press release of NEFA from adipose tissue,
leading to high plasma levels; and inappropriate
hepatic VLDL secretion which exacerbates the
rise in plasma TAG. Remnant particles of the
TAG-rich lipoproteins persist in the circulation
for longer, their smaller size increasing their
atherogenic potential.
Thus, insulin resistance may lie at the heart of
the abnormalities of lipoprotein metabolism
which are key features of the ‘metabolic syn-
drome’, i.e. low HDL cholesterol, high TAG
levels and possibly also a preponderance of small
dense LDL. It is not entirely clear, however,
which is the ‘chicken’ and which the ‘egg’ here
because an argument may be advanced for an
underlying role of abnormal fat metabolism—
secondary to the excessive delivery of TAG to
adipose tissue and muscle—in the pathogenesis
of insulin resistance. Either way, exercise may be
beneficial because of its potential to improve fuel
homeostasis through its effects on the assimila-
tion, mobilization and oxidation of fat fuels.
Alterations to lipoprotein metabolism result.
Effects of physical activity
Well-trained endurance runners, men and
women, possess lipoprotein profiles consistent
with a low risk of CHD (Durstine & Haskell
1994). HDL cholesterol is typically 20–30%
higher than in comparable sedentary controls.
Triglycerides are low, particularly when veteran
athletes (>40 years) are studied. Total cholesterol
concentrations stand out as low only when the
control group is large and representative of the
wider population. Athletes trained specifically
for strength and power do not differ from seden-
tary individuals in these ways.
Less athletic, but physically active, people also
show lipoprotein profiles which are consistent
with a reduced risk of cardiovascular disease.
For example, data from the Lipid Clinics
Prevalence Study showed that men and women
who reported some ‘strenuous’ physical activity
generally had higher HDL cholesterol levels
than those who reported none (Haskell et al.
1980). Differences were independent of age, body
mass index, alcohol use and cigarette smoking.
Even simple exercise like walking has been
linked to elevated HDL levels, with relationships
between distance walked per day and the con-
centration of HDL 2 , the subfraction that accounts
for most of the difference in total HDL choles-
terol between athletes and controls. In addition,
men and women who habitually walk 12–
20 km · week–1are only half as likely to possess an
unfavourable ratio of total to HDL cholesterol
(>5) as a comparable no-exercise group. Thus
cross-sectional observations of ordinary men and
women, and of everyday activity, provide a basis
for proposing that endurance exercise influences
lipoprotein metabolism.
Longitudinal studies are less consistent but,
for HDL cholesterol, the consensus is that, over
months rather than weeks, endurance exercise
involving a minimum expenditure of about
15 MJ · week–1(3580 kcal · week–1) causes an in-
crease and that the magnitude of this tends to be
greater when there is weight loss.
The majority of longitudinal studies have
employed rather high intensity exercise, most
frequently jogging/running, but evidence is
gradually becoming available that more accessi-
ble, self-governed exercise regimens may also be
effective (Després & Lamarche 1994). For
example, in previously sedentary middle-aged
women who had rather low levels of HDL
cholesterol (mean, 1.2 mmol · l–1) at base line,
walking briskly for about 20 km · week–1over a
year resulted in a 27% increase. Increases in HDL
cholesterol do not always mirror changes in
fitness, however. Figure 3.2 shows the main find-
ings of one study which examined the effect of
the intensity of walking in women over 24 weeks;
fast walking at 8 km · h–1 produced greater
improvements in fitness than walking the same
distance at slower speed, but increases in HDL
cholesterol did not differ between groups
walking at different speeds. Several other studies
have confirmed these findings.
Dietary modifications recommended to over-
weight people invariably combine energy intake
restriction with decreases in the intake of satu-
rated fats and cholesterol. Such changes can