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9. Adaptation to underfeeding

9.1. The range of adaptation
9.2. The response to semistarvation

Adaptation in energy expenditure is taken to signify a sustained change in physical activity progressively developing over a matter of days or weeks in response to a change in energy intake. A change in energy expenditure, which simply reflects the outcome of a change in body weight and in the mass of metabolically-active tissue, cannot be considered a true adaptation. Such a change in mass will buffer the effects of a sustained alteration in energy intake, but an additional adjustment in metabolic efficiency is needed for "adaptation" to be evident.

Table 3 shows that in the classic Minnesota studies of KEYS et al. (1950) on conscientious objectors, semistarved for six months, there must have been a substantial fall in energy output to allow the men to stabilize their reduced weight on about half their original usual energy intake. These magnificent studies showed that the basal metabolic rate fell by 15%, when expressed on a fat-free basis; this change was an early feature of semistarvation, that was complete by the end of three weeks of underfeeding. A further fall in BMR occurred, but this depended on the degree to which lean tissue was lost. Keys and his colleagues did measure the mechanical efficiency of exercise but not the thermic response to food; they noted the marked reduction in physical activity, so that in the final stages the individuals were observed to be extremely lethargic. Since some of the factors were not analyzed in detail, we cannot tell with certainty whether major changes in metabolic efficiency occurred or not.

Table 3. The changes in basal metabolic rate in relation to active tissue mass at different times during semistarvation

Data for Experiments 1 and 2 taken from GRANDE et al. (1958) and not as reproduced with slightly different numbers in GRANDE (1964).

Data for Experiment 3 taken from Table 166, KEYS et al. (1950), and assuming energy equivalent of oxygen 4.8 kcal/L.

Active tissue = body weight minus sum of fat, bone mineral and extracellular fluid derived by a formula from measures of total body water and extracellular fluid volume.

9.1. The range of adaptation

The Keys study showed that adult man responds to semistarvation over a period of several months until he re-enters a steady state on only half the usual intake: energy supply before and at the end of the 24 weeks' semistarvation was 14.5 MJ (3468 kcal) and 6.6 MJ (1570 kcal). In the last four weeks of semistarvation, weight was stable to within 0.5 kg. No other study has demonstrated adaptation so effectively. GRANDE (1958; 1964) has emphasized that adaptation is normally considered to include those adjustments in behaviour and in biochemical mechanisms which do not impair the body's function.

Table 4. The body composition and total energy exchange of semistarved and "undernourished" men

Data for New Guinea Kaul men taken from NORGAN et al. (1974), where lying values are taken as equivalent to basal metabolic rate. Specific dynamic action calculated assuming equal to 10% energy intake and cost of physical activity calculated by difference as suggested by TAYLOR and KEYS (1945). Energy turnover presumed to be that of energy expenditure and not the lower value for intake measured by Norgan et al., but taken in Minnesota experiments to be equal to intake since this is precisely controlled and weight steady under both conditions of measurement. Most of the Minnesota data is taken from Table 181 with values for body fat taken from Table 457.

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