Using Normal Fat Mass to Account for Body Size and Composition

Abstract

Aims: Allometry is the study of how body size is related to body function. In clinical medicine, size related differences in biological functions such as heart function (cardiac output, CO) and kidney function (glomerular filtration rate, GFR) are routinely scaled by body surface area (BSA). In contrast, a more fundamental theory (1) relates body mass to function, based on the energetic needs of cells and the structural overhead associated with larger mass. This biologically based allometric theory is consistent with previous "scaling laws" and has been confirmed by extensive cross-species observations in all domains of biology. Total body mass (TBM) can be thought of as being composed of fat mass (FAT) and (fat free mass (FFM). However, current allometric theory does not distinguish between these two components of body mass. The aim of this presentation is to show how allometric theory can be extended to describe body size and composition using the concept of normal fat mass (NFM). Methods: Normal fat mass is defined by NFM=FFM + Ffat x FAT Ffat is the fraction of FAT kg that contributes to functional body size relative to FFM kg. Results: For a wide variety of drugs, elimination and distribution are well described using theory based allometry and normal fat mass. NFM becomes FFM when Ffat=0 and TBM when Ffat=1.