Adaptation to low energy intakes: the responses and limits to low intakes in infants, children and adults

Reduction in energy intake below the acceptable level of requirement for an individual results in a series of physiological and behavioural responses, which are considered as an adaptation to the low energy intake. This abili ty of the human body to adapt to a lowering of the energy intake is without doubt beneficial to the survival of the individual. However, what is more controversial is the view held by some that the body can metabolically adap t in a beneficial manner to a lowered intake and consequently that the requ irements for energy are variable given the same body size and composition a nd physical activity levels. Much of this confusion is the result of consid erable evidence from studies conducted in well-nourished adults who, for ex perimental or other reasons, have lowered their intakes and consequently de monstrated an apparently enhanced metabolic efficiency resulting from chang es in metabolic rates which are disproportionate to the changes in body wei ght. Similar increases in metabolic efficiency are not readily seen in indi viduals who on long-term marginal intakes, probably from childhood, have de veloped into short-statured, low-body-weight adults with a different body c omposition. It would thus appear that the generally used indicator of metab olic efficiency in humans, that is a reduced oxygen consumption per unit fa t free mass, is fraught with problems since it does not account for variati ons in contributions from sub-compartments of the fat free mass which inclu de those with high metabolism at rest such as brain and viscera and those w ith low metabolism at rest such as muscle mass. Metabolic rate per unit fat free mass thus, does not reflect true variations in metabolic efficiency a nd is due largely to variations in body composition. This finding combined with the evidence that behavioural adaptation in habitual physical activity patterns which occurs on energy restriction is not necessarily beneficial to the individual raises doubts about the role of adaptation to low intakes in determining one's requirement for energy. The evidence is overwhelming that both in children and adults, changes in body size and composition as w ell as in levels of habitual physical activity may be the most important co nsequences of a lowered energy intake and cannot be assumed to be a part of a beneficial adaptation that influences energy requirements.