Age dimorphism in the association between growth-hormone status and the respiratory quotient

Objective: To investigate the impact of age on the association between the respiratory quotient (RQ) and growth-hormone (GH) secretion and to investigate the acute lipolytic response to an exogenous GH bolus. Research Methods and Procedures: A cross-sectional study of 36 non-obese healthy subjects (18 women and 18 men) from two age groups was used: "younger" (mean age, 29.5 years; range, 27 to 34 years) and "older" (mean age, 50.8 years; range, 47 to 59 years). Endogenous GH secretion by means of deconvolution analysis of 24-hour serum GH concentrations was measured every 20 minutes. Resting RQ was measured after a 12-hour overnight fast. The lipolytic response to an intravenous exogenous GH bolus (200 Ag) was assessed by measuring serum levels of free fatty acids as well as changes in RQ. Additional measurements included body composition (regional computed tomography scan and DXA) and physical fitness (Vo(2)max). Results: Resting RQ did not differ between the two age groups: 0.81 +/- 0.01 (young) vs. 0.82 +/- 0.01 (older; not significant). Several estimates of GH release correlated positively with RQ in the younger group, whereas a negative correlation was detected in the older subjects [GH production rate (mug/liter X kg) vs. RQ: r = 0.62, p < 0.01 (younger); r = -0.53; p = 0.02 (older)]. By regression analysis, 52% to 58% of the variation in RQ could be explained by GH status. After an exogenous GH bolus, the incremental response in nonesterified fatty acid was slightly higher in younger individuals (p = 0.09). Discussion: Resting RQ is significantly correlated with GH status. This association is positive in younger individuals and negative in older individuals. The lipolytic response to exogenous GH is moderately higher in younger compared with older individuals. GH status should be taken into account when investigating the residual variation in RQ.