Background: Body composition (BC) and energy expenditure (EE) are implicated as determinants of the biological drive to eat, but it remains to be demonstrated how these components affect energy intake (EI) at differing levels of body fatness. During weight loss (WL), changes in BC and EE may influence appetite and EI, but it is unknown whether the WL method (intermittent [IER] or continuous [CER] energy restriction) affects compensatory responses.
Objectives: This thesis examined i) the associations between BC, EE and EI during energy balance and energy deficit across a spectrum of body fatness, and ii) whether changes in BC and EE during WL influenced compensatory changes in appetite or EI.
Methods: A series of studies examined BC, EE, physical activity (PA), appetite and EI under laboratory and free-living conditions. These included cross-sectional studies (2 experimental chapters) and a controlled-feeding WL RCT (IER vs CER; 3 experimental chapters).
Results: During energy balance, associations between fat-free mass (FFM), resting metabolic rate and total daily EE with free-living 24-hour EI were moderated by body fatness. Associations between fat mass (FM) and test meal EI were non-linear, with altered strength and direction at higher body fatness. High-metabolic-rate organs better explained the between-subject variability in hunger than FFM. During WL, IER was not superior to CER in attenuating FFM losses or compensatory responses. Adaptive thermogenesis during WL, but not changes in BC or PA, was associated with increased free-living daily appetite. Changes in PA were associated with the mean rate of WL.
Conclusions: The coupling between EE and EI, as well the proposed inhibitory influence of FM on EI, weaken at higher body fatness. Furthermore, IER was not superior to CER in preserving FFM or attenuating compensatory responses during WL, but changes in PA may represent a modifiable factor that influences the rate of WL.
