drate feeding trial, blood glucose concentration
was slightly elevated over the placebo trial at all
time points, but no significant differences were
observed. Performance times for the 10¥91.4 m
training set at the end of training averaged 59.1 s
in the placebo trial and 59.9 s during the carbohy-
drate trial. The authors concluded that carbohy-
drate supplementation was not effective in
improving performance late in a swimming
practice because blood glucose remains stable
even without supplemental carbohydrate.
However, individual differences in their re-
sponses were presented. Two of the subjects did
614 sport-specific nutrition
experience a substantial decline in blood glucose
concentration during the training in the placebo
trial. During his placebo trial, one swimmer ’s
blood glucose concentration dropped from
4.3 mmol · l–1before the training to a low of 2.6
mmol · l–1immediately before the 10¥91.4 m per-
formance trial (Table 46.1). The ingestion of car-
bohydrate completely prevented this decline and
his performance time improved by 1.3 s from
64.3 s in the placebo trial to 63.0 s in the carbohy-
drate trial. Another subject experienced a drop in
blood glucose concentration from 5.6 mmol · l–1
pre-exercise to 3.7 mmol · l–1immediately before
the performance trial. Again, the carbohydrate
supplementation prevented this decline and per-
formance improved by 1.1 s from 58.4 to 57.3 s.
Thus, it seems that the carbohydrate supplemen-
tation protocol used in this study is effective in
improving late-practice performance but only
for those individuals who normally experience
declining blood glucose concentration during
the training.
Because it would be impossible for swimmers
to know if they normally experience declining
blood glucose concentration during training
without taking blood samples, coaches may
want to recommend carbohydrate supplementa-
tion for the entire team. In those individuals who
are able to maintain their blood glucose concen-
trations without supplemental carbohydrate,
there is little, if any, risk in consuming the carbo-
hydrate. Therefore, supplementing the entire
team would be one way of assuring that those
swimmers who need the extra carbohydrate
would get it. Alternatively, coaches could watch
their swimmers for signs of excessive muscle76543Blood glucose (mM
)4846 m swim Perf.0 20 40 60 80 100 120
Time (min)Fig. 46.3Blood glucose concentration throughout a
5486-m swim training session when fed placebo () or
carbohydrate () every 20 min. Perf., performance trial
of 10 ¥91.4-m swims. Adapted from O’Sullivan et al.
(1994).
Table 46.1Responses of two subjects who had declining blood glucose concentration during placebo and
carbohydrate trials. Adapted from O’Sullivan et al. (1994).
Pre-exercise Preperformance
Trial (mmol · l-^1 ) (mmol · l-^1 ) Time/91.4 m (s)Subject 1 Placebo 4.3 2.6 64.3
Carbohydrate 4.5 4.3 63.0
Subject 2 Placebo 5.6 3.7 58.4
Carbohydrate 5.7 7.2 57.3