Ghrelin reflects changes in body size, not energy availability /

by Boyle, Kristen E.

Boyle, Kristen E. M.S. June 2005. Biological Sciences. Ghrelin Reflects Changes in Body Size, Not Energy Availability (141 pp.) Director of Thesis: Anne B. Loucks Ghrelin, a peptide hormone secreted mainly by the stomach, has been implicated in the regulation of energy balance based on its acute meal-response fluctuations and its long-term fluctuations in obesity or anorexia. Long-term changes in ghrelin and body weight are accompanied by reciprocal changes in circulating leptin, insulin, and triiodothyronine levels and comparable changes in growth hormone. Previously, Loucks and Thuma1 reported doseresponse effects of 5 days of low energy availability treatment (defined and controlled as dietary energy intake minus exercise energy expenditure at 10, 20, and 30 kilocalories per kilogram of fat free mass per day (kcal/kg FFM·day)) on leptin and insulin in normal weight, young women, despite minimal weight loss across all treatments. The purpose of this experiment was to examine the effects of short-term low energy availability on ghrelin levels. Twenty-four-hour mean ghrelin levels were measured in 29 young women of normal body weight after 5 days of energy restriction (Study I). Ghrelin levels were increased minimally in all three treatments after 5 days of low energy availability and were not related to the degree of energy availability imposed. Based on the similar changes in ghrelin and body weight in Study I, the relationships between ghrelin and body weight and body composition, as well as various hormonal indicators of low energy availability, were examined in a meta-analysis of previous research reporting significant differences in these parameters using a the general linear model for regression analysis (Study II). Across a much larger range of change in both ghrelin and body weight, changes in ghrelin are related to body weight and are not independently related to leptin, insulin, or other parameters indicative of changes in energy availability. Approved: Anne B. Loucks Professor of Biological Sciences iv This thesis is dedicated to my exceptional family, for their unwavering spiritual, emotional, and financial support. To my father, for his constant reflection and advice, and to Leigh for making him so happy. To my mother and stepfather for their love and understanding. To my grandmother, Doris Boyle, for unconditional love and keeping a warm place for me to stay. To my grandfather, George Boyle, for continued support despite the distance. To my aunt Frannie, uncle Jack, and Jackie for their love and encouragement. To my favorite uncles, Brian and Luis, for their realism and love, not to mention the food and wine. To my uncle Terry who is always ready for a lively debate. To my cousin Mark for letting me know he’s thinking of me, even though I haven’t kept up my end. To my sisters, Lauren and Danielle, for fun times and lots of love. To my nephew Jordan and his exquisite pictures. To my brother Brendan and his beautiful children: Brenna and Brady. To my sister, Ashley, whom I hope still wants more letters after her name than mine. To my dog Rigby. And most notably, to my uncle Kevin, whose courage and determination have shown me that there are no impossibilities.