|Energy and Protein requirements, Proceedings of an IDECG workshop, November 1994, London, UK, Supplement of the European Journal of Clinical Nutrition (International Dietary Energy Consultative Group - IDECG, 1994, 198 pages)|
|Energy requirements: general principles|
Whether or not body composition plays, not a significant but an important role in estimating energy requirements, is a disputed question. The question may perhaps be simplified to enquiring whether using body mass, or fat-free mass, as the reference makes an important difference to the estimate.
We can consider the problem at two levels of fatness.
1. Moderate levels of fatness
This includes adults aged from 20 y up to 50-60 y, with a fat mass of, for men, up to 30% of the total body mass, with the equivalent for women of up to about 35% (or a BMI of 30 or so).
If the proportionate fat mass is only moderate there is little reason to expect that it will specifically influence energy metabolism, either at rest or while physically active, in ways which are unrelated to the actual body mass. The biochemical sources of energy to the body will not differ over this sort of range of fatness nor will those levels of fatness, within the range of average and normal activity, markedly influence the mechanical efficiency and thus the energy cost of movement.
The theoretical justification for the above statements is that adipose tissue has a metabolic rate which is not grossly dissimilar from the energy metabolism of the total fat-free mass. Therefore, within the above limits of fatness, the metabolic rate of a moderately overweight person of 60 kg body mass, would not be expected to be very different from a moderately lean, or from a 'normal' individual of the same body mass. The variations in relative fatness will not influence, in an important way, the energy metabolism per kg body mass. An adequate analysis of the likely influence of varying levels of fatness on energy metabolism has still not been carried out exhaustively and this is an important area for future research.
In the case of very lean populations, there may be confusing influences. If the leanness is of semi-permanent or long-term duration, but is compatible with a level of nutritional status which does not inhibit a 'normal' lifestyle, particularly with regard to physical activity, then BMR and energy requirements are unlikely to be influenced in any important way. On the other hand, if leanness has resulted from a comparatively recent negative energy balance, BMR may be significantly lowered, and it is at least possible that such a reduction would be one of the results of seasonal deficiencies in food availability (Durnin et al, 1990; Ferro-Luzzi, 1990; Schultink et al, 1990).
With greater levels of fatness, the situation is likely to become more complicated. There will certainly be an effect on the mechanical efficiency of movement but, although the biochemical energy transformations at rest may involve some basic differences from the normal, the possibility is that sources of variation both within and between individuals may mask any clear-cut effect. The considerable extent of the 95% confidence limits on inter-individual variability of BMR should always be kept in mind in relation to studies purporting to show the possible influences of body composition.
It is obviously necessary to take account of body mass, and as long as we are aware of the naively of expressing our data on BMR as energy per kg of body mass, the simplicity of the conversion and the fact that it will be unlikely to differ from energy per kg of fat-free mass (Garby et al, 1988; Henry et al, 1989) makes it a convenient and reasonable reference. It may appear that there are biologically sound reasons for making an allowance for differing body compositions and, at the simplest level, expressing data as energy per kg fat-free mass, but the probability is that this manoeuvre has, in fact, little biological justification and makes no important difference to any conclusions which may be drawn from the data. (No attempt has been made here to take into account more complex differences in body composition, i.e. related to organs or tissues, largely because of inadequate experimental information). A recent paper by Carpenter et al (1995), dealing with a meta-analysis of 13 studies using doubly-labelled water to measure total energy expenditure (TEE), seemed to show little relationship between TEE and adiposity, and a lower resting metabolic rate in women than in men, although the latter statement is made on the basis of only a very small sample of the published data. They also demonstrated the statistical dangers of using a simple ratio of TEE and body mass to compare data, but it is not clear from their paper how great an error is produced if the simple ratio is used.
In summary, in conditions of physical rest it is unlikely that body composition plays an important role in energy requirements. However, its role with regard to physical activity is more complex. Even if its influence on tissue energy metabolism is not of great practical importance, its influence on the amount of physical activity and on the energy cost of that activity might be appreciable.
3. Body mass of populations
Since obviously energy requirement, or at least BMR, is directly influenced by body mass, and populations of greater body mass will therefore have higher energy requirements, a careful appraisal of the actual or desirable body mass of all the relevant populations (infants, children, adults) becomes very important. This problem has not been considered as part of this analysis of 'general principles'.