Forbes’s theory provides a framework for describing the interaction between the major determinants of body composition change: the initial FM as well as the direction and magnitude of weight change. Forbes’s original equation only accounted for the initial FM but the new equation 8
presented here now includes the contribution of the body weight change. Interestingly, the new equation demonstrated only a weak dependence on the magnitude of the body weight change as indicated in Figures and by the effect of large changes of body weight. This explains why Forbes’s original equation for infinitesimal weight changes (equation 2
) worked so well for modest weight gain and loss.
However, large changes of body weight were predicted to have a significant influence on the composition of the weight change. Forbes’s original equation consistently underestimated the amount of FFM lost following bariatric surgery. Such surgical procedures are becoming increasingly popular for the treatment of obesity and it may be dangerous to erroneously assign a greater proportion of the observed weight loss to decreased body fat. The new equation 8
may provide the basis for better predictions of the relative loss of FFM versus FM following bariatric surgery. I have not adjusted Forbes’s original parameters to optimize the fit to the weight loss data, but such an optimized equation may be a valuable tool for the assessment of bariatric surgery patients.
The vast majority of the bariatric surgery patients depicted in Figures and were women, and the original parameterization of Forbes’s equation was derived from body composition studies of women. At first glance, it is surprising that the theory is at all applicable to men who typically have significantly higher FFM. However, the predicted body composition changes depend only on the shape of the logarithmic curve, but not its vertical position. If men have a similarly shaped curve, but shifted upwards corresponding to a higher FFM, then theory would hold for men as well as women.
Some investigators have assumed a constant composition of weight loss or gain (Dugdale & Payne, 1977
; Payne & Dugdale, 1977b
; Payne & Dugdale, 1977a
; Kreitzman, 1992
). The theory also provides the conditions for such an assumption to be valid. A linear relationship between FFM versus FM results in a body composition change that depends only on the slope of the line and is independent of the initial FM or the weight change (not shown). Thus, a group of subjects operating on an approximately linear part of the FFM versus FM curve would show very little dependence on initial FM or the weight change.
Forbes’s theory is a convenient model for body composition change in humans, but many questions still remain. For example, is it true that longitudinal changes follow the cross-sectional relationship of FFM versus FM? Furthermore, it is unclear whether the FFM versus FM curve would be followed over the entire time course of weight gain and loss, or only after the transients have dissipated and a new steady-state is achieved. Fortunately, these questions are amenable to both experimental as well as theoretical investigation and such studies will likely provide important new insights about how body composition is regulated in humans.