FACTORS AFFECTING ENERGY EXPENDITURE

by
D. R. Passmore

(c) Physical Activity

The 1950 Committee on Caloric requirements introduced the principle of calculating national requirements from the basis of the needs of a reference man and a reference woman. This principle was also used by the 1957 Committee. The method works and no alternative approach has been suggested and the 1971 Committee will probably proceed along the same lines.

Their problem is to decide at what level to set the reference figures. The previous FAO Committee put them at 3,200 and 2,200 kcal/day for the reference man and woman respectively. These certainly overestimate requirements for most persons in prosperous industrialized communities. The 1968 U.S.A. recommendations are for 2,800 and 2,000 kcal/day and the 1969 U.K. figures are 3,000 and 2,200 kcal/day. These figures are without doubt closer to results obtained in contemporary surveys of both food intake and energy expenditure than the FAO figures. There is also good evidence that many poor but apparently healthy peasant communities in the tropics utilize less energy than the FAO figures.

The 1957 FAO Committee introduced the conception of considering energy requirements under three heads (1) rest in bed, (2) working activities and (3) non-occupational activities. With increasing mechanization and larger supplies of power, energy needs for working activities. have been substantially reduced. There is no evidence that any community compensates for a decline in physical activity at work by increasing non-occupational or recreational activity.

There is an increasing body of medical opinion that supports the views that a sedentary way of life contributes to obesity, ischaemic heart disease and other degenerative diseases which are becoming more common and arising earlier in life in prosperous communities. A certain amount of the physical activity is essential for health; it would be impossible to state how much, but probably more than is taken in most industrial societies today. Recommended dietary intakes of energy are not only recommendations about food; they are also recommendations about a way of life. Thus the 1968 U.S.A. Food and Nutrition when recommending an intake of 2,800 kcal/day for a man of 25,400 kcal/day less than the 1957 FAO recommendation brought the figure into line with the contemporary way of life in their country, but by making the dietary recommendation, they implicitly recommended also the way of life. If degenerative diseases of middle age are to be prevented, more physical activity may be needed and more food required.

The 1971 Committee will have to decide whether to make recommendations, which would meet the needs either of contemporary society or of another type of society which might be more healthy.

(a) Body Size

Both the 1950 and 1957 Committee discussed this at length. The 1957 Report gives three equations relating size to requirements. The scientific basis for each of these equations was and remains dubious. Their use implies an accuracy that does not exist. Biological generalizations or laws have been able to relate food intake to body size in different species of mammals over a wide range from the mouse to the elephant. There is doubt as to whether the same laws apply to the narrow range in a single species. Certainly all the observations on individuals of homo sapiens indicate that increasing body size increases food intake proportionately less than the general increase when going from a small to a large species.

The use of the equations given in the 1957 report also necessitates the use of precise figures for the average weights of healthy men and women in a population. The accuracy of the figures used is often in doubt.

It is suggested that populations should be divided into four groups as follows:

Separate recommendations for each group should be made on an ad hoc basis. This would be more honest than making calculations from equations of dubious validity. There would be no loss of accuracy and a considerable simplification of the calculations for assessing requirements of individual countries.

(b) Age

The 1957 Report made subdivisions for age which have been followed by all subsequent FAO/WHO Expert Groups for nutrients. This subdivision is also used in all the work of the Nutrition Division of FAO. The U.S.A. and U.K. reports on recommendations have each modified this subdivision slightly, but differently, in the intention of relating recommendations more closely to physiological changes.

WHO in all its other work uses the age subdivision of the UN Demographic Year Book, which divides populations into 5 year age groups. The use of this subdivision by nutritionists would greatly facilitate the statistical work of the FAO Nutrition Division and also make easy the correlation of nutritional data with international demographic, economic and epidemiological data. In my opinion the advantages gained in this way far outweigh any loss of precision from the divisions being less accurately adjusted to physiological changes.

It is suggested that recommendations are made covering 8 age groups, thereby improving on Shakespeare by one. The exact range of some groups and their names obviously needs discussion. The suggested groups are:

The Infant. By international agreement this is defined as the age group from birth up to 1 year. Further discussion of this group is needed but is not discussed here.

The pre-school child, from 1 up to 5 years. This term is now widely used. The official starting age for school is 5 in most countries and going to school is likely to lead to a change of dietary habits.

The young school child, from 5 up to 10 years.

The period of puberty (?) from 10 up to 15 years. There has to be a special consideration of the nutritional needs of puberty. This convenient UN age group approximates closely to the period of puberty and it is suggested that a little loss of physiological accuracy is more than compensated for by the gain in statistical convenience.

Young adults (?) from 15 to 20 years.

The prime of life from 20 up to 35 (?) years. The subdivision of adults by decades, as formerly used by FAO, is too detailed. There comes a time when people begin to slow up and middle age may be said to have set in. When this occurs will be disputed, but it is exceptional to play football after the age of 35. This is suggested as a suitable point for middle age to be said to start.

Middle age from 35 (?) up to 60 (?) years. During this period of life most men and women are still working, but significantly less active than formerly. The age of retirement or giving up active work, of course, varies greatly, but 60 seems a sensible figure.

The elderly from 60 (?) up to 75 (?) years. In this group most people have retired, but are still capable of considerable recreational, domestic and other activities.

The aged over 75. At 75 most peoples activities become increasingly curtailed and life becomes more and more sedentary.

Adjustments of the previous recommendations to these divisions should not be difficult. For adults the decision as to the requirement of the reference man and woman is all important.

The 1957 Committee recommended for age group 16-19, 3,600 and 2,400 kcal for males and females. These are almost certainly too high, especially the figure for males. There have now been numerous surveys of the dietary intake and energy expenditure of young adults. These show that it is not uncommon for individual young men to require over 4,000 kcal, but there is no survey in which the average is as high as 3,600. I think that the 1957 figures should be lowered. As a guide to how much the thorough study in Melbourne, Australia, by McNaughton and Cahn (Brit. J. Nutr. 1970, 24, 331) should be useful.

Individual children vary greatly in their needs, which are very difficult to assess. The 1957 FAO recommendations have been in general accepted with only slight modifications in the 1968 U.S.A. and 1969 U.K. recommendations. The figures would need some adjustments if the suggested age classifications were accepted. In many countries paediatricians are becoming increasingly concerned with the problem of juvenile obesity. This may suggest that recommendations are on the high side. Im my opinion there is no reason to change substantially the old recommendations, but in making adjustments to new age subdivisions care should be taken not to raise the figures and small reductions should be acceptable.

(d) Pregnancy and lactation

The analysis of the 1957 Committee on the requirements for pregnancy and their recommendation are in my opinion, still sound.

For lactation the 1957 Committee recommended an additional 1,000 kcal/day. This was re-iterated in the 1968 U.S.A. recommendations but reduced to 500 kcal/day in the 1969 U.K. report. The U.K. Panel had a preview of a paper by Thomson, Hytten and Billewicz, now published (Brit., J. Nutr 24, 565). This reported diets survey on women from 6 to 10 weeks after delivery, of whom 23 were feeding their baby wholely by the breast and 32 by bottle. Both groups of women were losing weight, as is normal in the puerperium. Estimates of their energy balances in kcal/day is as follows:

No estimates were made of energy expenditure. However all the women who lived in Aberdeen were selected because of a good educational background and a good home. There is no reason to suspect that the physical activities of the two groups differed.

The conclusion is that the efficiency of human milk production is not 60 per cent, the guess of the 1957 FAO Committee, but approximates to 100 per cent. The estimate of efficiency from the Aberdeen data is 97 per cent and the 99 per cent confidence limits of the estimate are from 64 to 129 per cent.

There is thus reason to think that the 1957 Committee's estimate of efficiency is too low and it is certain that they were in error in neglecting completely the contribution of the the energy made available by the post-partum reduction of fat stores in the adipose tissue.

Hence the present recommendation for lactation requires lowering but whether to the level of the 1969 U.K. figure must be a matter for discussion.

(e) Climate

The section on Climate in the 1957 FAO Report is scientifically unsatisfactory. "The mean annual external temperature" remains an "oversimplification" and there appears no sign of any better alternative index. Fortunately the matter appears of little practical importance. The recommendation of a reduction of 5 per cent in requirements for every 10°C about the reference level has made sense and there would seem little reason to try to change it. The converse of an increase in requirements for temperatures below the reference is little use and has next to no scientific support. No one could notice, if it disappeared quietly.