There is considerable variability in BMR among individuals who are superficially similar, and since BMR accounts for a large portion of total energy metabolism, the extent of this variation can have important implications. For instance, two people who might appear to be physically similar in body weight and in body composition (with respect to the relative proportions of fat and lean) might differ in BMR by 300–400 kcal/day, meaning that one of them could eat food with an energy content of 300–400 kcal/day more than the other with the same end result for energy balance. In our present day Western lifestyle, this would impose a penalty on the biologically more efficient individual, whose BMR is comparatively low and who will therefore be somewhat unfairly constrained to a life of partial self-denial if obesity is to be avoided. The low BMR means that the total energy requirement of such a person will also be relatively low, compared with others with a similar pattern of physical activity, and will therefore be satisfied by a smaller total food intake — which may not be to that person's particular liking. The compensation for these people is a somewhat inadequate one in a privileged society; faced with a famine situation, they could expect to survive better and longer than those with average BMR.(ii) After a meal, the processes of digestion and absorption are initiated and continue for some hours. This involves an expenditure of some energy, equivalent to roughly 10% of the total calorie content of energy of the food and drink taken. The more food you eat, the more energy is used up in processing it.(iii) Physical activity obviously affects energy expenditure. Its influence on energy balance can go in either direction. With increased physical activity the extent of the greater energy expenditure will obviously depend on the intensity and duration of the exercise. If the energy expended is sufficiently large, this leads to a state of negative energy balance and part of the energy which would normally be supplied by the food will be obtained from the breakdown of energy stores (mostly from fat). The consequence is a reduction of body weight, and increased physical activity is indeed often prescribed as a treatment for obesity — albeit not always with marked success.
On the other hand, if physical activity is reduced, the requirement for energy is also diminished, and less food needs to be eaten — which often causes dissatisfaction. Because the situation is basically disheartening, the end result is often a gradual increase in fatness and body weight.
So far, this seems straightforward, but the effect of physical activity on energy balance is a source of some confusion. For example, if we know that Joe Bloggs regularly goes jogging for half an hour three times a week, we look on him as an active person expending much more daily energy than the average non-active person. However, the actual amount of extra energy used up at this level of exercise would amount to perhaps 200–300 kcal in the half hour; 600–900 kcal over the week or an average of about 90–130 kcal/day Out of a total daily energy expenditure of about 2700 kcals this is a relatively small quantity (equivalent, say, to the energy content of a pint of beer, or a small packet of hula hoops). It could influence energy balance only on a long-term basis. Other forms of activity which may be of considerable severity, but of short duration, may have a similar and surprisingly small effect.
One of the difficulties associated with maintaining energy balance in the kind of lifestyle of most people in our industrialized society is that obligatory physical activity, necessitated by the occupation, has become progressively diminished over the past few decades. Indeed, a great deal of time and ingenuity has gone into removing or minimizing any physical effort which would previously have been an essential part of the working day. The amount of physical activity indulged in by most people is therefore dependent on a voluntary decision, which, unhappily for a majority of people, requires a considerable mental effort.
Obviously, there are still a variety of jobs which involve whole body movement — working as a waiter or waitress, or in a shop, nursing, some types of construction work (as a joiner, bricklayer), and so on. The changing pattern of work over the years is interestingly illustrated by comparing present-day values with the rates of energy expenditure for various occupations given in a table in a book entitled ‘Energy, work and leisure’, published in 1967. People with jobs which at that time were regarded as requiring appreciable expenditure of energy — such as colliery clerks, factory workers, steel workers, farmers, coal miners — have no present-day equivalents because of the altered patterns of work: housework, which used to involve quite strenuous tasks, is reduced to minimal levels because of the development of household appliances.
All of these tendencies contribute to a situation where total energy expenditure is reduced and where therefore it becomes increasingly difficult to limit our intake of food to avoid the development of a positive energy balance, giving rise to overweight and, eventually, obesity.
Energy balance and age
ChildrenAt varying stages of the life cycle, physiological changes occur which may affect energy balance. In infancy and up to early adulthood, growth is a factor which would commonly be believed to have some influence. Mothers particularly would account for the ‘healthy appetite’ of their offspring as resulting from growth: ‘He's a growing boy’, they might say. Yet the actual amount of energy needed for normal growth is comparatively small. A 2-year-old child would normally expend daily a total of about 1200 kcal, of which only 30 kcal or so would satisfy the growth requirement. At the age of five, the total expenditure might be 1500 kcal/day with only 35 kcal for growth. Even at adolescence, when growth is occurring at its maximum rate, this is satisfied by no more than 50 kcal/day. The energy requirement for growth per se is therefore of little consequence for energy balance.
Of much greater import is the amount of energy that is required by the normal activity of children now, compared with previous years. There is considerable indirect evidence which seems to show, particularly in adolescents, a progressive reduction in energy expenditure: 13–15-year-olds expend several hundred kcal/day less than 13–15-year-olds did 50 years ago. Perhaps this diminution in total daily activity is not surprising when one considers the alteration in lifestyle during this period, with the considerable amount of time spent nowadays watching TV and using public and private transport instead of walking.
The elderlyAt the other end of the age range, from about 50 onwards, additional factors may affect energy balance. One of these is an apparent progressive reduction in BMR resulting from a diminishing quantity of active body tissue. However, there is some doubt about whether this lowering of overall energy metabolism is a normal ageing process, because of decreased metabolism of most of the body tissues, or whether it is dependent on an alteration in body composition due to relative physical inactivity. Where only comparatively small changes have taken place, as in the case of elderly individuals who have remained physically active throughout their whole life span and have retained good muscular and cardiovascular development, BMR does not change appreciably until advanced age — perhaps up to 70. A physically active existence is of great importance for the elderly. However, whether or not there is much diminution in the total energy expended by elderly people is something which can obviously be affected by health and mobility. Gross arthritic changes will inhibit activity to an extent where total energy expenditure becomes very low and even a moderate diet may produce a positive energy balance and the possible development of obesity. It may seem incongruous that obesity is a potential hazard in a situation where food intake is low and appetite is poor. It is nevertheless a real hazard and there may also be an inadequate intake of several nutrients, such as protein and some of the minerals and vitamins. Increasing physical activity has a two-fold benefit: the greater expenditure of energy necessitates a larger intake of food, making nutritional deficiencies less likely, and the improved muscle tone enhances the feeling of well-being and the capabilities of the elderly person.
The energy involved in gaining and losing weightIf there is a positive energy balance, with intake greater than expenditure, there will be a gain in body weight. Conversely, when energy expenditure is in excess of energy intake, body weight will become less. It is interesting to examine the actual amounts of energy represented by these weight changes. The weight which is added to or lost from the body does not consist only of fat itself but is mostly adipose tissue which is a complex mixture of fat (lipid), connective tissue, and fluid. One kilogram of lipid has an energy equivalent of about 9000 kcal. The energy content of the connective tissue and fluid is comparatively low, but these form 10–30% of the total mass of adipose tissue, which therefore has a lower energy equivalent than pure lipid: about 7000 kcal/kg. When the body is losing weight, each kilogram of adipose tissue which is being consumed has therefore provided 7000 kcal of energy. In the opposite circumstance, when the body is gaining weight, each kilogram of added adipose tissue increases the body energy stores by about 7000 kcal. However, the positive energy balance needed to effect this change is more than 7000 kcal/kg, because depositing adipose tissue is a chemical process which is metabolically rather inefficient, and adding an extra kilogram of adipose tissue to the body requires something like 10 000 extra kcal.
There are some interesting extrapolations which can be made from these data. Some slimming regimes advertise a loss of 10 lb (a little more than 4 kg) during a week of their therapy. Four kilograms of lost weight represents the equivalent of 28 000 kcal. If this amount of energy is lost during 7 days, this means a negative balance of 4000 kcal/day. Of course, any food which is eaten during this period will negate part of the energy deficit so that the negative balance of 4000 kcal/day will only be applicable if there is absolute starvation. An intake of, say, 1500 kcal/day will require an energy deficit of 5500 kcal/day. To increase the normal energy expenditure by this amount is virtually impossible for almost everyone (equivalent to continuous hill-climbing, say, at about 550 kcal/hour for 10 hours/day) and, in any case, could be continued for only extremely brief periods. A weight loss of 10 lb per week is therefore a gross distortion of what would actually happen. A more realistic possibility capable of being continued for many weeks would be to reduce energy intake by about 1000 kcal/day, thus producing an energy deficit of 7000 kcal per week, which is the energy equivalent of 1 kg of adipose tissue.
Extended effects of exercise on energy metabolismIt has been suggested that the increase in energy expenditure caused by physical activity continues for some time after the activity has stopped. This has not been convincingly demonstrated except in the case of severe and prolonged exercise which may have a limited effect over some hours.
Energy balance and pregnancyTheoretically, pregnancy would normally be expected to increase energy requirements. There is no doubt the development of the fetus, the placenta, the increase in breast and uterine tissue as well as in energy reserves, all need extra supplies of energy, and indeed it is possible to calculate with a fair degree of precision what the actual amounts should be. For a healthy mother leading the sort of lifestyle common in an industrialized country, the amount would be about 80 000 kcal. One would therefore expect an increase in energy intake of about this amount in order to maintain energy balance. Far from it! In many instances there is no increase whatever over the pre-pregnant level. There are no indications of any improved metabolic efficiencies: BMR remains at pre-pregnant levels and muscular efficiency stays the same. Although it is difficult to measure, it appears most likely that physical activity is subtly reduced, with an energy saving equivalent to the extra energy needs.
J. V. G. A. Durnin
Cottrell, R. (ed.) (1995). Weight control: the current perspective. Chapman and Hall, London.
Garrow, J. S. (1978). Energy balance and obesity in man, (2nd edn). Elsevier, Holland.
See also dieting; exercise; metabolism; obesity.
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