NUTRITION IN SPORT

have shown that when a fixed amount of exercise
is performed on a daily basis, performance is not
affected when only a moderate amount of carbo-
hydrate is consumed (Simonsen et al. 1991; Nevill
et al. 1993; Sherman et al. 1993).
Nevillet al. (1993) reported that power output
during 1 h of intermittent sprint exercise was
unchanged after the carbohydrate intake was
manipulated during the 24-h recovery (Fig. 41.6).
During the first trial, 18 games players per-
formed 30 maximum 6-s sprints, interspersed
with walking and jogging, on a non-motorized
treadmill. The subjects were then randomly
assigned to three equally matched groups and
repeated the test 24 h later, after consuming
either a high, low or normal carbohydrate diet
(79±3%, 47±8%, 12±1% carbohydrate, respec-
tively). Power output over the 30 sprints was not
different between trials; however, the high carbo-
hydrate group did perform better than the low
carbohydrate group over the first nine sprints.
However, although no performance decre-
ments were observed in the short term, an
increased carbohydrate intake is recommended
because it may improve performance after an

544 sport-specific nutrition

intensive training period (Simonsen et al. 1991).

Laboratory studies have shown that one 6-s

sprint reduces glycogen by approximately

44 mmol · kg–1dry matter (14%), and after 10

sprints, glycogen is reduced by 36% (Gaitanos et

al.1993). A sprinter may train intensively—say,

three to five times per week—which may cumu-

latively reduce glycogen stores, leading to glyco-

gen depletion. Performance during maximal

exercise may be reduced by 10–15% when

glycogen concentration falls below a critical

threshold (Jacobs et al. 1982). Although there is

no ergogenic benefit of carbohydrate loading in

the days prior to a single sprint, an adequate car-

bohydrate intake is recommended for sprinters

in training to support the intense daily training

sessions.

Dietary supplements and sprinting

Protein and amino acids

Anabolic steroids, used by bodybuilders to

increase lean muscle tissue, are illegal in sporting

competition, and may pose a number of health

risks. A variety of nutrients are believed to

provide an effective, safe and legal alternative

instead (M.H. Williams 1993). Amino acid sup-

plements have been advertised for strength ath-

letes because they are said to provide a safe

anabolic or muscle-building effect. The two most

commonly used amino acids are arginine and

ornithine because of their stimulatory effects on

human growth hormone (HGH) production

(Hatfield 1987; Williams 1989). It is well

documented that exogenous growth hormone

produces anabolic effects in growth hormone-

deficient animals and humans, but it is question-

able as to whether this same effect exists in

normal animals and humans.

However, many athletes believe that supple-

mentation with these amino acids stimulates

the release of HGH, which is thought to act by

increasing insulin-like growth factors (IGF1 and

IGF2). Thus, protein and nucleic acid synthesis is

stimulated in skeletal muscle (Lombardo et al.

1991). However, many well-controlled studies

800

400

700

600

500

Trial 1

Mean power (W)

Trial 2

Fig. 41.6Mean power output during 30 maximal 6-s
sprints during trial 1 and trial 2 for three dietary
groups: , high carbohydrate;
, normal
carbohydrate; , low carbohydrate (Nevill et al. 1993).