The accumulation of excess calories stored as adipose tissue requires an imbalance in the usually tight relationship between caloric intake and expenditure. While adiposity has a tendency to increase with age, total caloric intake either is unchanged (67) or declines (68) when assessed in longitudinal studies. Furthermore, the increment in adiposity with aging cannot be blamed on increased relative fat intake in the diet (69), since this too appears to decline with aging (67,68). However, one must always be careful interpreting data on reporting of calorie intake. As compared to doubly labeled water determination of calories intake, underreporting of calorie intake has been found in both male and female older subjects, with greater underreporting in heavier individuals (70). The ability to appropriately detect and respond to a change in body weight also appears to be impaired in older subjects. Two well-controlled feeding trials strongly suggest that in older subjects the regulation of energy intake is impaired in response to either an imposed increase or decrease in weight. After weight loss, older subjects did not appropriately increase their intake, and following weight gain they did not properly reduce their intake when compared to younger controls (71).
It is also likely that the age-related increase in obesity is in some way related to deficits in energy expenditure. This topic has been carefully reviewed (72), and the relationship between resting metabolic rate (RMR) and age appears to be curvilinear if sufficient numbers of older individuals are included. While three-fourths of this decline in RMR can be accounted for by decrements in FFM, one-fourth remains unexplained. Poehlman has suggested that this decline in RMR may be related to inactivity and has demonstrated normalization in older men after endurance training (73). Others have not been able to demonstrate an endurance training-related improvement in RMR in older subjects (74). While the thermic effect of feeding (TEF) may decline with age (75), this may be more related to inactivity than to age itself (72). Furthermore, variability in TEF does not predict subsequent weight gain (76).
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