The drive to eat: investigating the link between body composition, appetite and energy intake with ageing

This review aims to (1) provide an overview of research investigating the relationship between body composition, specifically fat-free mass (FFM) and fat mass (FM), appetite and energy intake (EI) and (2) to investigate potential mechanisms underlying these relationships, with a focus on ageing. Appetite and EI are influenced by complex, multifactorial pathways involving physiological, psychological, environmental, social and cultural factors. Early research investigating the association of body composition with appetite and EI focused on FM; however, the role of FFM in appetite control is gaining increasing attention. Studies have shown that FFM is positively associated with EI in younger populations, including infants, adolescents and adults. In contrast, FM appears to have no association or a weak inverse association with appetite/EI. However, research in older adults is limited, and the underlying mechanisms are not fully understood. It has been suggested that one way in which FFM may influence appetite and EI is by impacting resting metabolic rate (RMR). FFM, which includes metabolically active tissues including skeletal muscle and organs, represents the largest determinant of RMR and therefore may influence appetite and EI by ensuring the energetic requirements of crucial tissue-organs and metabolic processes are reached. Given that declines in FFM and RMR are common with ageing, they may be possible targets for interventions aimed at improving appetite and EI. While current evidence in older adults supports a positive association between FFM and appetite, further longitudinal studies are needed to explore this relationship in different contexts, along with the underlying mechanisms.