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The rate at which adult organisms ingest and expend energy can be predicted from their body weight. Kleiber has shown that animals and man utilize energy at a rate proportional to their body weight.' The logarithmic relationship between body weight and resting metabolism (Figure 1) forms the basis of "Kleiber 's rule": the energy needs of many different animal species are proportional to the three-fourths power of their body weight.1 Raising body weight to this three-fourths power provides an estimate of an animal's "metabolic body size."
The relationship between resting energy needs and metabolic body size holds only when an animal is at its normally maintained body weight. If the body weight declines, metabolic body size no longer accurately predicts energy needs. The rate of energy expenditure declines more quickly than would be predicted from the decline in metabolic body size.'·2 For example, although food restriction in two groups of rats caused an 8% and 11% weight loss, resting metabolic rate declined by 15% and 21%, respectively (Figure 2). This reduction in resting metabolism is substantially greater than would be expected due to the decline in metabolic body size. Similar findings have been reported in humans.
Energy is apparently expended at rates exceeding those expected on the basis of changes in metabolic body size when body weight rises above the level normally maintained. Both animals and man display elevated rates of thermogenesis after overconsumption or weight gain.
Adjustments in energy expenditure contribute to the stability of body weight that is characteristic of most adult animals and humans. Physiological mechanisms that change rates of energy expenditure in response to weight increase or decline blunt the effects of both ovemutrition in times of food abundance and undernutrition in times of famine. These actions may also explain why most dieters lose only a certain amount of weight even though they continue to restrict their intake, as well as why they regain their lost weight so rapidly when they forego the diet.
WEIGHT MAINTENANCE AT OBESE LEVELS
If these metabolic adjustments are so effective in restraining weight change, why are there wide variations in body weight within a species, and why is obesity so prevalent'? Do the physiological mechanisms controlling adaptive metabolic responses to weight gain fail to operate...