with regular aerobic exercise. The proposition
that exercise brings blood pressure down has
been tested experimentally. Valid conclusions
can only be drawn from studies including non-
exercising control subjects, because blood pres-
sures tend to fall with repeated measurements
when people become accustomed to the proce-
dure. Controlled exercise intervention trials have
found an average reduction of 3/3 mmHg
(systolic/diastolic) in normotensives, with
somewhat greater reductions in borderline
hypertensives and hypertensives, i.e. 6/7 mmHg
and 10/8 mmHg, respectively (Bouchardet al.
1994). These conclusions are based on resting
blood pressure measured in clinic or laboratory;
reductions in measures made during the normal
living conditions tend to be less consistent and
smaller but more evidence is needed. Moderate
intensity training (<70%V
.
o2max.) leads to reduc-
tions in systolic blood pressure which are up to
40% greater than those resulting from training at
higher intensity, possibly because of the lesser
response of the sympathetic nervous system.
The blood-pressure lowering effect of exercise
probably occurs very rapidly, possibly after as
little as 1 week of exercise training. Repeated
short-term effects during recovery from individ-
ual exercise sessions may therefore be important.
For example, in sedentary hypertensives, blood
pressure is reduced for up to 8–12 h after a single
exercise session. Longer training programmes
produce somewhat larger reductions in blood
pressures, however, suggesting that adaptive
effects of habitual exercise, i.e. training, may act
synergistically to enhance short-term effects.
Glucose/insulin dynamics
Diabetes mellitus afflicts about 2% of individuals
in Western populations. By far the most common
type is NIDDM, the incidence of which rises
steeply with age. It is characterized by the failure
of insulin to act effectively in target tissues such
as muscle, liver and adipose tissue. The pancreas
responds with enhanced secretion by its b-cells
and plasma insulin levels are chronically high.
Glucose intolerance (an abnormally high blood
glucose response to a standard 75 g oral load)
develops gradually, fasting plasma glucose and
insulin levels rising in parallel until the former
reaches 7–8 mmol · l–1(compared with normal
values of around 4–5 mmol · l–1). At this stage the
b-cells of the pancreas fail to maintain adequate
insulin secretion and so there is a progressive fall
in the fasting concentration. Profound glucose
intolerance then develops and the condition
worsens to overt NIDDM, the severity of
which is determined by the inadequacy of b-cell
function.
Resistance to insulin-stimulated glucose
uptake is the most important precursor of
NIDDM and a common characteristic occurring
in approximately 25% of the population. It is a
prominent feature of obesity. Normal glucose
tolerance is maintained but at the expense of
hyperinsulinaemia, which leads to multiple der-
angements of metabolism—for example, high
plasma levels of triacylglyceride (TAG) and low
levels of high-density lipoprotein (HDL) choles-
terol. In the longer term, these result in damage
to blood vessels, with increased risk of develop-
ing CHD, hypertension and problems of the
microcirculation, including renal disease and
retinal damage.Risk of NIDDM
Prospective studies show an inverse relationship
between energy expenditure in leisure time
activity and the risk of subsequently developing
NIDDM (Kriska et al. 1994). For example, among
male ex-students of the University of Pennsylva-
nia the incidence of NIDDM decreased by some
6% for each 2.1 MJ (500 kcal) expended per week
in physical activity. US male physicians who
exercised ‘vigorously’ at least once per week
experienced only 64% of the risk of developing
NIDDM, compared with those who exercised
less frequently. Findings have been similar for
middle-aged women, those taking part in vigor-
ous exercise experiencing only two thirds of the
risk seen in other women. There are indications
that the influence of physical activity may be
particularly strong in those who are overweight.