NUTRITION IN SPORT

indirectly affected by which compound (Fig.
28.5). Due to this uncertainty, the reader should
note that when the term ‘caffeine’ is used in this
chapter, it could be any of the methylxanthines.
Another complication of studying caffeine
ingestion is the variability of individual
responses, affecting central, metabolic and exer-
cise performance responses to caffeine. This
problem affects all categories of subjects, but is a
larger problem with less aerobically fit individ-
uals. Chesley et al. (1998) reported a variable
glycogen sparing response to a high caffeine
dose (9 mg · kg–1) in untrained men. Only 6/12
subjects demonstrated glycogen sparing during
15 min of cycling at approximately 85% V

.
o2max.,
whereas the sparing response was more uniform
in a group of trained men (Spriet et al. 1992). Vari-
ability is also present in all groups of caffeine
users, including mild and heavy users, users
withdrawn from caffeine and non-users. There-
fore, while mean results in groups of subjects and
athletes predict improved athletic performance,
predictions that a given person will improve are
less certain.
There has been a recent report comparing the
effects of 4.5 mg caffeine · kg–1, given in ‘pure’

388 nutrition and exercise

capsule form or in two mugs of strong coffee

(Grahamet al. 1998). Caffeine in capsule form

resulted in the usual metabolic and performance

effects, but the ingested coffee produced less of a

response in plasma adrenaline concentration and

little or no effect on performance, even though

the plasma caffeine concentrations were identi-

cal. It appears that the hundreds of additional

chemicals in coffee negated the usual ergogenic

benefit. On the other hand, there have been

reports where caffeine administration in coffee

produced strong ergogenic performance effects

(Wiles et al. 1992). Therefore, while it is common

to equate caffeine with coffee, it should be noted

that rarely is coffee the method of administration

in research studies and it may be misleading to

equate the two.

The study of caffeine ingestion and exercise

performance has been generally limited to male

subjects. There has been little systematic study of

the response of females to caffeine ingestion at

rest and during exercise. It will be important to

control for menstrual status in future studies, as

oestrogen may affect the half-life of caffeine.

Other considerations of

ingesting caffeine

Caffeine dose

Caffeine is a ‘controlled or restricted substance’

with respect to the IOC. Athletes are permitted

up to 12mg caffeine · ml–1urine before it is consid-

ered illegal. This allows athletes who normally

consume caffeine in their diet to continue

this practice prior to competition. An athlete can

consume a very large amount of caffeine before

reaching the ‘illegal limit’. A 70-kg person could

drink three or four mugs or six regular-size

cups of drip-percolated coffee approximately 1 h

before exercise, exercise for 1–1.5 h, and a subse-

quent urine sample would only approach the

urinary caffeine limit. A caffeine level above 12

mg·ml–1suggests that a person has deliberately

taken caffeine in capsule or tablet form or as sup-

positories, in an attempt to improve perfor-

mance. Not surprisingly, only a few athletes have

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

–60 0 5 15 Exh.

Time (min)

Paraxanthine (

μM

)

6 mg

9 mg

3 mg

Placebo

Fig. 28.5Plasma paraxanthine concentrations during
exhaustive (Exh.) cycling at 80% V
.
o2max.following
the ingestion of placebo or 3, 6 and 9 mg caffeine · kg–1
body mass 1 h prior to exercise. Exhaustion occurred
between 50 and 62 min in all trials. From Graham and
Spriet (1995), with permission.