Endocrine Abstracts

Aims: Endogenous and exogenous oestrogens have previously been shown to improve postprandial lipid metabolism, probably due to up-regulation of hepatic LDL receptors. We used stable isotope techniques to investigate the contribution of muscle oxidation of dietary lipid to this improvement.

Methods: Matched groups of healthy premenopausal (n=8) and postmenopausal (n=8) women and postmenopausal women taking oestrogen replacement therapy (HRT; n=8) were given a mixed meal labelled with 1,1,1-¹³C-tripalmitin. Plasma triglyceride (TAG), ¹³C-palmitic acid in the TAG fraction (¹³C-PA; index of chylomicron clearance) and breath ¹³CO₂ (index of muscle oxidation of dietary lipid) were measured.

Results: The combined oestrogenised groups (premenopausal + HRT) had lower postprandial TAG and ¹³C-PA levels and higher breath ¹³CO₂ levels in the postprandial period than the non-oestrogenised (postmenopausal) group (TAG area under curve (AUC) mean+/-SD 7.5+/-3.0 vs 13.7+/-4.8 mmol.l^-1.6h^-1, p=0.007; ¹³C-PA AUC 26.7+/-11.8 vs 45.5+/-20.5 micrograms.ml^-1.6h^-1, p=0.009; breath ¹³CO₂ AUC 22.3+/-4.8 vs 17.1+/-3.5%admin dose.24h^-1, p=0.012). Both the oestrogenised subgroups (premenopausal and HRT) showed lower postprandial TAG and ¹³C-PA levels (premenopausal vs postmenopausal: TAG AUC 7.3+/-3.3 vs 13.7+/-4.8 mmol.l^-1.6h^-1, p=0.036; ¹³C-PA AUC 25.9+/-10.6 vs 45.5+/-20.5 micrograms.ml^-1.6h^-1, p=0.031. HRT vs postmenopausal: TAG AUC 7.7+/-2.8 vs 13.7+/-4.8 mmol.l^-1.6h^-1, p=0.007; ¹³C-PA AUC 27.6+/-13.6 micrograms.ml^-1.6h^-1 vs 43.9+/-21.6 micrograms.ml^-1.6h^-1, p=0.05). The premenopausal group had higher postprandial breath ¹³CO₂ levels than the postmenopausal group (AUC 24.1+/-5.0 vs 17.1+/-3.5%admin dose.24h^-1, p=0.006), whilst there was a non-significant rise in breath ¹³CO₂ in the HRT group compared to the postmenopausal group (AUC 20.5+/-4.0 vs 17.1+/-3.5%admin dose.24h^-1, p=0.09).

Conclusions: Our study suggests that increased muscle oxidation of dietary fat contributes to the improvement in postprandial lipid metabolism due to endogenous and exogenous oestrogens.