to its slower rate of absorption, which may cause
gastrointestinal distress and impaired perfor-
mance (Murray et al.1989a). Interestingly, the
combination of fructose and glucose results in
higher rates of exogenous CHO oxidation than
ingestion of either sugar alone (Adopo et al.
1994). This may be a consequence of activation of
two intestinal transport mechanisms, resulting in
greater appearance of ingested CHO from the
gastrointestinal tract. Such a finding is consistent
with observations of greater fluid absorption
from rehydration beverages containing more
than one CHO (Shi et al.1995). Whether the
increased exogenous CHO oxidation results in
enhanced exercise performance has not been
tested. Galactose is less available for oxidation
when ingested during exercise (Leijssen et al.
1995) and soluble corn starch is oxidized to a
greater extent than insoluble starch during exer-
cise due to its higher amylopectin/amylose ratio
(Sariset al.1993). The physical form of the
ingested CHO does not exert a major influence
since liquid and solid CHO supplements elicit
similar metabolic responses during exercise
(Lugoet al.1993; Mason et al.1993).
Amount of CHO
There is no clear dose–response relationship
between the amount of CHO ingested during
exercise and subsequent exercise performance
(Mitchellet al.1989; Murray et al.1989b, 1991).
Ingestion of CHO at a rate of 13 g · h–1is insuf-
ficient to alter the glucoregulatory hormone
response to prolonged exercise or time to fatigue
(Burgess et al.1991). The addition of a small
amount of CHO to a rehydration beverage was
shown to increase exercise time to fatigue, com-
pared with water alone, and to be more effective
than a larger amount of CHO (Maughan et al.
1996). This may be the result of better rehydra-
tion, due to the stimulatory effect of a hypotonic
glucose solution on intestinal fluid absorption,
and subsequent maintenance of a higher plasma
volume, rather than a metabolic effect of the
ingested CHO. Ingestion of CHO at a rate of 26
and 78 g · h–1increased 4.8-km cycle performance
to a similar extent, following 2 h of exercise at
65–75%V.
o2peak(Murrayet al.1991). No differ-
ences in physiological responses to exercise were
observed between ingestion of 6%, 8% and 10%
sucrose solutions, but performance was only
enhanced with 6% (Murray et al.1989b). There is
likely to be little benefit in ingesting CHO solu-
tions more concentrated than 6–8% because this
does not result in increased rates of exogenous
glucose oxidation (Wagenmakers et al.1993) and
increases the risk of impaired gastrointestinal
function and reduced fluid delivery. The absorp-
tion of ingested CHO could potentially limit
exogenous CHO oxidation which is observed to
peak at 1–1.3 g · min–1(Hawleyet al.1992). Such
values are similar to those reported for gastric
emptying and intestinal absorption of glucose
from a 6% glucose–electrolyte solution under
resting conditions (Duchman et al.1997). Obvi-
ously, the important goal of CHO replacement is
to provide sufficient CHO to maintain blood
glucose and CHO oxidation without causing
impaired fluid delivery. Ingesting CHO at a rate
of 30–60 g · h–1has been repeatedly shown to
improve exercise performance. This CHO intake
can be achieved by ingesting commercially avail-
able sports drinks, at a rate of 600–1200 ml · h–1,
with the added benefit of providing fluid and
reducing the negative effects of dehydration
(Coyle & Montain 1992; American College of
Sports Medicine Position Stand 1996).Timing of CHO ingestion
The beneficial effects of CHO ingestion are likely
to be most evident during the latter stages of pro-
longed exercise when endogenous CHO reserves
are depleted. Indeed, ingestion of CHO late in
exercise, approximately 30 min prior to the point
of fatigue, produced increases in exercise time to
fatigue similar in magnitude to those seen with
ingestion of CHO early or throughout exercise or
with intravenous infusion of glucose at the point
of fatigue (Coggan & Coyle 1989; Coggan et al.
1991; Tsintzas et al.1996b). In contrast, ingestion
of CHO at the point of fatigue is not as effective in
enhancing endurance capacity (Coggan & Coyle
1987, 1991); however, delaying CHO intake until
late in exercise, despite increasing blood glucose