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A working definition adopted by the FAO/WHO/UNU Expert Consultation (1985) on adaptation states that it is 'a process by which a new or different steady state is reached in response to a change or difference in the intake of food and nutrients'. This definition attempts to deal with both short-term and long-term adaptation; the word 'new' having relevance to short-term responses to acute changes in a subject who is in balance, while the word 'different' refers to long-term changes in individuals or groups exposed habitually to different environmental or nutritional conditions. Three general points were made by this Report in relation to both types of adaptation:
(1) The concept of a 'steady state' is relative, and the time-scale over which a state may be considered steady or stable varies for different functions.
(2) Adaptations are of different kinds: metabolic, biological/genetic, and social/behavioural.
(3) It follows from the above that adaptation must imply a range of steady states, and hence it is impossible to define a single point within the range that represents the 'norm'. Implicit in this is the understanding that different adapted states may have advantages and/or disadvantages.
The concept of a range of adapted states, each with possible advantages and disadvantages, while implying a respect and understanding for different biological and cultural situations, can also serve to condone the acceptance of double standards and the endorsement of the status quo.
An adaptive response is an inevitable consequence of a sustained perturbation in the environment and may be genetic, physiological and/or behavioural. These different types of adaptation are not mutually exclusive; they interact with each other at several levels. Every adaptation has a potential cost. Reduced physical activity in a child may reduce the interaction with the environment needed for normal development. Reduced physical activity in adults, with no apparent biological cost, may have serious economic and social consequences. At some point, adaptation will begin to have both biological and social costs. The processes and costs involved may be overt or covert, reversible or irreversible, and transient or permanent. Adaptation, both in the short-term and in the long-term, is a relatively slow process and should be distinguished from the rapid regulatory role of homeostatic mechanisms. A homeostatic response in a biological system may neither have additional costs to the organism nor lead to compromise in its function, capability, or performance, in contrast to an adaptive response which may do both in order to further the survival of the individual.
Metabolic adaptation
The suggestion that the energy metabolism of individuals is variable and adaptable, and that allowances need to be made for this when making estimates of human energy requirements, has been based on several important publications that have drawn attention to the possibility of such physiological variability in energy utilisation between individuals (Durnin et al, 1973; Edmundson, 1980) and within individuals (Sukhatme & Margen, 1982; Sukhatme & Narain, 1983). Healy (1989) criticized the validity of Sukhatme's approach based on autocorrelation. Norgan (1983) critically evaluated the following four types of evidence that have been adduced to illustrate this variation which is purported to result in adaptation in human energy metabolism:
(1) in any group of 20 or more similar individuals, energy intake can vary as much as two fold (Widdowson, 1962);
(2) large numbers of apparently healthy active adults exist on energy intakes that are lower than their estimated energy requirement (Durnin, 1979);
(3) the efficiency of work and work output is variable per unit energy intake (Edmundson, 1979); and
(4) observations based on studies of experimental or therapeutic semistarvation (Benedict et al, 1919; Keys et al, 1950; Grande, 1964; Apfelbaum, 1978) and overfeeding of humans (Sims, 1976; Norgan & Durnin, 1980).
Differences in body size, levels of physical activity and systematic errors in the estimation of energy intakes may provide explanations for most of these observations (Norgan,1983). However, what is implied or explicitly stated by the proponents of metabolic adaptation is that metabolic efficiency and mechanical work efficiency of the individual are variable and show an adjustment to variations in the levels of energy intake. The decrease in oxygen utilisation of the residual active tissue mass of an individual seen during experimental (or therapeutic) semistarvation (Keys et al, 1950) constitutes the most important biological argument for metabolic adaptation. On the basis of these observations it has been assumed that an enhanced metabolic efficiency is also a characteristic of individuals who are habitually on diets that are low in energy content.
Several physiological mechanisms, chiefly hormonal, operate to account for the changes in the metabolic activity of the tissues to enhance their metabolic efficiency, when a well nourished individual's energy intake is restricted (Shetty, 1990). The activity of the sympathetic nervous system is toned down, signalled by a decrease in energy flux, while the energy deficit lowers insulin secretion and initiates changes in peripheral thyroid metabolism. The latter are characterised by a reduction in the biologically active T3 and an increase in the inactive reverse T3. The reduction in the activities of these key thermogenic hormones acts possibly in a concerted manner to lower cellular metabolic rate. Changes in other hormones such as glucagon, growth hormone and glucocorticoids may influence these changes and at the same time, in association with the insulin deficiency, promote endogenous substrate mobilization which will lead to an increase in circulating free fatty acids and ketone bodies. The elevated free fatty acid levels, alterations in substrate recycling and protein catabolism will also influence this process. These hormonal and metabolic changes that accompany energy restriction aid the survival of the organism during restricted availability of exogenous calories. Hence these physiological changes that are associated with body weight and body composition changes have been considered as an indication of metabolic adaptation which is seen in a previously well nourished individual and which are aimed at increasing the 'metabolic efficiency' of the residual tissues in the body at a time of energy deficit.
It has been assumed that the physiological and metabolic responses of an adult on a low plane of habitual intake are similar to, and can be explained on the basis of, the sort of physiological changes that occur during experimental or therapeutic semi-starvation in previously well nourished adults. Ferro-Luzzi (1985) summarised the thinking at that time on ways in which an individual on habitually low intakes could respond to a sustained energy imbalance by metabolic adaptation. Metabolic adaptation was represented as a series of complex integrations of several different processes that occur during energy deficiency. These processes were expected to occur in phases which could be distinguished, and it was suggested that a new equilibrium could be established at a lower plane of energy intake. At this stage, individuals who had gone through the adaptive processes that occur during long-term energy deficiency, were expected to exhibit more or less permanent sequelae or costs of adaptation, which included a smaller stature and body mass, an altered body composition, a lower BMR, a diminished level of physical activity and possibly a modified or enhanced metabolic efficiency of energy handling by the residual tissues of the body. However, a large number of measurements made during the last decade, in subjects in environments that predispose to low energy intakes, do not confirm the existence of an enhanced metabolic efficiency (Srikantia, 1985; McNeill et al, 1987; Soares & Shetty, 1991). It would therefore appear that an increase in metabolic efficiency in the BMR component of energy expenditure, which has been hitherto considered to be the cornerstone of the beneficial, metabolic adaptation to long-term energy inadequacy, is of doubtful existence. It is more likely that a lower BMR per kg body mass in the chronically undernurished is an arte-fact attributable to the changes in body composition, more specifically the disproportionate reduction in muscle tissue with a normal or even increased non muscle or visceral organ size (Shetty, 19933, possibly enhanced by an increase in number of infective episodes in individuals living in such environments. Hence it is highly unlikely that metabolic: adaptation is of any relevance in chronic energy deficiency, as opposed to a situation where normal individuals are energy restricted.
Behavioural adaptation
The behavioural adaptations in physical activity patterns that accompany low energy intake are related to the individual's allocation of time and energy to different productive and leisure activities and to the biological as well as the economic consequences of these altered behavioural patterns. When there is both a fall in energy intakes and an increased demand for energy expenditure at work, for instance during seasonal agricultural activities, individuals adjust the time they allocate to different tasks; more time is given to work activities and less time and energy to productive tasks at home or socially desirable or pleasurable activities (Immink, 1987).
Lower energy intakes and stunting in preschool children were associated with lower levels of physical activity (Rutishauser & Whitehead, 1972). An analysis of physical activity patterns during voluntary reduction in food intake showed that the behavioural response to a deficient intake and associated weight loss was a change in the pattern of activity: lower effort discretionary activities replaced those which needed greater effort, while obligatory activities were not affected (Gorsky & Calloway, 1983). Rural Guatemalan men were able to carry out the specific agricultural task allocated to them, but took a longer time doing it (Torun et al, 1989); they also took a longer time to walk home and spent nearly 3 h resting or taking a nap or indulging in very sedentary activities during the rest of the day. Rural women in India and Africa with marginal energy intakes and low BMIs have been observed to spend fewer hours working per day and more time resting than better-off individuals in the same socio-economic milieu (Ferro Luzzi et al, 1992, Shetty & James, 1994). Waterlow (1990) computed the saving in energy that may result from doing a task (e.g. walking a certain distance) slowly rather than quickly, at the cost of having less time for other activities. He drew attention to the fact, which could be relevant, that slow muscle fibres are more efficient than fast ones in terms of ATP used per unit force developed.
Appreciable increases in both activity at work and in discretionary activities without concurrent changes in body weight were reported in male agricultural workers whose diet was supplemented (Viteri & Torun, 1975). There was also an improvement in their sense of wellbeing. Similar improvements in subjective well-being with very small body weight increases have been seen in lactating Gambian women when provided with supplementary food (Whitehead et al, 1978). All these studies support the existence of behavioural adaptation in the spontaneous, free-living physical activity of adults which may limit their work output, economic productivity and income-generating ability, at the same time restricting their socially desirable and discretionary or even their obligatory physical activity. This latter behavioural adaptation becomes an important survival strategy. Recommendations for energy requirements have to take into consideration the energy needs to cope with the cost of behavioural adaptation in adults.