Metabolic effects of oestrogens: impact of the route of administration

To investigate whether oestrogen modulates GH secretion and action in adult life, we studied the impact of oestrogen replacement on circulating GH and IGF-I levels in post-menopausal women. Since the liver is the major source of circulating IGF-I and the oral route of oestrogen delivery causes non-physiologic effects on hepatic proteins, we compared the effects of oral and transdermal route of delivery. Oral ethinyl oestradiol administration resulted in a significant fall in mean IGF-I levels and a 3-fold increase in mean 24h GH. Transdermal administration of 17beta oestradiol resulted in a slight increase in serum IGF-I but no change in mean 24h GH levels. To determine whether differences in oestrogen type rather than in the route of delivery caused the different effects on the GH/IGF-I axis, we compared the effects of three oral oestrogen formulations. Ethinyl oestradiol, conjugated equine oestrogen and oestradiol valerate each induced a fall in IGF-I and a rise of mean 24h GH levels in post-menopausal women. To determine the metabolic significance of oestrogen-induced changes on GH/ IGF-I, we compared the effects of 24 weeks each of oral and transdermal oestrogen on energy metabolism and body composition in 18 post menopausal women in an open-label randomised cross-over study. When compared to the transdermal route, oral oestrogen reduced lipid oxidation, increased fat mass and reduced lean body mass. Oestrogen causes distinct, route dependent effects on the somatotrophic axis. The dissociation of the GH/IGF-I axis by the oral route is likely to arise from impaired hepatic IGF-I production which causes increased GH secretion through reduced feedback inhibition. The route of oestrogen therapy confers divergent effects on substrate oxidation and body composition. The suppression of lipid oxidation during oral oestrogen therapy may increase fat mass while the fall in IGF-I may lead to a loss of lean body mass. The route dependent changes in body composition observed during oestrogen replacement therapy may have important implications for post-menopausal health and oestrogen use in general.