 mm,whereas
the blood glucose level only reached 6 mmwhen
the liquid glucose supplement was consumed
orally. Despite this large difference in blood
glucose, the rates of muscle glycogen resynthesis
were virtually identical at the end of the recovery
periods. The results of Reed et al. (1989) therefore
support the hypothesis that glycogen resynthesis
is not limited by glucose availability when ade-
quate carbohydrate is consumed.
Prior research studies employing glucose infu-
sion (Ahlborg et al. 1967b; Bergström & Hultman
1967c; Roch-Norlund et al. 1972), however, have
generally demonstrated greater rates of glycogen
synthesis than those reported by Reed et al.
(1989). Possibly accounting for the difference in
synthesis rates are the different rates of glucose
infusion. The rates of glucose infusion in the
earlier studies were much faster and plasma
glucose concentrations two to three times higher
than those reported by Reed et al. (1989). It is
likely that plasma insulin concentrations in the
earlier studies were greater as well, although
these results were not reported.
It was of interest to note that in the study by
Reedet al. (1989) the plasma insulin response
during the infusion treatment was similar to that
produced by the liquid supplement, and there-
fore could account for the similar rates of glyco-
gen storage for these two treatments. The blood
insulin concentration plays a major role in deter-
mining the rate of muscle glycogen storage.
Insulin stimulates both muscle glucose transport
and activation of glycogen synthase. The results
raised the possibility that increasing the insulin
response to a carbohydrate supplement could
increase the rate of muscle glucose uptake and
glycogen storage.protein plus carbohydrate
Certain amino acids are effective secretagogues
of insulin and have been found to synergistically
increase the blood insulin response to a carbohy-
drate load when administered in combination
(Floydet al. 1966; Fajans et al. 1967). Of the 20
amino acids normally found in protein, the most
effective insulin secretagogue is arginine (Fajans
et al. 1967). When infused with carbohydrate,
arginine has been found to increase the insulin
response fivefold above that produced by the
carbohydrate or arginine alone. However, we
have found the use of amino acids to be impracti-
cal when added to a carbohydrate supplement
because they produce many unwanted side-
effects such as mild borborygmus and diarrhoea.
Protein meals and supplements also have been
found to enhance the insulin response to a carbo-
hydrate load and do not produce the unwanted
side-effects of the amino acids (Rabinowitz et al.
1966; Pallota & Kennedy 1968; Spiller et al. 1987).
For example, Spiller et al. (1987) demonstrated an