Influence of cyclic changes in ovarian hormones on resting metabolic rate and appetite in women
Energy balance, when energy intake (EI) equals energy expenditure (EE), is key in the maintenance of body weight and composition. Pre-menopausal women seem to experience fluctuations on both sides of the energy balance equation and this seems to respond to changes in the ovarian hormones, estradiol (E2) and progesterone (P4) within a menstrual cycle (MC). In light of the current prevalence of overweight and obese in the female population, it seems imperative to have a better understanding of the influence of the ovarian hormones on energy regulation in pre-menopausal women. This thesis aimed to: examine the fluctuations in salivary and plasma ovarian hormones within a MC in naturally cycling (NC) women and hormonal contraceptive (HC) users; investigate whether resting metabolic rate (RMR) significantly changed throughout the MC in NC women and in HC users; explore the association between RMR and the ovarian hormones; and investigate whether appetite responses to the same breakfast changed throughout the MC phases. Ovarian hormones in plasma experienced a greater fluctuation than in saliva within a MC and this affected the correlation and the ratio between the two collection specimens. Increases in RMR in the luteal (LPh) compared to the other MC phases in NC women were observed, but were not statistically significant despite showing clinically meaningful fluctuations. Salivary P4 contributed to the variance of RMR seen in the LPh. Finally, gastric emptying (GE) time and PYY response to a standardized breakfast changed significantly across the MC; the LPh had the fastest GE time and the smallest PYY response of all the phases. P4 and E2-P4 ratio were significantly correlated to GE time. The findings suggest that NC women might experience changes in their inherent regulatory mechanisms by which energy homeostasis is achieved within a MC. Whether these regulatory changes are beneficial or not in the maintenance of body weight in the long term remains unknown.
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