15 min, a reduction in SMA flow of 43% was
observed (Qamar & Read 1987).
In other studies, the venous portal flow was
monitored by Doppler to give an indication of
splanchnic blood flow. In one study in which
upright cycling was conducted at 70% V
.
o2max.,
and environmental temperature was 26.8±0.2°C
and humidity was 58±2%, a reduction in portal
flow after 60 min of exercise of 80±7% was
observed (Rehrer et al. 1992a). These results are in
line with Rowell’s earlier work using ICG clear-
ance with intensive upright exercise.
Kenney and Ho (1995) demonstrated a differ-
ence in redistribution of blood flow with exercise
between old and young subjects, even when
matched for V
.
o2max.and exercising at the same
intensity. Younger (26±2 years) subjects experi-
enced a 45±2% reduction in estimated splanch-
nic flow (ICG clearance) during cycling at 60% of
V
.
o2peakat an ambient temperature of 36°C. Older
(mean, 64±2 years) subjects only experienced
a 33±3% decrease. A decrease in blood supply
and relative ischaemia to the splanchnic area
decreases the absorption of actively transported
nutrients, e.g. glucose (Varro et al. 1965).
However, a decrease in flow of less than 40% is
compensated for by an increase in oxygen extrac-
tion, and absorptive capacity is not altered. A
greater decrease than this results in a reduced
oxygen supply and reduced absorption. This
leads one to wonder if the decreased splanchnic
flow caused by exercise may inhibit normal
absorption of nutrients during exercise.
Relatively few studies have assessed the com-
bined effects of exercise and nutrient ingestion
on splanchnic blood flow. It has been known for
some time that ingestion and absorption of nutri-
ents results in an increased blood flow to the
intestinal tract at rest. The effect of exercise and
nutrient ingestion/absorption has been little
studied. One early study did look at the blood
flow within the SMA in dogs after a meal, at rest
and during exercise (Burns & Schenck 1969).
Exercise consisted of treadmill running on an
incline until panting began (after about 5–
10 min). An indwelling electromagnetic flow
probe, which was surgically placed several days
246 nutrition and exercise
prior to experiments, was used to measure flow.
A 20% decrease in SMA flow with exercise was
observed, and this was lessened (14% decrease)
when the exercise was performed 3 h after
feeding. Another similar study with dogs was
performed in which they were exercised on a
treadmill for 4 min at 1.5 km · h–1 (Fronek &
Fronek 1970). Although no effect of exercise on
SMA flow in the fasted state was observed, exer-
cise immediately following meal ingestion
resulted in a relatively reduced blood flow in
contrast to that seen with feeding at rest.
Qamar and Read (1987) found that mild exer-
cise (walking, 15 min at 5 km · h–1, 20% incline)
reduced the increase in SMA blood flow
observed after a meal in man. The flow, however,
was still greater than that observed at rest in
the fasted state. Strenuous exercise of longer
duration (70% of V.
o2max.for 60 min) has been
observed to cause an attenuation of the increased
flow through the portal vein observed with
glucose (100 g · h–1) ingestion (Rehrer et al. 1993).
Flow increased to 195±19% of the resting, fasted
value after glucose ingestion and decreased to
61 ±15% with glucose ingestion during exercise.
Although this type and intensity of exercise
with glucose ingestion did result in a significant
decrease in portal flow, it is of a lesser magnitude
than that observed with similar exercise in
the fasted state (Rehrer et al. 1992a). Thus, when
glucose is ingested during exercise, the reduc-
tion in flow is lessened to such a degree that
one would not expect glucose absorption to be
inhibited.
The redistribution in splanchnic blood flow
observed as a result of exercise and meal inges-
tion is regulated by hormonal and neural stimuli,
with the sympathetic nervous system playing a
central role. Several studies have demonstrated a
relationship between sympathoadrenergic acti-
vation and splanchnic blood flow alterations
with exercise (Chaudhuri et al. 1992; Iwao et al.
1995; Kenney & Ho 1995). In particular, an
inverse correlation between portal venous flow
and plasma noradrenaline concentration has
been observed with measurements taken at rest
and during mild and intensive exercise in man