Lambet al. (1990) tested whether a diet in
which 80% of calories came from carbohydrate
was superior to a 43% carbohydrate diet in sup-
porting the daily training of collegiate swim-
mers. Both diets provided 19.6 MJ · day–1(4680
kcal · day–1) and were maintained for 9 days.
During the last 5 days of each diet, swimmers
performed intervals of various distances ranging
from 50 m up to 3000 m and mean swim velocities
were recorded. These authors found no signifi-
cant differences between the two diets in perfor-
mance of the interval sets. However, they did
note that the swimmers who regularly consumed
a high carbohydrate diet tended to perform
better than those who generally consumed a low
carbohydrate diet. A possible reason for the lack
of difference in performance between the high
and moderate carbohydrate diets could be that
even in the moderate carbohydrate diet, enough
carbohydrate was supplied to support the
demands of their training. At 19.6 MJ · day–1(4680
kcal · day–1) and 43% carbohydrate, these subjects
were consuming an average of 503 g carbohy-
drate · day–1. Costill and Miller (1980) reported
that muscle glycogen repletion is proportional to
the mass of carbohydrate consumed until carbo-
hydrate intake reaches approximately 600 g ·
day–1. Therefore, it is possible that the 935 g of
carbohydrate that were provided in the 80% car-
bohydrate diet did not stimulate any greater rate
of glycogen repletion than the 503 g of carbohy-
drate provided in the 43% carbohydrate diet.
In light of the observations of Costill et al.
(1988a), wherein four of the 12 swimmers
studied failed to eat enough carbohydrate to
prevent chronic muscle glycogen depletion
during 8970 m · day–1training, it would seem
prudent to recommend that swimmers consume
a diet that (i) meets the energy requirements of
training and (ii) provides at least 600 g carbohy-
drate · day–1.
Carbohydrate ingestion during
training sessions
A number of studies have shown improved
endurance performance when carbohydrate is
ingested at frequent intervals during the exercise
(Coyleet al. 1983, 1986; Coggan & Coyle 1987;
Daviset al. 1988; Tsintzas et al. 1993). Typically,
the exercise modes studied in these investiga-
tions have been either cycling or running. The
hypothesized benefit of carbohydrate ingestion
is improved maintenance of blood glucose
throughout the duration of the activity and/or
muscle glycogen sparing leading to increased
carbohydrate availability at a time when the low
endogenous carbohydrate supplies generally
limit muscular performance.
To determine if carbohydrate ingestion during
exercise would have similar beneficial effects on
the performance of swimming training bouts,
O’Sullivan et al. (1994) measured swimming
performances during a standardized training
session once while ingesting a placebo and once
while ingesting a liquid carbohydrate supple-
ment. In each of these trials, the nine male colle-
giate swimmers performed a 5944-m training
session composed of a mixture of low- and high-
intensity interval training bouts. The final 914 m
of the training session was 10¥91.4 m swims
with 20 s rest between each as a performance
trial. Performance was measured as each
swimmer’s average velocity during the first 5,
second 5, and for the whole set of 10¥91.4 m.
During the carbohydrate supplementation trial,
the swimmers were given 1 g · kg–1of glucose
polymers in 50% solution 10 min into the training
session, and 0.6 g · kg–1of glucose polymers in
20% solution every 20 min thereafter according
to the feeding schedule of Coggan and Coyle
(1988). The placebo trial was the same as the
carbohydrate trial, but an artificially sweetened
placebo drink was substituted for the carbohy-
drate drink. The trials were conducted 7 days
apart in a randomized, double-blind manner.
Blood samples were taken before the training
session, immediately before each feeding, and at
the conclusion of the 10¥91.4 m performance
trial. During the placebo trial, blood glucose con-
centration remained fairly stable throughout the
first 100 min of the training but rose significantly
during the 10¥91.4 m performance test to a final
level of 6.3 mmol · l–1(Fig. 46.3). In the carbohy-