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The route of estrogen replacement therapy has a major impact on the growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis. Estrogen administration by the oral, but not the transdermal route, reduces IGF-I and increases GH levels in postmenopausal women. To investigate whether these perturbations have metabolic consequences, we compared the effects of 24 wk each of oral (Premarin 1.25 mg) and transdermal (Estraderm 100TTS) estrogen on energy metabolism and body composition in 18 postmenopausal women in an open-label randomized crossover study. Energy expenditure, lipid oxidation (lipid(ox)), and carbohydrate oxidation (CHOox) were measured by indirect calorimetry in the fasted and fed state before and after 2 and 6 mon treatment. Lean body mass, fat mass, and total body bone mineral density were measured by dual X-ray absorptiometry before and after 6 mon treatment. Mean (+/-SE) Luteinizing hormone levels fell to comparable levels during oral and transdermal estrogen, and bone mineral density was significantly increased by both treatments. Mean IGF-I was significantly lower during oral estrogen (77+/-7 versus 97+/-7 microg/liter, P < 0.05) treatment. Lipid(ox) 30-60 min after a standardized meal was significantly lower (36+/-5 versus 54+/-5 mg/min, P < 0.01) and CHOox higher (147+/-13 versus 109+/-12 mg/min, P < 0.05) with oral compared with transdermal estrogen. Oral estrogen resulted in a 1.2+/-0.5 kg (P < 0.05) increase in fat mass and a 1.2+/-0.4 kg (P < 0.01) decrease in lean mass compared with transdermal estrogen. Lean body mass (0.4+/-0.2 kg) and fat mass (0. 1+/-0.4 kg) did not change significantly during transdermal estrogen. In summary, when compared with the transdermal route, oral estrogen reduces lipid(ox), increases fat mass, and reduces lean body mass. The route of estrogen therapy confers distinct and divergent effects on substrate oxidation and body composition. The suppression of lipidox during oral estrogen therapy may increase fat mass although the fall in IGF-I may lead to a loss of lean body mass. The route-dependent changes in body composition observed during estrogen replacement therapy may have important implications for postmenopausal health.