The acute effects of exercise on appetite perceptions, gut hormones and food intake in females

In recent years there has been growing interest in the role of gut hormones in regulating appetite, energy balance and weight control. Prominent among these hormones is the hunger hormone ghrelin which is the only circulating hormone currently known to stimulate appetite. A variety of hormones are known to suppress appetite and notable among these is peptide YY (PYY). Both ghrelin and PYY exist in more than one form with acylated ghrelin and PYY3-36 representing the biologically active forms of these hormones i.e. the form of each hormone with the most potent effects on appetite. Many studies have investigated ghrelin responses to exercise in male participants and some studies have also examined PYY responses. Far fewer studies have examined ghrelin and PYY responses in female participants and this was the primary purpose of the studies reported here. This thesis comprises four main experimental chapters which collectively sought to clarify whether there is any evidence to support the hypothesis that appetite, gut hormone and food intake responses differ in female compared with male participants. A total of 123 participants took part in the studies reported in this thesis. The first of these studies was cross-sectional in nature and compared fasting appetite, plasma acylated ghrelin and dietary restraint questionnaire values (among other variables) in 34 males and 33 females. No significant differences were observed between sexes for any of these variables. In the second study, appetite, plasma acylated ghrelin and ad libitum food intake responses to cycling exercise were examined in 13 female participants taking the oral contraceptive pill in both the luteal and follicular phases of the menstrual cycle. Although fasting hunger and prospective food consumption values were higher in the follicular than the luteal phase there was no difference in appetite, plasma acylated ghrelin and food intake responses to exercise between menstrual cycle phases. In the third study, appetite, plasma acylated ghrelin, plasma PYY3-36 and food intake responses to energy deficits created via diet and exercise were compared in 13 young, healthy female participants who completed three separate trials (control, exercise deficit and food deficit) in a random order. The findings revealed that, as with male participants, females experience compensatory appetite, gut hormone and food intake responses to dietary induced energy deficits but not to exercise induced energy deficits (over the course of a nine hour observation period). The final study reported in this thesis compared appetite, plasma acylated ghrelin and ad libitum food intake responses to a one hour run in 10 male and 10 female participants. Suppressions of both hunger and plasma acylated ghrelin were noted during exercise but there was no significant difference in the responses of males and females during or after exercise. Collectively, the studies reported here suggest: 1) that fasting appetite and plasma acylated ghrelin concentrations do not differ between male and female participants; 2) that appetite, ghrelin and food intake responses to cycling exercise do not differ according to the phase of the menstrual cycle in females; 3) that dietary restriction is more likely to elicit compensatory feeding responses than elevated exercise levels in females and 4) that males and females do not differ in their acute appetite, ghrelin and food intake responses to an acute bout of running exercise. Hence the studies reported here do not support the hypothesis that exercise will be less effective for controlling appetite and food intake in females than in males.