The effect of gender and body composition method on the apparent decline in lean mass-adjusted resting metabolic rate with age

Background Declining resting energy expenditure (REE) is a hallmark of norm al aging, but the cause of this decline remains controversial. Some, but no t all, studies have shown that the decline in REE with age is eliminated af ter adjustment for fat-free mass (FFM). Methods. We examined the effect of four body composition methods used to as sess FFM (underwater weighing [UWW], bioimpedance analysis [BIA], tritium d ilution, and total body potassium [TBK]) on the relationship between REE an d age in 30 healthy men and 101 healthy women aged 18 to 87 years. Results. The decline in REE with age was significant in women (-80.3 kJ/d/y , p <.003) but not in men(-46.9 kJ/ d/y, p =.328). After adjustment for FFM , the decline in REE with age persisted when FFM was measured by BIA, UWW, or tritium dilution, but no decline was seen when TBK was used to adjust fo r FFM. In both women and men, fat mass was significantly associated with RE E after adjusting for age and FFM. Conclusion. It is the decline in cell mass with age, detectable by TBK but not by other methods, rather than any metabolic alteration, that explains t he decline in FFM-adjusted REE with age.