Changing perspectives on aging and energy requirements: Aging, body weight and body composition in humans, dogs and cats

The drivers of the age-related decline in maintenance energy requirement in humans are primarily reduced physical activity and a decrease in basal metabolic rate, which is largely driven by changes in body composition. Most studies indicate that there is a significant loss of lean body mass and a concomitant increase in fat mass with advancing age. The causal factors appear to be changes in physical activity and a reduction in the activity of growth hormone. Sustained physical activity and/or administration of growth hormone have been shown to offset age-related changes in lean:fat ratios in humans and in rats. Very little information is available on dogs, but current data suggest that aging is accompanied by a decrease in lean:fat ratios. The rate and extent of change is similar to that observed in aging humans and it is assumed that the same causal factors are responsible. On this basis, it is likely that basal metabolic rate declines in older dogs. New evidence suggests that the situation is very different in cats, with no apparent change in lean:fat ratios with advancing age. This is probably related to constant activity levels throughout life and suggests that basal metabolic rate probably does not decrease as cats age. On the basis of this evidence, there is no reason to reduce energy provision to the majority of older cats.