Plasma leptin concentrations, basal metabolic rates and respiratory quotients in young and older adults

OBJECTIVE: To-examine the interrelationship of circulating leptin concentra tions, basal metabolic rates (BMR) and respiratory quotients (RO) in young and older adults. DESIGN: Cross sectional study. SUBJECTS: Seventy-six Australian men and women, 48 young (< 35 y) and 28 ol der ( greater than or equal to 50 y). MEASUREMENTS: Fasting plasma leptin concentrations by RIA, BMR and RO by in direct calorimetry, percentage body fat (BF%). fat mass (FM) and fat-free m ass (FFM) from total body water (TBW) based on deuterium dilution, waist an d hip circumferences from anthropometry. RESULTS: Older subjects had significantly higher BF%, FM and waist-to-hip r atio (WHR), but significantly lower FFM and absolute BMR as compared to you nger subjects. Absolute leptin concentrations were 60% higher in older subj ects but did not achieve statistical significance, There was, however, a si gnificant gender x age group interaction in leptin concentrations. This was reflected in a significant inverse relationship between age group and lept in in women when data was controlled for waist circumference (r= -0.38, P= 0.028), or FM (r= -0.36, P- 0.042), A similar relationship was not observed in men on controlling for BF% or FM. Log transformed plasma leptin was bes t explained by a model that included BF%, gender, age-group, gender x age-g roup and WHR r= 0.75, adjusted r(2)=0.56, standard error of estimate (SEE)= 0.73ng/ml). BMR was best explained by FFM, FM and age group r=0.94, adjuste d r(2)=0.87; SEE = 429 kJ/day). On controlling for BF%, WHR and FFM, leptin was negatively related to RO only in older men (r= -0.67, P= 0.033). There was no relationship of leptin to BMR in the groups studied. CONCLUSION: The study demonstrates an age-related modification of the gende r bias in leptin, and a gender-specific inverse relationship between leptin and RO in older people. The decline in leptin and the lack of a relationsh ip between RO and leptin in older women may indicate an increased risk of w eight gain relative to older men.