FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS	ESN:FAO/WHO/UNU/ EPR/81/2 August 1981
	WORLD HEALTH ORGANIZATION
	THE UNITED NATIONS UNIVERSITY

Item 2.1.1. of the Provisional Agenda

Joint FAO/WHO/UNU Expert Consultation on Energy and Protein Requirements

Rome, 5 to 17 October 1981

REFLECTIONS ON THE CONCEPT OF ADAPTATION TO ENERGY INTAKE

by

L. Garby

University of Odense Denmark

Widdowson (1947) suggested that the observed huge individual differences in calorie intakes may be explained by individual variation in machine efficiency. This variation may be a result of the energy intake itself, i.e. an adaptation to energy intake. The subject has received some attention (see Garrow, 1974 and 1978), but the available data are not sufficient to allow conclusions with respect to recommendations of energy intakes.

There is a need to discuss the design, analysis and interpretation of experiments to evaluate the possible importance of the mechanism suggested by Widdowson.

Adaptive regulation.

The normal adult organism can be considered to be in a steady state in which all of its variables (concentrations, pressures, electrical potentials, fluxes, etc.) have given, constant magnitudes. A sustained perturbation of the external milieu will, in general, induce changes in the magnitude of some of the variables while others are kept (almost) constant. It is often possible to “understand” such a behaviour in terms of simple control systems in which a given variable is said to be regulated (for a purpose), and this is the approach taken here.

Consider a simple water bath whose temperature is regulated by feed-back control of its heating element. Since we have built the apparatus, we know that the water temperature is the regulated variable. The system has two controller elements: (i) the effector element (the heating device) and (ii) the dissipative element (the heat conducting walls). We assume first that the intrinsic properties, i.e. the input-output characteristics, of the controller elements are independant of the external temperature and of the water temperature (as is true of ordinary laboratory water baths). This simple control system is said to have non-adaptive regulation. We next assume that the apparatus is built in such a way that its heat conducting walls receive input from the external temperature (and/or the water temperature) such that their heat conductance now depends on and changes with the external temperature (in such direction as to assist the system in keeping the water temperature constant). The control system is now said to have adaptive regulation in that one of the controller elements (the dissipative element) now has an input-output characteristic which is dependant upon the perturbation (or its effect on the regulated variable).

In living organisms, we can sometimes agree to appoint a certain variable as the regulated variable and we can then often identify the feed-back loops and the controller elements. In such cases, changes in the input-output characteristics of the controller elements are adaptive changes and we can speak of adaptation to the perturbation.

Food energy transformations.

Food energy intake represents an amount of work performed on the body by the surroundings. This work is put to use in the body through coupled processes in which energy is transformed to appear in different forms (chemical, electrical, osmotic, elastic, etc.). While all the work (with the exception of the mechanical external work) is eventually dissipated as heat, the end result is the maintenance of generalized potential differences within the body and between the body and its surroundings. Such potential differences may e.g. be the electrochemical potential difference of Na ions in cells and extracellular fluid, the chemical potential difference of glycogen molecules in liver cells and plasma and the chemical potential difference of carbon dioxide in blood and the atmosphere. I suggest that it is a good idea to consider these potentials as regulated variables. It follows that the processes directly responsible for the creation of the potentials (the effector elements) and the processes which relax the potentials (the dissipative elements) are the controller elements) in the regulation. Regulation of the potentials through feed-back on the dissipative elements is therefore adaptive and the concept of regulation through efficiency is readily appreciated.

The dissipative elements of the energy transactions in the body are essentially short-circuiting devices, controlled to prevent the potentials from becoming abnormal. Although they are of several different kinds (and many remain to be identified), they may, somewhat arbitrarily and for convenience only, be divided into two groups: biochemical and mechanical. The first group includes sodium and calcium leak permeabilities, glucose and fatty-acid-substrate cycles and proton short-circuit in mitochondria in brown adipose tissue. The second group includes shivering but also such “uneconomical” muscle movements that are of a “behavioural” nature.

Very little is known with respect to the absolute and relative importance of these elements in the regulation of the potentials and it is also clear that we are largely ignorant with respect to which of the many possible potentials that have priority in different situations.

Attempts towards an operational definition of adaptation to energy intake in humans.

The work performed by the surroundings on the body, i.e. the energy intake, can be measured relatively accurately. On the other hand, the work performed within the body, i.e. the useful work responsible for the maintenance of the generalized potentials, cannot even be roughly estimated. Therefore, the efficiency of the energy transformations cannot be measured. However, many of the important manifestations of the energy transactions can be observed, e.g. the process of walking at a given speed and angle, and, for a given, well-defined, manifestation (or task), differences in efficiency (between and within subjects) will be expressed as differences in energy expenditure. Now, the energy expenditure for a given task will, in general, depend on a large number of factors, say, age, sex, body weight, body composition, environmental factors, etc. (=confounding factors), and, in order to separate out the effect of energy intake, i.e. the adaptive change in which we are interested, we need to know the effects on energy expenditure of changes in the confounding variables for all the tasks that we consider important.

A direct experimental approach for determination of the magnitude of adaptation thus suggests itself: measurements of the energy expenditure for a number of well-defined tasks, in one and the same subject before and after a perturbation in energy intake, can be made very accurately in calorimeters large enough to permit realistic tasks. The approach requires that the effect on energy expenditure of changes in confounding variables, in this case changes in body weight and body composition, can be evaluated separately.

A second possibility to evaluate the importance of adaptation to energy intake is afforded by measurements of the energy expenditure for well-defined tasks in subjects with different habitual energy intakes. This approach puts more severe demands on the knowledge of the effects of confounding variables.

A third, more theoretical, possibility is as follows. Consider two populations with different energy intakes. Assume that one can argue that the “performance” of the two populations are the same. It follows that the population with the low energy intake has adapted and also that supplementation of this population with extra food will not change its “performance”.

References

Garrow, J.S.: Energy Balance and Obesity in Man. North-Holland/American Elsevier Publishing Company, Amsterdam, London, New York 1974.

Garrow, J.S.: Energy Balance and Obesity in Man. Elsevier/ North-Holland Biomedical Press, Amsterdam, New York, Oxford 1978.