| Ghrelin, a 28 amino acid polypeptide primarily secreted from the stomach, appears to be directly involved with energy intake and body weight maintenance and thus has been implicated in the regulation of short and long-term energy balance. Our previous findings have shown that circulating ghrelin responds in a compensatory manner to changes in body weight in healthy young women to regulate energy homeostasis. To further assess the role of ghrelin in the regulation of long-term energy balance, the first study identified the association between baseline circulating ghrelin and key parameters that either regulate energy homeostasis or indicate metabolic status.{09}While specific metabolic, body composition, and energy balance parameters were examined, baseline circulating ghrelin was found only to be positively associated with resting metabolic rate. The relation of periodic changes in circulating ghrelin to changes in key energy balance parameters in response to an energy deficit-imposing diet and exercise program was also examined. Results indicate that greater increases in ghrelin were associated with greater decreases in leptin, IGF-1, body weight, and feelings of fullness following a meal. In examining the acute time course of ghrelin change with these parameters, ghrelin appears to respond inversely and concurrently to that of leptin, IGF-1, and body weight. While circulating ghrelin is responsive to acute alterations in energy homeostasis, the meal-related pattern and diurnal rhythm of ghrelin may also play a significant role in daily regulation of energy balance. Thus, the purpose of the second study was to characterize and quantify the meal-related patterns of ghrelin and determine whether meal timing and/or meal calorie content alter these patterns. Total ghrelin was found to exhibit specific meal-related patterns. However, the lunch response, specifically the pre-prandial rise and post-prandial decline, were significantly smaller compared to the dinner response. The meal-related pattern of ghrelin was also examined in relation to meal calorie content and no differences were found in any of the meal-related characteristics of ghrelin when comparing larger vs. smaller meal calorie levels. Furthermore, the post-prandial lunch trough was found to be the best predictor of the subsequent dinner peak. Several studies have also examined the meal-related pattern and diurnal rhythm of ghrelin in acute and chronic energy imbalance states and found alterations consistent with the idea of energy balance restoration. Thus, the third and final study examined whether an energy-deficit imposing diet and exercise program would lead to alterations in the meal-related pattern and diurnal rhythm of ghrelin. Significant elevations were observed in the lunch peak, dinner peak, postprandial dinner decline, and the diurnal peak following the intervention in those who lost weight. Taken together, these findings suggest that circulating levels, the meal-related pattern, and the diurnal rhythm of ghrelin are responsive to acute and/or chronic energy imbalances and thus appear to be involved in the regulation of short and long-term energy balance in normal weight, healthy young women. |
