Potential effects of fat mass and fat-free mass on energy intake in different states of energy balance.

Recently models have attempted to integrate the functional relationships of fat mass (FM) and fat-free mass (FFM) with the control of human energy intake (EI). Cross-sectional evidence suggests that at or close to EB, FFM is positively related to hunger and EI, whereas FM either shows a weak negative or no association with ad libitum EI. Further analysis suggests that the effects of FFM and FM on EI may be mediated by resting metabolic rate (RMR). These studies suggest that energy turnover is associated with EI and the largest determinant of energy requirements in most humans is FFM. During chronic positive EBs both FM and FFM expand (but disproportionately so), increasing energy demands. There is little evidence that an expanding FM exerts strong negative feedback on longer term EI. However, during chronic negative EBs FM, FFM and RMR all decrease but appetite increases. Some studies suggest that proportionate loss of FFM during weight loss predicts subsequent weight regain. Taken together these lines of evidence suggest that changes in the size and functional integrity of FFM may influence appetite and EI. Increases in FFM associated with either weight gain or high levels of exercise may 'pull' EI upwards but energy deficits that decrease FFM may exert a distinct drive on appetite. The current paper discusses how FM and FFM relationships influence appetite regulation, and how size, structure and functional integrity of FFM may drive EI in humans (i) at EB (ii) during positive EB and (iii) during negative EB.