Memorandum by Adrianne Hardman (OB 73)
MY BACKGROUND
I have 30 years of experience with research
in exercise metabolism, energy expenditure and adaptations to
training. I was a member of the Scientific Advisory Board for
the 1992 Allied Dunbar National Fitness Survey. I have published
extensively and was the only UK participant in the 2000 Consensus
Symposium convened by Health Canada and the US Centers for Disease
Control and Prevention Dose-response issues concerning physical
activity and health. My book Physical activity and health:
the evidence explained is to be published in September.
THE ROLE
OF PHYSICAL
ACTIVITY IN
ENERGY BALANCE
The Committee will have heard evidence for the
rapid increase in the prevalence of obesity in the UK. They may
also be familiar with the evidence that proxy measures of physical
inactivity are more closely related than indices of dietary change
to changes in the prevalence of obesity (Prentice and Jebb 1995).
This implicates inactivity as the dominant factor in the aetiology
of obesity. Prospective studies lend support to this proposition,
showing that people who report low levels of physical activity
are more prone to weight gain than those who report high levels.
For example, in one large study, men and women with low levels
of leisure time activity were more than two and a half times as
likely as highly active people to experience a clinically important
weight gain (>5 kg) over 10 year follow-up (Haapanen et
al 1997).
Factors that have probably contributed to the
decrease in levels of physical activity were summarized in the
report of National Audit Office (2001) and will be familiar to
the Committee. The effects of these changes are not trivial; total
daily energy expenditure has declined by around 20% ( 800 Calories)
during the second half of the 20th Century—the equivalent
of walking 10 miles.
THE FOLLOWING
NOTES:
— explain the relationships between
physical activity[1]/exercise[2],
energy expenditure and body weight; and
— provide illustrative examples of
the potential of physical activity to improve weight regulation
and prevent weight gain.
Basic principles
An individual's weight remains stable only if
they remain in energy balance over a long period. That is:
energy intake from the diet = total energy expenditure.
Just a small imbalance between energy intake
and expenditure makes a large difference to body fat stores in
the long term. For instance, a positive energy imbalance of just
1% will increase body weight by around 20 kg (more than 40 lb)
during adulthood.
Three components make up total energy expenditure,
ie the resting metabolic rate, the energy associated with digesting,
absorbing and storing food and the energy expended during physical
activity. Thus:
Total energy expenditure = resting metabolic
rate + thermic effect of food + energy expended in activity.
The resting metabolic rate accounts for 60-70%
of total energy expenditure and the energy costs of feeding are
about 10%. The energy expended in physical activity is the most
variable component, contributing 10-15% in sedentary people but
30-40% in normally active people. (It can be much larger in some
athletes or in individuals engaged in endurance challenges.) Importantly,
it is the only component of total energy expenditure that one
can increase voluntarily, through behaviour. Even walking at an
ordinary pace increases energy expenditure three-fold.
Factors that influence the energy expenditure
of physical activity
(i) For weight-bearing activities, energy
expenditure is proportional to body weight.
For a 70 kg (11 stone) individual,
walking at 3 mph (a "normal" speed) expends about 4
Calories/min gross or 2.8 Calories net (more than at rest). For
a 90 kg individual, this rises to 5 Calories/min gross, or 3.5
Calories/min net.
(ii) Energy expenditure increases with the
intensity of activity.
A 70 kg individual expends 3 Calories/min
walking at 2 mph, 4 Calories/min at 3 mph and 8.6 Calories/min
at 5 mph.
(iii) Energy expenditure increases in proportion
to the duration of activity.
(iv) Exercise induces a small increment in
energy expenditure in addition to that incurred during the exercise
because the metabolic rate remains elevated for some time afterwards.
For activities such as walking, this is about 15% of the net energy
expenditure during exercise.
THUS: For the typical range of walking speeds,
the net increment in energy expenditure, ie above the resting
level, associated with walking one mile is 60 Calories for a 70
kg individual, 50 Calories for a 57 kg individual and 75 Calories
for a 90 kg person.
What determines an individual's perception of
how "hard" exercise is—and thus their capability/willingness
to engage in it?
This depends on the individual's capacity for
exercise, usually measured as his/her maximal oxygen uptake. The
level of "physiological stress" elicited by a specific
activity depends on what proportion of the individual's total
capacity this demands. Activity demanding less than 45% of capacity
is regarded as light, 45-60% as moderate and levels above that
as vigorous or hard. The maximal oxygen uptake varies a lot between
individuals, is higher in men than in women. It declines on average
by 10% per decade after the age of 30.
Thus, walking at 3 mph demands an oxygen uptake
that constitutes less than 30% of capacity for an average young
man and is therefore light exercise. For a 75-year-old, with a
lower capacity, walking at this speed may represent 60% or more
of total capacity and thus be moderate to vigorous exercise.
Is physical activity an effective way to lose
weight?
A lot of activity is needed to expend the energy
in body fat stores. In the short-term increasing activity looks
an unpromising means of weight loss. For example:
— Adipose tissue (body fat) contains
about 7,740 Calories per kg (3,518 per lb). Our 70 kg person therefore
has to walk 129 miles in order to expend the energy stored in
one kg of fatty tissue (81 miles to expend the energy in one pound
of fat. Since a typical confectionery bar contains 300 Calories,
he/she has to walk five miles to expend this amount of energy.
BUT—Obesity takes years to develop. The
average 35 year-old man gains 0.2-0.8 kg every year (0.4-1.8 lb
per year). Gains in women tend to be even greater. Even small
amounts of activity, taken regularly, have considerable influence
on energy balance long-term.
— By increasing energy expenditure
by walking just one mile extra every day, our 70 kg individual
expends an additional 21,900 Calories in one year—the equivalent
of 2.8 kg (6.2 lb) of fatty tissue[3].
What tends to happen in practice?
— Overall, experiments on weight loss
through exercise have found only modest reductions in body mass—of
the order of 0.09 kg per week (0.2 lb per week). This is equivalent,
of sustained, to a loss of more than 4.5 kg or 10 lb in a year.
Such studies typically last months rather than years, however,
and the rate of weight loss may be expected to decrease over time.
Physical activity in people who are obese
— Weight-bearing activity is fatiguing
for obese persons. For many, even walking constitutes vigorous
exercise, because it demands a high proportion of their poor functional
capacity. They also experience problems with joints. Thus, although
obese individuals expend more Calories per mile than normal weight
people, they find it difficult to expend a lot of energy through
physical activity.
— Activities like swimming or cycling,
where body weight is supported, may be better tolerated.
What type of activities are effective?
Aerobic or endurance activities usually lead
to the greatest energy expenditure. This does not have to be vigorous
or sporting, however (Table). For weight regulation, it is the
total energy expenditure that matters. To improve weight regulation,
activity needs to be regular and frequent. It has to become a
habit. Walking in particular can be recommended (Morris and Hardman
1997). It is readily incorporated into the lifestyle and incurs
little risk of injury—one main reason why people give up
exercise regimens based on sports.
Activity
| Rate of energy expenditure (Calories/min)
|
Duration/occasions
per week
| Total net energy expenditure (Calories)
|
Casual soccer | 8.7
| 60 min, twice a week | 897
|
Tennis | 7.7 |
60 min, twice a week | 777 |
Walking ("normal" pace, 3 mph)
| 4.5 | 30 min, twice a day, to and from work, 5 days/week
| 982 |
Cycling ("commuting" pace, 5.5 mph)
| 4.5 | 30 min, twice a day, to and from work, 5 days/week
| 982 |
| |
| |
SUMMARY
— Over months or years, increasing physical activity
is associated with modest weight loss.
— More importantly, high levels of physical activity
improve weight regulation and contribute to avoidance of weight
gain.
— Current recommendations for physical activity
are widely interpreted as 30 minutes of moderate activity per
day. This is probably not sufficient to avoid further increases
in population levels of overweight/obesity.
REFERENCES
Haapanen, N, Miilunpalo, S, Pasanen, M, Oja, P and Vuori,
I (1997). "Assocation between leisure time physical activity
and 10-year body mass change among working-aged men and women."
International Journal of Obesity 21: 288-296.
Morris, J N and Hardman, A E (1997). "Walking to health."
Sports Medicine 23: 306-332.
National Audit Office (2001) Tackling obesity in England.
The Stationery Office.
Prentice, A M and Jebb, S A (1995). "Obesity in Britain:
gluttony or sloth?" British Medical Journal 311: 437-439.
1
Physical activity-any bodily movement that is produced by the
contraction of skeletal muscle that substantially increases energy
expenditure. Back
2
Exercise (or exercise training)-a subcategory of leisure-time
physical activity in which planned, structured and repetitive
bodily movements are performed to improve or maintain one or more
components of physical fitness. Back
3
Actual loss of weight/fat will be less than this if an associated
increase in energy intake reduces the energy deficit. Back
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