Fat-free mass and resting metabolic rate are determinants of energy intake: implications for a theory of appetite control

Any explanation of appetite control should contain a description of physiological processes that could contribute a drive to eat alongside those that inhibit eating. However, such an undertaking was largely neglected until 15 years ago when a series of independent research programmes investigated the physiological roles of body composition and appetite. These outcomes demonstrated that fat-free mass (FFM), but not fat mass, was positively associated with objectively measured meal size and energy intake (EI). These findings have been accompanied by demonstrations that resting metabolic rate (RMR) is also positively associated with EI, with the influence of FFM largely mediated by RMR. These findings re-introduce the role of drive into models of appetite control and indicate how this can be integrated with processes of inhibition. The determinants of EI fit into an evolutionary perspective in which the energy demands of high metabolic rate organs and skeletal tissue constitute a need state underlying a tonic drive to eat. This approach should lead to the development of integrated models of appetite that include components of body composition (FFM) and energy expenditure (RMR) as tonic biological signals of appetite alongside other traditional tonic (adipose tissue derived) and episodic signals (gastrointestinal tract derived). This article is part of a discussion meeting issue 'Causes of obesity: theories, conjectures and evidence (Part I)'.