amino acids as substrates (Hultman & Nilson
1971; Nilsson & Hultman 1973).
The rate of hepatic glucose release during exer-
cise in the postabsorptive state has been shown
to be mainly a function of exercise intensity (Fig.
6.2) (Hultman 1967; Wahren et al. 1971; Ahlborg et
al.1974; Ahlborg & Felig 1982). The uptake of
gluconeogenic precursors by the liver is only
marginally increased during the initial 40 min of
submaximal exercise but increases further as
exercise continues (Ahlborg et al. 1974). Most
(more than 90%) of the glucose release is derived
from liver glycogenolysis resulting in a decline
and ultimately depletion of liver glycogen stores.
Direct measurements of liver glycogen concen-
tration in the postabsorptive state and following
60 min of exercise at 75% of maximal oxygen con-
sumption showed a 50% decrease in the liver
glycogen concentration with exercise (Fig. 6.3).
This corresponded to a glycogen degradation
rate of 4.2 mmol · min–1(assuming 1.8 kg of liver)
and suggested that the liver glycogen store
would have been depleted within 120 min of
exercise at this intensity.
The exact mechanisms responsible for the
regulation of liver glucose release at the onset
and during exercise are still unresolved.
However, it is known that the decline in blood
insulin concentration and increases in adrenaline
and glucagon with increasing exercise duration
together with afferent nervous feedback from
contracting muscle will stimulate liver glucose
release (for more complete information, see Kjaer
1995).carbohydrate metabolism in exercise 91
6 5 4 3 2 1 0Hepatic glucose production(mmol.min–1)0
(rest)10 20 30 40 120 180 240
Work time (min)85%75%55%30%Fig. 6.2Hepatic glucose release
during exercise at 30%, 55%, 75%
and 85% of maximal oxygen
consumption in men. From
Ahlborg et al. (1974), Ahlborg and
Felig (1982), Wahren et al. (1971)
and Hultman (1967).
Liver glycogen (mmol.kg–1 wet wt)4000300200Rest100Exercise
Fig. 6.3Hepatic glycogen concentration in men at rest
following an overnight fast (n=33) and following an
overnight fast and 60 min of exercise at approximately
75% of maximal oxygen consumption in a second
group of subjects (n=14). Biopsy samples were
obtained at the same time of day in both groups of
subjects. From Hultman and Nilsson (1971).