training places a great demand not only on
dietary energy, but also on the carbohydrate
needs of these athletes. Maglischo (1993) has esti-
mated that dietary carbohydrate needs of swim-
mers range between 500 and 800 g · day–1. Thus,
a swimmer who consumes a diet providing
16.8 MJ · day–1(4300 kcal · day–1) with 50% of the
energy from carbohydrate will be consuming
approximately 500 g · day–1of carbohydrate and
therefore may not meet the carbohydrate
demand of the daily training. Clearly, these ath-
letes should make carbohydrate intake a priority
in their daily diet.
To determine the amount of muscle glycogen
depletion that can occur during typical swim
training bouts, Costill et al. (1988b) examined
muscle glycogen levels of male collegiate swim-
mers before and after swimming either 2743 or
5486 m. Each swimmer performed the 5486 m of
training twice; once by doing 60¥91.4 m swims
and once by performing 12¥457.2 m swims.
Biopsies were taken from the anterior deltoid
before, at the half-way point of the training
session (2743 m), and at the end of each training
session and analysed for glycogen concentration.
Additional biopsies were taken after 8 h of recov-
ery and ingestion of 112 g carbohydrate to assess
the amount of glycogen repletion that might
occur in this amount of time. When the training
sessions were performed with repeated 91.4-m
swims, muscle glycogen concentration declined
by 68% at 2743 m, and by 87% at 5486 m (Fig.
46.2). Using repeated 457.2-m swims, muscle
glycogen declined by 54% at 2743 m, and by 63%
at 5486 m. The greater amount of glycogen deple-
tion with 91.4-m repeats than with 457.2-m
repeats was accounted for by a significantly
faster swimming speed during the 91.4-m
repeats (ª7% faster than during the 457.2-m
repeats). In recovery, glycogen repletion was 52%
complete after 8 h and ingestion of 112 g of
carbohydrate.
These findings show the large loss of muscle
glycogen that can occur during a single training
session among competitive swimmers. When
one considers that many swimmers perform this
kind of training on a daily basis, and in many
612 sport-specific nutrition
instances twice per day, the probability of
chronic glycogen depletion is great, especially
considering the incomplete glycogen repletion in
the 8 h of recovery. Chronic glycogen depletion
may then result in poor performances in subse-
quent training sessions and in competitions that
may follow a period of such training. An obvious
solution to this problem is to train only once
per day and consume a diet containing at least
500 g · day–1of carbohydrate. Less frequent train-
ing would likely not deplete glycogen from
working muscles as much as twice per day train-
ing, and the higher intake of dietary carbohy-
drate would tend to accelerate glycogen
repletion in the 24 h between training sessions,
especially if a large quantity of carbohydrate is
ingested during the first 2 h after the training is
finished (MacDougall et al. 1977; Ivy et al. 1988).14008060Pre-exerciseMuscle glycogen (mmol.kg–1)12010040202743 5486
Distance per interval set (m)Fig. 46.2Muscle glycogen utilization during 2743- and
5486-m interval swim training using repeated 91.4-m
( ) or 457-m ( ) swims. Adapted from Costill et al.
(1988b).