Weight gain has become increasingly prevalent over the past two decades, with currently 65% of American adults classified as overweight. Unsurprisingly, the incidence of weight-related illnesses has also risen. The primary objective of this dissertation was to examine individual differences in adult weight gain and loss in order to identify the physiological mechanisms that predispose individuals to gain weight in adulthood, or alternatively, to maintain a healthy body weight throughout their lives. This thesis specifically addressed three common forms of weight change in adult women: menopausal weight gain, unintentional slow progressive weight gain, and weight change resulting from dietary adjustments.