NUTRITION IN SPORT

the existence of restrictive cellular membranes
(Fitch 1977).
Creatinine has been established as the sole
end-product of creatine degradation being
formed non-enzymatically in an irreversible
reaction (Fitch & Sinton 1964; Fitch et al. 1968). As
skeletal muscle is the major store of the body
creatine pool, this is the major site of creatinine
production. Daily renal creatinine excretion is
relatively constant in an individual, but can vary
between individuals (Fitch 1977), being depen-
dent on the total muscle mass in healthy individ-
uals (Heymsfield et al. 1983). Once generated,
creatinine enters circulation by simple diffusion
and is filtered in a non-energy-dependent
process by the glomerulus and excreted in urine.

Effect of dietary creatine

supplementation on muscle

creatine concentration

In normal healthy individuals, muscle creatine
is replenished at a rate of approximately

368 nutrition and exercise

2 g · day–1 by endogenous creatine synthesis

and/or dietary creatine intake (Walker 1979).

Oral ingestion of creatine has also been demon-

strated to suppress biosynthesis, an effect which

has been shown to be removed upon cessation of

supplementation (Walker 1979). Conversely, the

absence of creatine from the diet has been shown

to result in low rates of urinary creatine and

creatinine appearance (Delanghe et al. 1989).

Augmented creatine retention occurs during

subsequent dietary creatine supplementation in

vegetarians, suggesting that endogenous synthe-

sis may not match creatine requirements in these

individuals (Green et al. 1997). In this respect, cre-

atine could be viewed as an essential constituent

of a ‘normal’ diet.

Early studies demonstrated that creatine

ingestion resulted in a small increase in urinary

creatinine excretion. In general, urinary creati-

nine excretion rose slowly during prolonged

creatine administration and, upon cessation,

around 5 weeks elapsed before a significant fall

in creatinine excretion was observed (Benedict &

Diet Blood Muscle, heart and brain

Creatine

Creatine

Creatine-P Creatine

Creatine

Creatine

Creatinine

Creatine

Creatinine (2 g.day–1)

ADP ATP

70% 30%

Urine

Liver

Serine

Glycine transamidinase Guanidinoacetate methyltransferase

Glycine Guanidinoacetate

Ornithine SAM SAH

ADO

HCYS

CYS

Methionine

Arginine

FUM HOH Urea

CAP

Argininosuccinate Citrulline

ASP

Fig. 27.1The biosynthesis of creatine. Italics indicate enzymes. Adapted from Walker (1979).