Age‐related decrease in energy expenditure at rest parallels reductions in mass of internal organs

This study explores to what extent the mass of internal organs may impact the age‐related decrease in energy expenditure at rest (EErest). The relationship between direct measurements of EErest in elderly women and predicted EErest based on equations deriving from the metabolic activity in tissue from younger women were also elucidated. Body composition of elderly women was measured by an impedance method. EErest was measured by the Douglas bag method after an overnight fast. These data were compared with predicted values of EErest based on equations derived from studies in younger women. The mass of internal organs was obtained from autopsy material. Young women (mean age 31.7 years, range 14–60, n = 104) and elderly women of 65 years (n = 22), 75 years (n = 26), and 85 years (n = 31) participated in this study. Autopsy data were obtained from women (n = 238) from the same birth cohorts as the elderly women who died at ages 42–87 years. EErest showed a progressive age‐related decline, which appeared to parallel a similar reduction in the mass of internal organs derived from autopsy material of women who died at the same ages. In contrast, FFM was identical in the group of 65 and 75‐year‐old women, but was lower in the 85‐year‐old women. Predicted and measured EErest revealed a strong correlation in elderly women. Modest reductions in the mass of internal organs with a high metabolic rate appear to contribute markedly to the decline in EErest observed in aging. Further, it is also possible to predict EErest from the body composition of elderly women using equations developed from younger women. Am. J. Hum. Biol. 14: 486–493, 2002. © 2002 Wiley‐Liss, Inc.