Validity of a Generalized Equation for Setting Target Body Weight

Exercise training produces changes in lean and fat weight, yet the original equation that computes target body weight (TBW) assumes that all weight loss is fat. We developed a generalized equation (GTBW) that accounts for various percentages of weight loss due to fat (WF%). GTBW = (CF -WF% × CBW)/(DF% – WF%), where CF is current fat weight, CBW is current BW, and DF% is the desired percent of body fat. We studied the validity of GTBW and original TBW (OTBW) in 38 fitness participants who exercised 3 days a week, with 20 minutes of aerobic and 20 minutes of strength training, for 10 weeks. Final post weight (PW) was the criterion measure. Percent fat was computed from three sex-specific skinfold sites, age and weight. WF% (191 ± 242%) from a separate group (n = 46) was used in both estimates. WF% greater than 100% reflects increased lean weight that was greater than the decrease in fat. DF% (2.6 ± 1.9%) was the %fat loss from this same group and was used in the OTBW and GTBW. Correlations between OTBW (r = 0.97) and GTBW (r = 0.99) versus PW were significant (p < 0.05). Residual plots for OTBW (r = 0.31) and GTBW (r = 0.17) were not significant. Mean differences showed that OTBW was significantly smaller (p < 0.01) than PW, while GTBW and PW were not different. Subjects increased (p < 0.05) lean mass by 0.82 ± 1.56 kg. The increase in lean mass violated the basic assumption for setting target weights using the OTBW formula. OTBW estimate produced significantly lower TBW and may have negative motivational outcomes. We conclude that the GTBW formula should be used when setting TBW.