absorption but also is influenced by the gastric
emptying rate. Thus it is impossible to conclude
if it is decreased gastric emptying or intestinal
absorption which is responsible for the reduction
in absorption. Further, deuterium accumulation
in the plasma does not give information as to the
net absorption. Alterations in intestinal secretion
are not represented by deuterium accumulation
(Gisolfiet al. 1990). However, it is assumed that
the osmolality of the intestinal contents, as a
function of beverage composition, is primarily
responsible for changes in secretion. There is no
evidence to suggest that exercise affects intesti-
nal secretion. Thus, in the study by Maughan et
al.(1990) in which the same beverage was admin-
istered during exercise at various intensities, one
may expect that the secretion rate is constant.
One other problem with using a tracer to eval-
uate the appearance of a substance is that one
measures the concentration in the plasma which
is also influenced by the rate of disappearance,
i.e. the rate at which the substance is taken up
from the circulation by the tissues. During exer-
cise the rate of mixture of the tracer with the total
body water pool is nearly instantaneous and the
different body compartments may be treated as
one pool (N.J. Rehrer and R.J. Maughan, unpub-
lished observations). This mixing of the tracer
into the body water pool is delayed at rest.
However, this could not account entirely for
the difference in plasma deuterium accumula-
tion during exercise in the study in question
(Maughan et al. 1990), since during exercise
the concentration of the label in the plasma con-
tinued to rise throughout the exercise protocol,
rather than peaking early and decreasing with
exercise duration, as one would expect if a
delayed rate of efflux accounted for the reduced
rate of plasma accumulation of the deuterium.
In conclusion, no consistent clear picture of
the effect of exercise upon intestinal absorption is
evident. Beverage composition, in particular car-
bohydrate concentration and osmolality, has a
greater influence on net absorption.
A few studies have looked at intestinal perme-
ability during or after exercise. Both Moses et al.
(1991) and Oktedalen et al. (1992) have shown a
decreased functioning of the mucosal barrier
with running. More recently, Ryan et al. (1996)
have shown that intestinal permeability is
reduced to a greater degree when exercise and
aspirin ingestion are combined than with aspirin
ingestion at rest. This apparent lack of mainte-
nance of intestinal membrane integrity may be
related to alterations in blood flow to the intesti-
nal region during strenuous exercise.Splanchnic blood flow
Rowellet al. (1964) and others (Clausen 1977;
Qamar & Read 1987; Rehrer et al. 1992a; Kenney
& Ho 1995; Seto et al. 1995) have demonstrated a
decrease in blood flow to the intestinal tract as
a result of physical exercise. Early studies, with
limited exercise, indicated that splanchnic blood
flow could be altered with exercise. Bishop et al.
(1957) showed a decrease in arteriovenous dif-
ference in oxygen content over the liver, with
supine cycling ergometry. Wade et al. (1956)
demonstrated reduced bromosulphalein clear-
ance in patients recovering from pulmonary
infections, undertaking ‘light’, supine exercise
(7–8 min of leg lifts). Rowell et al. (1964) were the
first to quantify this decrease during upright
treadmill exercise, using indocyanine green
(ICG) clearance as an indication of splanchnic
flow. They showed decreases of up to 84% with
exercise.
A number of the more recent studies have
made use of pulsed Doppler ultrasound to
measure blood flow. In some cases the superior
mesentery artery is measured and in others the
portal vein. The measurements of the superior
mesenteric artery (SMA) are typically done at
rest prior to or after exercise as accurate measure-
ments during exercise are made difficult due to
the increased force of contraction of the heart
during intensive exercise. This gives large fluctu-
ations in the aortic flow which results in a super-
imposed flow on top of the true mesentery flow.
Nevertheless, the measurements that are taken
shortly after exercise has stopped do show a
reduction in SMA flow. In one study, in which
subjects walked at 5 km · h–1up a 20% incline for