Hypohydration
Exercise undertaken by individuals who begin
exercise in a hypohydrated state has been shown
to be impaired relative to that possible when
fully hydrated at the beginning of exercise (see
Chapter 16). However, in addition to these
adverse effects on performance, hypohydration
increases the likelihood of heat illness, and exer-
cise in this state is only likely to accelerate and
exacerbate these effects (Sutton 1990).
Postexercise carbohydrate
replacement
Many of the issues relating to carbohydrate
replacement during exercise are relevant to
carbohydrate replacement after exercise and a
full discussion of this topic can be found in
Chapter 8.
The primary aim of carbohydrate ingestion
following exercise is to promote glycogen resyn-
thesis and restoration of the muscle and liver
glycogen utilized during exercise. This is of
particular importance when a further bout
of exercise is to be undertaken and therefore is of
significance to all athletes in training and in com-
petition where more than one game or round is
involved. Several factors will influence the rate at
which glycogen resynthesis occurs after exercise.
The most important factor is undoubtedly the
amount of carbohydrate consumed: the type of
carbohydrate and the time of ingestion are less
important, but also have an effect.
Amount of carbohydrate to be ingested
The general pattern for glycogen synthesis after
exercise is one of an increasing rate with increas-
ing amount of carbohydrate consumed up to a
certain rate of resynthesis after which there is no
further increase with increasing quantities of
carbohydrate ingestion. This has been demon-
strated in studies where subjects were fed differ-
ent amounts of glucose or maltodextrins every
2 h after exercise (Blom et al. 1987; Ivy et al. 1988a).
The results showed that muscle glycogen synthe-
sis occurred at a rate of 2 mmol · kg–1·h–1when
25 g of carbohydrate was ingested every 2 h, and
that the replenishment rate increased to 6 mmol ·
kg–1·h–1 when 50 g was ingested every 2 h.
However, muscle glycogen synthesis did
increase to more than about 5–6 mmol · kg–1·h–1
even when very large amounts (up to 225 g) of
carbohydrate were ingested every 2 h.
Further, with intravenous glucose infusion of
100 g every 2 h, a muscle glycogen synthesis
of about 7–8 mmol · kg–1·h–1has been reported
(Reedet al. 1989). This is not significantly greater
than the rates achieved with oral intake, and
suggests that the failure to keep increasing gly-
cogen synthesis with increasing carbohydrate
consumption is not caused by a limitation in
substrate availability imposed by the gastroin-
testinal tract. Also, increasing the amount of
carbohydrate ingested will increase the rate of
delivery to the intestine for absorption (see
Chapter 18).
Therefore, it seems that the maximum rate of
muscle glycogen synthesis after exercise is in the
region of 5–8 mmol · kg–1·h–1, provided that at
least 50 g of glucose is ingested every 2 h after
exercise.Carbohydrate type and form of ingestion
Glucose and sucrose ingestion both give rise to
similar glycogen synthesis rates when consumed
after exercise. Fructose alone, however, seems
only to be able to promote glycogen synthesis
after exercise at a much lower rate of approxi-
mately 3 mmol · kg–1·h–1(Jenkinset al. 1984; Blom
et al. 1987). This is likely to be because of the rela-
tively slow rate with which the liver converts
fructose to blood glucose, and even when fruc-
tose is consumed in large amounts, the entry of
glucose into the blood does not reach a rate of
50 g every 2 h. Although the use of fructose as a
carbohydrate source is often promoted for ath-
letes, it is poorly absorbed in the small intestine
relative to many other sugars, and ingestion of
large amounts is likely to result in diarrhoea
(Maughanet al. 1989).
There is some evidence that carbohydrates