NUTRITION IN SPORT

0.05). However, this was not the case for those
subjects who ingested creatine in combination
with carbohydrate (r=0.058, n=9; P>0.05),
where the initial muscle creatine concentration
was found to have little association with the
extent of muscle creatine accumulation when
creatine was ingested in combination with carbo-
hydrate. Evidence was also presented in the
studies of Green et al. (1996a, 1996b) to indicate
that the augmentation of muscle creatine accu-
mulation following carbohydrate ingestion
occurred as a result of a stimulatory effect of
insulin on muscle creatine transport, and that
this effect outweighed the positive effect that
exercise has on muscle creatine accumulation.
The exact mechanisms by which muscle contrac-
tion and insulin stimulate muscle creatine trans-
port are currently under investigation. As muscle
creatine is elevated to above the Kmconcentra-
tion reported for muscle creatine transport when
creatine alone is ingested, it is possible that
insulin operates by increasing the V

.
max.of crea-
tine transport. This could perhaps be achieved by
insulin stimulating sodium–potassium, adeno-
sine triphosphatase (ATP)-dependent, pump
activity, and thereby sodium-dependent creatine
transport. Interestingly, other hormones have
also been shown to stimulate muscle creatine
transport (Odoom et al. 1996).

Health risks associated with dietary

creatine supplementation

There have been anecdotal reports of creatine

supplementation being linked with kidney

damage and muscle cramps. At the time of

writing this author is unaware of any definitive

data to support these conclusions. Creatine sup-

plementation does cause an increase in urinary

creatinine excretion, which is often used as an

indicator of kidney function, but this increase

correlates well with the increase in muscle crea-

tine observed during supplementation and

reflects the increased rate of muscle creatine

degradation to creatinine rather than any abnor-

mality of renal function (Hultman et al. 1996).

Furthermore, chronic high-dose creatine supple-

mentation (20 g · day–1for 5 days followed by

10 g · day–1for 51 days) has been reported to have

no effect on serum markers of hepatorenal func-

tion and routine clinical chemistry (Almada et al.

1996; Earnest et al. 1996). It should be stressed,

nevertheless, that the long-term health risks

of chronic creatine ingestion are presently

unknown. Equally, however, the regimen of

ingesting 20 g · day–1 for 5–6 days has been

reported to have no known side-effects, provid-

ing the creatine is dissolved prior to ingestion

(undissolved creatine may cause slight gastroin-

creatine 371

200

180

160

140

120

100

200

180

160

140

120

100

Creatine concentration (mmol

.kg

–1

d.m.)

(a) (b)

Pre-

ingestion

Post-

ingestion

Pre-

ingestion

Post-

ingestion

Fig. 27.4Mean and individual
values for total muscle creatine
concentration before () and
following () 5 days of: (a)
creatine (20 g · day–1) ingestion,
and (b) creatine (20 g · day–1) and
carbohydrate (370 g · day–1)
ingestion.