(introductory text...)

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.

Figure 1. Energy expenditure of normal-weight adults in a whole-body calorimeter. Data selected from studies involving light (o) or moderate activity (·) only. FFM = fat-free mass.

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

	Exp. 1 (n = 12)		2 (n = 14)		3 (n = 12)
Day	0	14	0	19	0	168
BMR MJ/d	7.29 ± 0.40	5.73	6.62 ± 0.66	5.49	6.59	4.20
kcal/d	1742 ± 96.5	1370	1582 ± 158	1311	1575	1004
Body weight kg	71.6 ± 9.2	65.4	69.1 ± 10.1	62.4	67.5	51.7
Active tissue mass kg	44.9 ± 5.1	42.2	43.4 ± 4.9	40.8	38.8	28.7
BMR kJ/kg	162.3	135.9	152.5	134.5	169.8	146.4
Decrease active tissue kg (%)	2.7	(6.0)	2.6	(6.0)	10.1	(26.0)
Decrease BMR kJ per kg active tissue (%)	26.4	(16.3)	18	(11.8)	23.4	(13.8)
Decrease BMR kJ/d	1556	(21 4)	1134	(17.1)	2380	(36.3)

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.