RESISTANCE EXERCISE AND GROWTH-HORMONE ADMINISTRATION IN OLDER MEN - EFFECTS ON INSULIN SENSITIVITY AND SECRETION DURING A STABLE-LABEL INTRAVENOUS GLUCOSE-TOLERANCE TEST

To assess the effects of 16 weeks of heavy resistance exercise trainin g (RE) on insulin sensitivity and secretion in healthy older men aged 64 to 75 years (N = 15), stable-label ([6,6,H-2(2)]glucose) intravenou s glucose tolerance tests (IVGTTs) were performed before and 7 days af ter the last bout of exercise. Glucose disappearance rate (R(d)) and a n index of insulin sensitivity (Si) were derived using the minimal mo del of labeled glucose disappearance, and insulin secretion parameters were derived from C-peptide and glucose concentrations measured durin g the IVGTT, using a minimal model of C-peptide secretion and kinetics . Each subject trained at an intensity of 70% to 95% maximum strength 4 d/wk for 16 weeks on Nautilus (DeLand, FL) weight-training equipment . In conjunction with exercise, six men received daily injections of r ecombinant human growth hormone ([rhGH] 12.5 to 24 mu g/kg/d) and the other nine received placebo injections. GH/placebo injections were adm inistered in a double blind randomized fashion. The RE program was sup ervised and progressive in nature, consisting of both upper- and lower -body exercises, and significantly increased muscle strength (P < .05) with no additional benefit from rhGH except for a tendency toward a g reater increase in fat free mass (FFM) in the RE + GH group (P = .06). Peak glucose R(d) increased following RE (P < .01), and there was a t rend for an improved Si- (ie, from 6.79 +/- 1.14 to 8.42 +/- 0.89 x 1 0(4) per min/[mu U/mL], P = .06). Peak glucose R(d) and Si were uncha nged in the RE + GH group following treatment. First- and second-phase insulin secretion were not affected by RE or RE + GH. Glucose toleran ce, quantified as the glucose disappearance constant (K-g) between 10 and 32 minutes of the IVGTT, was unchanged by exercise or hormone trea tment. These findings support those of a recent study that used the hy perinsulinemic-euglycemic clamp technique (Miller et al, J Appl Physio l 77:1122-1127, 1994), and suggest that when healthy older men engage in RE, whole body glucose R(d) and Si are improved, and these benefic ial effects are not only due to the acute effects of the last bout of exercise. Additionally, in six subjects who received GH, glucose R(d) and Si were not significantly improved following the RE program. Alth ough this may suggest that GH can diminish improvements in glucose R(d ) and Si that result from RE, further study is needed to confirm this observation. (C) 1996 by W.B. Saunders Company