There is great attachment to biological modelling. When it works it is great but many have fallen by the way side
Hall in Brit J of Nutrition 2007, 97, 1059-63 ( Body fat and free fat mass inter-relationships :Forbes theory revisited.

Twenty years ago Forbes suggested that body fat mass and fat free mass are in a sense companions and a change in one induces a change in the other and in the same direction. That there is an empirical, non-linear relationship between Free Fat Mass and body Fat Mass using cross-sectional body composition data and that longitudinal changes of body composition were described by movement along the cross-sectional curve
From this hypothesis Forbes expressed a mathematical expression for the Free Fat Mass proportion of body weight change as a function of the initial body Fat Mass.. Forbes’s mathematical expression is strictly valid only for infinitesimal weight changes.
Hall has looked at this to account for macroscopic weight changes. The new equation predicts that the composition of weight change depends on both the direction and magnitude of the weight change in addition to the initial body fat mass.
The new equation was compared with experimental data from underfeeding and overfeeding in humans.
Hall also discusses the relationship between Forbes’s theory and an alternative representation of body composition change that assumes the existence of an energy partitioning parameter, called the P-ratio The P-ratio defines the fraction of an energy imbalance accounted for by changes of the bodv’s protein reserves. The P-ratio depends on the initial body composition s well as the direction and magnitude of weight change.

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Martin Eastwood

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