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ExerciseNutrition and Health

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ExerciseNutrition and Health
Chapter 3
Exercise, Nutrition and Health
ADRIANNE E. HARDMAN
Introduction
By virtue of its mass and unique potential
to increase metabolic rate, skeletal muscle is
man’s largest ‘metabolic organ’. Energy expenditure is increased profoundly during exercise with the body’s large muscles and
individuals who engage regularly and frequently in such exercise have enhanced energy
requirements. These are met through increased
nutrient intake, particularly of carbohydrate, so
that the relative contributions of macronutrients
to energy intake may be altered. This in itself
may constitute a more healthy diet but, in addition, the metabolic handling of dietary fats and
carbohydrates is improved, changes which help
reduce the risk of developing several chronic
diseases, specifically atherosclerotic vascular diseases, non-insulin-dependent diabetes (NIDDM,
also known as adult-onset or type II diabetes)
and possibly some cancers (Bouchard et al.
1994).
An example of an association between disease
risk and energy turnover is given in Table 3.1,
which shows average daily energy intakes in
prospective studies of coronary heart disease
(CHD). Men who subsequently had fatal attacks
showed lower levels of energy intake than
survivors, an apparent paradox in the light of
the increase in CHD risk associated with
overweight, obesity and the deleterious metabolic sequelae of these. One explanation is
that the men with higher energy intakes were
more physically active, and that their exercise
afforded a level of protection against CHD,
compared with more sedentary men who ate
less.
Thus, the transition from a sedentary to
an active state is associated with a higher
energy turnover, with important implications
for the transport, storage and utilization of the
body’s metabolic fuels. All of these are altered
in the trained state such that regular exercisers
experience a lower risk of what has been
called ‘metabolic, hypertensive cardiovascular
disease’. Higher energy turnover may also be
associated with improved weight regulation
because food intake appears to be more closely
coupled to energy expenditure with more
exercise.
Rather than prolonging life, regular exercise
protects against premature death, with an estimated increase in longevity in men on average of
one or two years (Paffenbarger et al. 1986). Moreover, lower all-cause mortality has recently been
reported for physically active women (Blair et al.
1996), although evidence is much less extensive.
People who take exercise also maintain a better
quality of life into old age, being less likely
than sedentary individuals to develop functional
limitations.
This chapter will identify some of the health
gains which accrue from the biological interactions between exercise and the body’s metabolism of dietary carbohydrate and fats. For
discussion of the evidence for a specific role of
diet in promoting health, the reader is referred to
other sources (WHO 1990).
39
40
nutrition and exercise
Table 3.1 Average daily energy intake (MJ) and future risk of coronary heart disease. Adapted from Wood (1987).
Cohort
Heart disease victims
Survivors
English banking and
London bus workers
Framingham, Massachusetts
Puerto Rico
Honolulu, Hawaii
11.11
12.00
9.91
9.30
8.99
10.97
10.02
9.70
Atherosclerotic vascular diseases
Pathological changes to the arterial wall give rise
to atherosclerotic plaques, complex structures
which result from proliferation of the smooth
muscle cells and collagen, with deposition of
cholesterol-rich lipid. These probably begin as
fatty streaks which develop when lipid-laden
macrophages accumulate after the integrity of
the endothelium is breached and blood components are exposed to collagen in the wall of the
artery. The clinical outcome depends on the
site(s) and extent of the lesion: in coronary arteries, myocardial blood flow is reduced, leading
to chest pain on effort (angina) and a risk of
thrombotic occlusion (heart attack) and/or disturbances in the electrical coordination of contraction; blood supply to the limbs is impaired
when the arteries supplying the legs are
narrowed, imposing severe limits on walking
capability; and stroke occurs when there is
thrombolytic occlusion of a cerebral artery or a
local haemorrhage from a vessel with atherosclerotic damage. Links with nutrition are clear from,
for example, the association between levels of
saturated fat in the diet and the risk of CHD.
Coronary heart disease
Epidemiological studies have shown significant
associations between indices of both physical
activity (a behaviour) and physical fitness (a
set of characteristics arising from the regular
pursuance of this behaviour) and risk of the commonest manifestation of atherosclerosis, CHD —
a disease responsible for one in four male deaths
and one in five female deaths in the UK in 1994.
We must be careful in our interpretation of associations, however, because exercisers may be
constitutionally different from sedentary people
in ways which decrease the likelihood of their
developing the disease. Complementary scientific evidence of plausible mechanisms has much
to contribute and the role of exercise in this will
be discussed later.
More than 50 population studies have compared the risk of CHD in physically active men
with that of their sedentary counterparts. Careful
scrutiny of their findings shows that sedentary
men experience about twice the risk seen in
active men (see Whaley & Blair 1995). This relative risk is of the same order of magnitude as that
associated with hypertension (systolic blood
pressure > 150 mmHg vs. < 120 mmHg), smoking
(≥ 20 cigarettes · day–1 vs. no smoking) and high
serum total cholesterol levels (> 6.9 mmol · l–1 vs.
£ 5.6 mmol · l–1). Estimates of the protective effect
of exercise are highest in those studies with the
soundest design and methodology and no study
has found a higher risk in active men. The effect is
independent of hypertension, smoking and high
total cholesterol levels.
Early studies compared groups of men with
different levels of occupational work. For
example, postal workers who walked and cycled
delivering mail and dock workers with high
levels of habitual on-the-job energy expenditure
experienced less heart disease than colleagues in
less physically demanding jobs. Leisure time
physical activity has also been studied and an
inverse, graded relationship between leisure
time physical activity and CHD was found
among graduates (alumni) of Harvard and
Pennsylvania universities (Paffenbarger et al.
exercise, nutrition and health
1986); the risk of first attack was one quarter to
one third lower in men who expended more than
8.36 MJ · week–1 (2000 kcal · week–1) in physical
activity (sports, garden work, walking, stairclimbing, etc.) than in classmates whose
exercise energy expenditure was lower, i.e. high
total energy expenditure in exercise was a
determinant of risk.
By contrast, prospective study of English civil
servants found no association between total
exercise energy expenditure and risk of heart
attack (Morris et al. 1990); only men reporting
‘vigorous’ exercise experienced a lower risk than
sedentary men. Vigorous was defined as exercise
likely to involve peaks of energy expenditure of
31 kJ · min–1 (7.5 kcal · min–1) or more. This is
about the rate of energy expenditure of a middleaged man of average weight during fast walking,
so it is not surprising that men who reported
that their usual speed of walking was ‘fast’
(> 6.4 km · h–1) experienced a particularly low rate
of attack. Low rates were also reported for men
who did considerable amounts of cycling.
Increasingly, studies have measured physical
fitness rather than, or as well as, physical activity.
Their findings are broadly similar, i.e. a two- to
threefold increase in the risk of cardiovascular
death in men when comparing the least fit with
the most fit groups (Whaley & Blair 1995). The
41
limited data available suggest an effect of at least
this magnitude for women.
Given the diverse methodologies and cohorts
studied, the clarity with which the inverse,
graded relationship between level of physical
activity or fitness and risk of mortality from CHD
emerges is noteworthy. Figure 3.1 summarizes
the findings of seven studies in which either
leisure time activity (questionnaire) or fitness
(laboratory exercise test) was assessed prior to a
follow-up period of 7–17 years. The precise
pattern differs between studies, but it is clear
that, whilst men with only moderate levels of
activity or fitness experience some degree of protection, higher levels tend to confer greater
protection. Some studies, however, suggest that
the relationship may be curvilinear — CHD risk
decreasing steeply at the lower end of the continuum, reaching an asymptote in the mid-range.
Thus, for men in the age group most studied
.
(approximately 40–60 years), values for Vo2max.
of around 35 ml · kg–1 · min–1 have been proposed
as being sufficient to confer a worthwhile — not
necessarily optimal — decrease in risk; evidence
for women is scanty, but a comparable value is
probably at least 2 or 3 ml · kg–1 · min–1 lower.
Two aspects of the evidence strengthen the
argument that the relation of activity and fitness
with CHD risk may be causal. First, only current
Reduction in coronary
events/mortality
100
80
+
60
40
+
+
+
+
+
Paffenbarger et al., 1986
Morris et al., 1990
Blair et al., 1989
Leon, 1991
Ekelund et al., 1988
Sandvik et al., 1993
Shaper & Wannamethee, 1991
20
0
Physical activity/fitness level
Fig. 3.1 The relationship between the level of physical activity (Paffenbarger et al. 1986; Ekelund et al. 1988; Morris
et al. 1990; Leon 1991; Shaper & Wannamethee 1991) or fitness (Blair et al. 1989; Sandvik et al. 1993) and risk of
coronary heart disease among men in prospective studies. Adapted from Haskell (1994).
42
nutrition and exercise
and continuing activity protects against heart
disease; men who were active in their youth but
became sedentary in middle-age experience a
risk similar to that of men who had never been
active. Second, men who improved either their
physical activity level or their fitness level
between one observation period and another
some years later experienced a lower risk of
death than men who remain sedentary. To put
these levels of risk reduction into perspective,
taking up physical activity was as effective as
stopping smoking.
The role of exercise intensity in determining
CHD risk is still uncertain. Several key studies
have shown substantial reductions in risk with
accumulation of physical activity, most of which
was at a moderate intensity (see Haskell 1994).
However, other evidence argues that more vigorous physical activity may provide unique benefits. These uncertainties should not, however,
detract from the wealth of evidence, gathered
over a long period and in different populations,
that identifies physical inactivity as a major risk
factor for CHD.
Mechanisms by which exercise might confer a
lower risk of CHD include effects on blood pressure, weight regulation, lipoprotein metabolism
and insulin sensitivity — all of which are discussed below. Another suggestion, arising from
the evidence referred to above that only current
exercise protects against CHD, involves an effect
on the acute phase of the disease — the thrombotic component, for example. This possibility
is supported by associations between exercise
habits and haemostatic factors and is an area justifying more research.
Stroke
Atherosclerotic damage to cerebral arteries is a
prominent feature of stroke, so an effect of habitual exercise on the risk of having a stroke is
plausible, but there is little direct evidence. In the
British Regional Heart Study (Wannamethee &
Shaper 1992), the age-adjusted rate for strokes
showed a steep and significant inverse gradient
with physical activity category in men with or
without heart disease or stroke at baseline; the
risk in moderately active subjects was less than
half that reported for inactive men. Data from the
Honolulu Heart Program (Abbott et al. 1994)
show an association between the risk of stroke
and a physical activity index in older middleaged men (55–68 years) but not in younger men
(45–54 years); the excess incidence of haemorrhagic stroke in inactive/partially active men
was three- to fourfold. For thromboembolic
stroke, among non-smokers the risk for inactive
men was nearly double that for active men but
there was no effect in smokers.
Hypertension
About 16% of men and 14% of women in
England have hypertension (systolic blood pressure > 159 mmHg and/or diastolic > 94 mmHg).
It is a major public health problem; even mild to
moderate elevations in blood pressure substantially increase the risk of developing CHD,
stroke, congestive heart failure and intermittent
claudication in both men and women.
There is some evidence that high levels of
physical activity decrease the risk of developing
hypertension (see Paffenbarger et al. 1991). For
example, of 5500 male Harvard alumni free of
hypertension at entry to the study, 14% developed the disease during 14 years’ observation.
Contemporary vigorous exercise alone was associated with lower incidence, chiefly among men
who were overweight-for-height. Similar conclusions arise from study of fitness levels in relation
to risk of hypertension: during follow-up of 6000
men and women over 1–12 years (median, 4) the
risk of developing hypertension was 1.5 times
greater for those with low fitness (the bottom
75% of the sample) than for those deemed to
have high fitness (the top 25%).
The rationale for a role for exercise in the prevention of hypertension is that, during exercise,
there is marked dilation of blood vessels in active
skeletal muscle, decreasing resistance to flow.
This persists during the recovery period, possibly contributing to the chronic lowering of (arterial) blood pressure which is often associated
exercise, nutrition and health
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 nonexercising control subjects, because blood pressures tend to fall with repeated measurements
when people become accustomed to the procedure. 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 (Bouchard et 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% Vo2max.) leads to reductions 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 individual 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
43
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 derangements of metabolism — for example, high
plasma levels of triacylglyceride (TAG) and low
levels of high-density lipoprotein (HDL) cholesterol. In the longer term, these result in damage
to blood vessels, with increased risk of developing 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 Pennsylvania 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 vigorous 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.
44
nutrition and exercise
Potential mechanisms
The primary targets for insulin-stimulated
glucose disposal are skeletal muscle and adipose
tissue and influences on their glucose transport
and metabolism dictate whole-body responsiveness to insulin. Muscle, representing some 40%
of body mass, is probably the more important
tissue.
Insulin-mediated glucose uptake into skeletal
muscle proceeds by a series of steps, the first of
which is insulin binding to receptors on the outer
surface of the cell membrane. Glucose transport
is achieved via ‘facilitated diffusion’, a process
which involves a mobile protein carrier (GLUT4) which facilitates its transport across the membrane and is thought to be rate-limiting. Besides
its action on glucose transport, insulin inhibits
glycogenolysis, promoting glycogen synthesis.
Muscle glycogen is reduced during exercise, creating the need for enhanced uptake and storage
and raising the possibility that improved responsiveness of this tissue to insulin might exert an
important influence on the body as a whole,
explaining the lower incidence of NIDDM in
physically active people.
It is more than 25 years since the first report of
markedly lower plasma insulin concentrations in
endurance-trained middle-aged men — both in
the fasted state and after an oral glucose load —
than in comparable sedentary men. These findings have generally been interpreted as a sign of
increased insulin sensitivity in peripheral
tissues, since hepatic glucose output is suppressed after glucose ingestion. Later studies
have confirmed this, measuring reduced insulin
secretion and a shift in the insulin/glucose disposal response curve, promoting glucose transport and storage. Whole-body non-oxidative
glucose disposal during glucose infusion is
higher in endurance-trained athletes than in
sedentary controls. Total activity of glycogen
synthase and insulin-stimulated activation of the
enzyme is enhanced as trained muscle adapts to
the increased intracellular availability of glucose
by developing an enhanced capacity for glucose
storage as glycogen.
The mechanisms by which training enhances
glucose uptake in skeletal muscle are local rather
than systemic and probably involve changes in
levels of the muscle glucose transporter GLUT-4.
Endurance-trained athletes possess higher levels
of GLUT-4 than sedentary controls and levels are
markedly higher in trained than in untrained
muscle from the same individual, in association
with a higher insulin-stimulated glucose uptake.
Glucose uptake depends on its rate of delivery
to the tissue, however, as well as that tissue’s
responsiveness to insulin. Insulin stimulates
increases in blood flow to muscle in a doseresponsive manner and this effect could, speculatively, be enhanced in athletes because of
improved capillarization.
Following each exercise session, glucose
uptake into skeletal muscle increases. This is
partly an insulin-independent contractile effect
which persists for several hours afterwards but,
in addition, the response to insulin of the glucose
transport system is improved. This usually
lasts longer, for at least 48 h. As stated above,
these may be responses to the need to replenish
muscle glycogen; certainly exhaustive, intermit.
tent glycogen-depleting exercise at 85% Vo2max.
results in increased non-oxidative glucose disposal when measured 12 h later.
When endurance-trained people refrain from
training, their enhanced insulin action is rapidly
reversed. The timescale of this reversal is not
clear; training effects have been reported to
persist for as little as 36 h but more typically for
about 3 days, so that levels seen in sedentary
people are approached within 1 week. Could the
good insulin sensitivity which characterizes athletes be attributable to residual effects of their last
training session, rather than to any long-term
adaptive effects? The answer to this question is
‘probably not’. Studies have compared the
response of a trained leg to a single session of
exercise with that of an untrained (contralateral)
leg to identical exercise; insulin action was
improved in the trained leg but there was no
effect in the untrained leg. The effect of training
on insulin-mediated glucose disposal in muscle
has therefore been described as a genuine adaptation to training — but short-lived.
Whole-body insulin sensitivity is directly and
exercise, nutrition and health
positively related to muscle mass and, long-term,
there may be additional effects of regular activity
if muscle mass increases. By contrast with the
effects of endurance training, which leads to
predominantly qualitative changes in muscle
insulin/glucose dynamics, the main effect of
strength, and perhaps sprint, training may be
to increase the quantity of muscle. Indeed,
increases in lean body mass gained through
strength training have been reported to be
closely related to reductions in the total insulin
response during an oral glucose tolerance test.
Net effect of training
Laboratory study has shown that exercise
increases insulin sensitivity and decreases
glucose-stimulated b-cell insulin secretion. It
does not follow, however, that training spares
insulin secretion and blood glucose levels in real
life because training necessitates an increase in
food intake. A study from Copenhagen makes
the point well (Dela et al. 1992). These workers
compared trained male athletes with untrained
controls during their (different) ordinary living
conditions as well as in the laboratory (Table 3.2).
The higher daily energy intakes of the athletes —
mean of 18.6 MJ · day–1 (4440 kJ · day–1), compared
with 12.5 MJ · day–1 (2986 kcal · day–1) for the
45
sedentary men — reflected mainly differences in
carbohydrate intake (678 vs. 294 g · day–1). Following oral glucose loads comprising identical
fractions of daily carbohydrate intake, the areas
under the plasma glucose and insulin concentration vs. time curves did not differ between athletes and untrained men. The two groups also
had identical 24-h glucose responses during a
day when they went about their normal activities
(including one or two training sessions for the
athletes). It seems that training, rather than
sparing the pancreas, elicits adaptations in the
action of insulin which allow the necessary
increases in food intake without potentially
harmful hyperglycaemia and overloading of bcells. During the day of normal activity, arterial
insulin concentrations were, however, some 40%
lower (NS) in athletes because of enhanced
hepatic clearance. As insulin may directly promote both atherosclerosis and hypertension, a
lower circulating level of the hormone may in
itself be advantageous.
The example just presented is an extreme case,
with the athletes (one 800-m runner, one 1500-m
runner and five triathletes) consuming some 50%
more food energy than the sedentary comparison group. It is not atypical, however, as other
researchers have found almost 80% of the
increased energy intake associated with high
Table 3.2 Integrated glucose and insulin responses to the same absolute oral glucose load (1 g · kg-1 body mass), to
the same relative oral glucose load (27.7% of usual daily carbohydrate intake) and to food consumed under
ordinary living conditions during a 24-h period. Adapted from Dela et al. (1992).
Glucose
Insulin
(mmol · l-1 · 3 h-1)
(pmol ·ml-1 · 3 h-1)
Same absolute load
Untrained
Trained
1277*
1040
58*
24
Same % daily carbohydrate intake
Untrained (1 g · kg-1 body mass)
Trained (2.3 g · kg-1 body mass)
1277
1173
58
44
(mol · l-1 · 24 h-1)
(pmol · ml-1 · 24 h-1)
24-h responses, ordinary living
Untrained
Trained
*Significantly different from trained, P < 0.05.
7.3
7.4
175
124
46
nutrition and exercise
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 hydrophobic molecule which is transported through
the watery plasma in particles called lipoproteins. Lipoproteins comprise a core of fatty
material (cholesteryl esters as well as TAG)
surrounded by a relatively hydrophilic coat comprising phospholipid, free cholesterol and one or
more protein molecules known as apolipoproteins. 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 implications 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 capillary 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
a variety of tissues, according to their needs.
Plasma total cholesterol concentration, in epidemiological study shown to be strongly and
positively related to the risk of CHD, predominantly 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 underlying the inverse relationship between HDL cholesterol 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 chylomicron remnants and LDL remain in the circulation for longer, becoming correspondingly
smaller and more readily taken up into atherosclerotic 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 concentrations following an oral fat load than
healthy controls.
Insulin plays an important role in fat metabolism, coordinating events during the postprandial 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 metabolism is disordered: there is failure to stimulate
LPL, so TAG-removal rate falls; failure to sup-
exercise, nutrition and health
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 syndrome’, 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 assimilation, 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 sedentary 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
47
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 concentration of HDL2, the subfraction that accounts
for most of the difference in total HDL cholesterol between athletes and controls. In addition,
men and women who habitually walk 12–
20 km · week–1 are 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 increase 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 accessible, 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–1 over 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 findings 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 overweight people invariably combine energy intake
restriction with decreases in the intake of saturated fats and cholesterol. Such changes can
48
nutrition and exercise
6
+16%
.
∆ VO2max (ml.kg–1.min–1)
4
+9%
2
+4%
∆ HDL cholesterol (mmol.l–1)
0.12
0.09
+6%
+6%
0.06
+4%
0.03
+1%
0
0
–2
–6%
–4
Controls
Strollers
Brisk
walkers
Fast
walkers
(a)
Controls
Strollers
Brisk
walkers
Fast
walkers
(b)
.
Fig. 3.2 Changes in (a) maximal oxygen uptake (Vo2 max.) and (b) serum high-density lipoprotein (HDL) cholesterol
concentration in control subjects (n = 10/13) and in three groups of previously sedentary women who walked 4.8
km · day–1 for 24 weeks. One group walked at 4.8 km · h–1 (n = 17/18, strollers), one group at 6 km · h–1 (n = 12, brisk
walkers) and one group at 8 km · h–1 (n = 13, fast walkers). Adapted from Duncan et al. (1991).
reduce HDL levels and, given the inverse
association between HDL cholesterol and the
risk of CHD, theoretically may diminish the
anticipated beneficial effects of decreased low
density lipoprotein cholesterol. Exercise may be
one way to offset a diet-related fall in HDL cholesterol. Comparison of two different interventions in sedentary overweight men and women,
i.e. a low energy, low fat diet alone with the same
diet plus exercise (brisk walking and jogging)
showed that the addition of exercise to the low
fat diet resulted in more favourable changes in
HDL cholesterol than diet alone; in men, diet
plus exercise provoked in a greater rise in HDL
cholesterol than did diet only; and in women
only the diet-plus-exercise group showed a
favourable change in the ratio of LDL cholesterol
to HDL cholesterol.
It was mentioned above that changes in
lipoproteins tend to be greater when an exercise
regimen is accompanied by weight loss. There is
also an effect which is independent of weight
change, which appears to be linked to adaptations in skeletal muscle. During exercise there is a
net efflux of HDL2 across a trained leg, but not
across the contralateral untrained leg (Kiens &
Lithell 1989). The rate of HDL2 synthesis is positively and strongly related to the rate of VLDL
degradation. As the rate-limiting step in VLDL
degradation is LPL activity, this points to skeletal
muscle LPL as an important determinant of the
effects of exercise on lipoprotein metabolism.
Postprandial lipoprotein metabolism
High levels of muscle LPL activity, leading to an
enhanced metabolic capacity for TAG may therefore explain the elevated HDL cholesterol levels
in physically active people. Endurance trained
men and women show high levels of plasma and
muscle LPL activity, together with high rates of
TAG clearance (compared with sedentary con-
exercise, nutrition and health
trols). The high LPL levels probably arise from
enhanced capillarization in the muscle of athletes
because the enzyme is bound to the luminal
surface of capillary endothelium.
There are also short-term effects of recent exercise on postprandial TAG clearance. During
recovery TAG clearance rates are increased,
reducing the postprandial rise in plasma TAG
concentration. The effect is greater after moder.
ate intensity exercise (60% Vo2max.) than after low
.
intensity exercise (30% Vo2max.) of the same duration probably because of its greater energy
expenditure; if energy expenditure is held constant the effects on lipaemia of low and moderate
intensity exercise are strikingly similar (Tsetsonis
& Hardman 1996). These short-term benefits
may therefore be potentially greater for trained
.
people because their higher Vo2max. values
and greater endurance capability allow them to
expend more energy than untrained individuals
before becoming fatigued.
People spend the majority of their lives in
the postprandial state and exercise-induced decreases in postprandial lipaemia may be clinically important in the long term. When TAG
clearance is good, the postprandial rise in TAG is
reduced and TAG-rich particles will remain in
the circulation for shorter periods, decreasing the
atherogenic stimulus. Clinical evidence is consistent with this view because case-control studies
have shown that postprandial TAG levels accurately predict the presence or absence of coronary artery disease.
Energy balance
In the UK, overweight (body mass index 25–30
kg · m–2) and obesity (body mass index > 30 kg ·
m–2) are a serious problem. More than 50% of
men and more than one third of women in the
age group 45–54 are overweight, whilst nearly
20% of both sexes are obese. Figures are even
worse in the US, where mean body weight
increased by 3.6 kg between 1976/80 and
1988/91. The health hazards of carrying excess
weight are well documented so its prevalence
rightly gives rise to concern. Recent findings
49
have particularly emphasized the importance of
the regional distribution on body fat in relation to
the risk of atherosclerotic metabolic disease. As
with so many aspects of human health, there is
substantial genetic control but environmental
factors — diet, physical activity — modify these
influences profoundly.
The energy stores of the body are, of course,
determined by the balance between energy
intake and energy expenditure and any exercise
contributes to energy expenditure. Although for
most people the expenditure in habitual exercise
rarely accounts for more than 20% of the total,
physical activity is the only way in which energy
expenditure can be increased voluntarily. Its
importance in helping to control body weight
and body fat content — for individuals or for populations — is still a matter of debate, despite the
fact that there is a fairly consistent negative relationship between level of activity and body mass
index or skinfold thicknesses.
The energy stored in 1 kg of adipose tissue
is approximately 32.4 MJ (7740 kcal). Energy
expenditure during weight-bearing activities
depends on body mass; for example, walking
or running 1.6 km expends (net) about 220 kJ
(52 kcal) for a 50-kg person, but about 350 kJ
(84 kcal) for an 80-kg person, i.e. about 4.2 kJ · kg–1
body weight · km–1 (1 kcal · kg–1 body weight ·
km–1). Theoretically, therefore (and disregarding
the small postexercise elevation of metabolic rate
which, in non-athletes, probably never exceeds
10% of exercise expenditure), walking an extra
mile every day for a year would expend (net) an
estimated total of 80–128 MJ (19 100–30 580 kJ),
i.e. the energy equivalent of 2.5–4 kg of adipose
tissue. Resting metabolic rate decreases, however, as body mass falls and energy intake will
be stimulated, offsetting this deficit. As planned
exercise increases, there may also be a spontaneous decrease in the physical activities of everyday living. The situation is far from simple.
What tends to happen in practice? The consensus in the literature is that relatively small
increases in physical activity (for example,
walking 3.2 km · day–1, three times per week,
adding up to 2.1–2.5 MJ or 500–600 kcal gross) are
50
nutrition and exercise
not associated with changes in body fatness over
3–6 months (Haskell 1991). Above this amount of
exercise, there tends to be a consistent loss of
body fat, 0.12 kg · week–1 for men (a little less for
women), total exercise energy expenditure being
the variable most strongly related to the body
mass change. Thus, the natural adjustments to
increased exercise levels reduce, but do not eliminate, the theoretical energy deficit. For example,
in a study where sedentary men followed a programme of jogging for 2 years with no instructions about dietary intake, energy intake rose
over the first 6 months by about 1.3 MJ · day–1
(310 kcal · day–1). This compensation, however,
did not increase further, remaining less than the
energy expenditure of exercise so that a gradual
loss of body fat occurred.
Physical activity is increasingly viewed as an
important adjunct to restriction of dietary
energy. For example, the addition of exercise to a
low energy diet has been reported to enhance
weight and fat loss and prevent a fall in resting
metabolic rate and it may also help with the
intractable problem of weight maintenance after
weight loss. The most important role for activity
is probably that which is least well explored, i.e.
prevention of weight gain. Some information
on the relationship of activity with longer-term
weight change in the general population is available from the NHANES-I Epidemiologic Follow-
up Study in the USA; this found that the risk of
major weight gain (> 13 kg) over a 10-year period
was twice as high among inactive men and
sevenfold higher among inactive women, compared with men and women of high activity level
(Williamson et al. 1993).
Exercise may influence the distribution of
body fat as well as the amount. In population
studies, individuals practising vigorous activities on a regular basis have lower waist-to-hip
ratios than others, even after the effect of subcutaneous fat is adjusted for. Training has sometimes been reported to decrease this ratio even in
the absence of a reduction in body weight. One
reason may be that the metabolic state of the visceral fat depot is such that it should be readily
mobilized during weight loss.
For individuals who are overweight, the
health gains from increased physical activity
should not be judged solely by the extent of
change in body fatness; several prospective
studies have shown that overweight men and
women who are physically active have lower
rates of morbidity and mortality than comparable sedentary people.
Fat balance
The energy balance equation (change in energy
stores = energy intake – energy expenditure) has
Fig. 3.3 Sport offers an
opportunity to people who wish
to take exercise for health reasons
rather than as a competitive
outlet. Photo courtesy of Ron
Maughan.
exercise, nutrition and health
traditionally provided the theoretical framework
for understanding of the nature of energy
balance in humans. More recently, alternative
approaches have been proposed which take
account of how different fuels are partitioned
among metabolic pathways. The body responds
differently to overfeeding with different nutrients, suggesting that balance equations for separate nutrients might be more informative.
Protein balance is achieved on a day-to-day
basis, with oxidation of intake in excess of needs;
and carbohydrate intake stimulates both glycogen storage and glucose oxidation, with negligible conversion to TAG under dietary conditions
of industrialized countries. In marked contrast,
fat intake has little influence on fat oxidation so
that energy balance is virtually equivalent to
fat balance and there is a strong relationship
between fat balance and energy balance even
over a period as short as 24 h. Chronic imbalance
between fat intake and fat oxidation may therefore predispose to increased fat storage.
This line of thinking leads to the conclusion
that physical activity has greater potential to
influence body energy stores than would be
deduced on the basis of the tradtional energy
balance equation. Fat oxidation is of course
enhanced during submaximal exercise, and more
so in people who are well trained. It is also
enhanced for some hours afterwards, even when
the postexercise elevation of metabolic rate has
disappeared (Calles-Escandon et al. 1996). The
response to a fatty meal is changed, with greater
postprandial fat oxidation (Tsetsonis et al. 1997).
There might be synergistic benefits of increased
exercise if, as discussed above, there is an
increased appetite for high carbohydrate foods.
Conclusion
Substantial elevations in mortality are seen in
sedentary and unfit men and women. With
regard to CHD, a biological gradient has been
documented convincingly, although its exact
pattern remains unclear; high levels of rather
vigorous endurance exercise may be necessary
for optimal benefit but some studies show that
51
risk decreases steeply at the lower end of the
physical activity (or fitness) continuum, reaching
an asymptote in the mid-range. Detailed information about the influence of either the amount
of exercise or the independent effect of intensity
is not available for the relation of physical activity with the development of either hypertension
or NIDDM.
Information on the mechanisms by which
activity decreases the risk of these diseases is
incomplete, but adaptive changes in the metabolism of fat and carbohydrate, giving rise to ‘metabolic fitness’, are undoubtedly involved. Many of
the health gains associated with high levels of
physical activity can be explained through the
consequences of increased exercise for the intake
and metabolism of these macronutrients.
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