Study of the Compensatory Mechanisms of Energy Balance during and After Weight Loss

Abstract

A number of strategies to lose weight are available. However, a high inter-individual variability is commonly observed in terms of weight loss and its maintenance in individuals enrolled in different interventions. This high variability is mainly explained by individual differences in the activation of compensatory mechanisms triggered by energy deficits. Increases in appetite ratings as well as the rewarding effects of foods are some of the consequences commonly observed from weight loss induced by caloric restriction. On the other side of the energy balance equation, resting energy expenditure (REE) was also found to decrease as consequence of weight loss. Numbers might in fact decrease beyond what could be expected from changes in body weight and composition, highlighting an adaptation in thermogenesis in some individuals. These changes were previously found to be associated with the magnitude of weight loss. However, it is not clear whether different rates of weight loss have a different impact on the compensatory mechanisms described above. Moreover, other questions regarding weight loss maintenance deserve further investigations. For example, the role of exercise, more specifically resistance training (RT), on weight loss maintenance needs additional attention. Accordingly, the present thesis aimed to investigate the effects of caloric restriction on compensatory mechanisms that occur during and after weight loss. We first aimed to determine whether the rate of weight loss differently influence physiological and psychological variables related to energy balance. Secondly, we aimed to elucidate whether early changes in the above mentioned adaptations in energy expenditure (EE) and energy intake (EI) variables predict final outcomes (fat mass - FM and weight loss). Finally, we aimed to determine whether RT promoted greater weight loss maintenance. In Article I and II, we investigated whether different rates of weight loss play a role in EI and EE related-variables. We noted significant increases in fasting appetite measures, as well as increases in satiety measures. REE decreased over time, as did the relative reinforcing value of fruit. No significant group interaction was observed illustrating that different rates of weight loss has no impact on the magnitude of adaptations in EI and EE after weight loss. In article III we demonstrated that early changes in fasting and postprandial appetite measures in response to caloric restriction were associated with greater body weight and FM loss in women. Indeed, greater increases in fasting appetite were associated with greater FM loss, contrary to our hypothesis. However, increases in postprandial appetite were associated with greater FM and body weight loss, independently of changes in eating behaviours. Taken together, articles I and III demonstrated that caloric restriction has a significant impact on increases in appetite and reduction in REE as soon as the in the first week of intervention. Those changes remain significant until the end of the program. In article IV it was shown that 1-year of resistance training (2x/ week) after 6-month of caloric restriction was not sufficient to promote better weight and FM loss in post-menopausal women. Furthermore, the results demonstrated that RT did not improve the differences between measured and predicted measures in REE observed as consequence of weight loss. The picture that emerges is that, increases in appetite and decreases in REE can be observed since the first week of caloric restriction and remain significant until the end of the program, independently of the rate of weight loss. Feeding-related variables such as fasting and postprandial appetite and RRV of a snack food are better predictors of final FM loss, even after adjusting for changes in eating behaviours. In addition, our study demonstrated that different rates of weight loss do not have an independent aspect on either physiological or psychological aspects related to energy balance.