The influence of fat mass on fat-free mass in healthy adults

Background: Fat-free mass (FFM) tends to vary in response to changing fat mass (FM). Values of typical (positive) ratios ΔFFM/ΔFM (where Δ indicates a change or difference) are necessary for understanding the physiological mechanisms and the quantitative relationships amongst FM, body mass and height in ‘ordinary’ adult populations. That malnutrition and body building also affect FFM has clouded the extensive literature. Aims: These are to quantify the effect for healthy men and women using published data, to integrate results from the diversity of past studies and to establish the value of an approach through regression analysis. Methods: Published results were re-worked as necessary. Some estimates of ΔFFM/ΔFM are for dieting or over-fed individuals. Others were obtained from regression equations for population samples using an algebraic model that distinguishes a FM-dependent component of the FFM (FFMF) and a FM-independent component that correlates with stature and is directly affected by other factors. Results and conclusions: Although ΔFFM/ΔFM rises with falling FM in starvation, the evidence for such an inverse relationship in adults of normal or high FM is mostly invalid. In them, the ratio is generally independent of fatness. Estimates from different studies vary, but steady-state values of ΔFFMF/ΔFM probably average about 0.2-0.3 in women and 0.4-0.5 in men. The regression method could easily be applied to existing data sets and be adapted to include other variables