positively related to muscle mass and, long-term,
there may be additional effects of regular activity
if muscle mass increases. By contrast with the
effects of endurance training, which leads to
predominantly qualitative changes in muscle
insulin/glucose dynamics, the main effect of
strength, and perhaps sprint, training may be
to increase the quantity of muscle. Indeed,
increases in lean body mass gained through
strength training have been reported to be
closely related to reductions in the total insulin
response during an oral glucose tolerance test.
Net effect of training
Laboratory study has shown that exercise
increases insulin sensitivity and decreases
glucose-stimulatedb-cell insulin secretion. It
does not follow, however, that training spares
insulin secretion and blood glucose levels in real
life because training necessitates an increase in
food intake. A study from Copenhagen makes
the point well (Dela et al. 1992). These workers
compared trained male athletes with untrained
controls during their (different) ordinary living
conditions as well as in the laboratory (Table 3.2).
The higher daily energy intakes of the athletes—
mean of 18.6 MJ · day–1(4440 kJ · day–1), compared
with 12.5 MJ · day–1 (2986 kcal · day–1) for the
sedentary men—reflected mainly differences in
carbohydrate intake (678 vs. 294 g · day–1). Fol-
lowing oral glucose loads comprising identical
fractions of daily carbohydrate intake, the areas
under the plasma glucose and insulin concentra-
tion vs. time curves did not differ between ath-
letes and untrained men. The two groups also
had identical 24-h glucose responses during a
day when they went about their normal activities
(including one or two training sessions for the
athletes). It seems that training, rather than
sparing the pancreas, elicits adaptations in the
action of insulin which allow the necessary
increases in food intake without potentially
harmful hyperglycaemia and overloading of b-
cells. During the day of normal activity, arterial
insulin concentrations were, however, some 40%
lower (NS) in athletes because of enhanced
hepatic clearance. As insulin may directly pro-
mote both atherosclerosis and hypertension, a
lower circulating level of the hormone may in
itself be advantageous.
The example just presented is an extreme case,
with the athletes (one 800-m runner, one 1500-m
runner and five triathletes) consuming some 50%
more food energy than the sedentary compari-
son group. It is not atypical, however, as other
researchers have found almost 80% of the
increased energy intake associated with highexercise, nutrition and health 45
Table 3.2Integrated glucose and insulin responses to the same absolute oral glucose load (1 g · kg-^1 body mass), to
the same relative oral glucose load (27.7% of usual daily carbohydrate intake) and to food consumed under
ordinary living conditions during a 24-h period. Adapted from Dela et al. (1992).
Glucose Insulin(mmol · l-^1 ·3h-^1 ) (pmol ·ml-^1 ·3h-^1 )
Same absolute load
Untrained 1277* 58*
Trained 1040 24
Same % daily carbohydrate intake
Untrained (1 g · kg-^1 body mass) 1277 58
Trained (2.3 g · kg-^1 body mass) 1173 44
(mol · l-^1 ·24h-^1 ) (pmol · ml-^1 ·24h-^1 )
24-h responses, ordinary living
Untrained 7.3 175
Trained 7.4 124*Significantly different from trained, P<0.05.