NUTRITION IN SPORT

Metabolic responses to CHO

ingestion during exercise

Ingestion of CHO during prolonged, strenuous
exercise results in higher blood glucose levels
and rates of CHO oxidation late in exercise (Fig.
8.1) (Coyle et al.1986; Coggan & Coyle 1987).
Liver glucose output is reduced by CHO inges-
tion (Fig. 8.2) (Bosch et al.1994; McConell et al.
1994) and while the tracer method used cannot
distinguish between liver glycogenolysis and
gluconeogenesis, it is likely that there is a signifi-
cant liver glycogen sparing effect of CHO inges-
tion. A reduction in splanchnic gluconeogenic
precursor and oxygen uptake during prolonged,
low-intensity exercise following CHO ingestion

suggests a lower rate of gluconeogenesis

(Ahlborg & Felig 1976) and this has recently been

confirmed in experiments using tracers to esti-

mate rates of gluconeogenesis (Jeukendrup et al.

1999). Muscle glucose uptake during exercise, as

measured by tracer-determined glucose Rd, is

increased by CHO ingestion (McConell et al.

1994). This is consistent with previous observa-

tions of increased leg glucose uptake during low-

intensity exercise (Ahlborg & Felig 1976) and

elevated rates of glucose disposal and oxidation

during strenuous exercise when blood glucose

availability is increased (Coggan et al. 1991;

Coyleet al.1991; Bosch et al.1994; Hawley et al.

1994; Howlett et al.1998).

Most, if not all, studies utilizing prolonged

carbohydrate replacement during exercise 113

CHO

Placebo

CHO

Placebo

*

*

*

* *

*

*

*

*

0 1 234

0 1 2 3 4

(a)

Plasma glucose (m

M)

6

5

4

3

2

2.6

2.2

1.8

1.4

Carbohydrate oxidation (g.min

–1

)

(b) Exercise time (h)

Fig. 8.1(a) Plasma glucose and
(b) rates of carbohydrate (CHO)
oxidation during exercise to
fatigue at 70–74% V
.
o2peakwith
ingestion of either a placebo ()
or CHO () solution every 20
min. Values are means±SEM
(n=7). *, difference from CHO,
P<0.05. Adapted from Coyle et al.
(1986).