volume training was from carbohydrate. Similar,
if smaller, effects on carbohydrate intake also
tend to occur with more modest, non-athletic,
levels of exercise. For example, when a group of
middle-aged men took up jogging, their average
daily energy intake increased by about 1.25 MJ ·
day–1(300 kcal · day–1) after 2 years (12.5%) and
this was almost all from carbohydrate (an
increase of 70 g · day–1, about 30%) (Wood et al.
1985).
Lipoprotein metabolism
The body’s major energy store is TAG, a hydro-
phobic molecule which is transported through
the watery plasma in particles called lipo-
proteins. Lipoproteins comprise a core of fatty
material (cholesteryl esters as well as TAG)
surrounded by a relatively hydrophilic coat com-
prising phospholipid, free cholesterol and one or
more protein molecules known as apolipopro-
teins. The main categories are (in order of
increasing density): chylomicrons, very low
density lipoproteins (VLDL), low-density
lipoproteins (LDL) and HDL. A brief outline of
their metabolism helps understand both the
influence of exercise and the potential implica-
tions for health.
The function of chylomicrons is to carry TAG
and cholesterol derived from the diet. Their main
role is to deliver TAG to peripheral tissues and
cholesterol to the liver. Secreted by the cells of the
intestinal wall, they enter the bloodstream via
the lymphatics. As they pass through the capil-
lary beds of adipose tissue and muscle, their TAG
is hydrolysed by the enzyme lipoprotein lipase
(LPL), the non-esterified fatty acids (NEFA)
released mostly being taken up by the tissues.
As TAG is lost, the chylomicrons shrink and
cholesterol-rich remnant particles are removed
by hepatic receptors.
By contrast, VLDL distribute TAG from the
liver to other tissues. Like chylomicrons, they are
a substrate for LPL and become TAG-depleted as
they pass through capillary beds. Their remnants
are LDL which carry (in ester form) some 70% of
the cholesterol in the circulation, delivering it to
46 nutrition and exercise
a variety of tissues, according to their needs.
Plasma total cholesterol concentration, in epi-
demiological study shown to be strongly and
positively related to the risk of CHD, predomi-
nantly reflects LDL cholesterol.
HDL provide a means by which cholesterol is
routed from peripheral tissues to the liver where
it is disposed of safely, mainly via synthesis into
bile acids. HDL receive unesterified cholesterol
which is released as excess surface material
during the degradation of TAG-rich particles,
but also incorporate cholesterol from the body’s
cells when this is present in excess of needs. This
pathway has been termed ‘reverse cholesterol
transport’ and may be the mechanism underly-
ing the inverse relationship between HDL cho-
lesterol and the risk of CHD. In women, for
example, an increase of 0.26 mmol · l–1 (about
20%) in HDL cholesterol is associated with a
42–50% decrease in CHD risk. An alternative
explanation is that low HDL-cholesterol may be
a marker for some defect in the metabolism of
TAG-rich lipoproteins which means that chy-
lomicron remnants and LDL remain in the cir-
culation for longer, becoming correspondingly
smaller and more readily taken up into athero-
sclerotic lesions. There is clear evidence of this
for LDL, but also increasing awareness that the
chylomicron remnant may also be atherogenic,
not least because it may contain 30 times as many
cholesterol molecules as a typical LDL particle.
The view that atherogenesis is a postprandial
phenomenon is gaining support and patients
with known coronary artery disease show a more
marked and prolonged rise in plasma TAG con-
centrations following an oral fat load than
healthy controls.
Insulin plays an important role in fat metabo-
lism, coordinating events during the postpran-
dial period. LPL activity in adipose tissue is
stimulated and mobilization of NEFA is
depressed through inhibition of hormone
sensitive lipase and plasma NEFA levels fall
markedly.
When insulin sensitivity is poor, fat metabo-
lism is disordered: there is failure to stimulate
LPL, so TAG-removal rate falls; failure to sup-