Metabolic slowing with massive weight loss despite preservation of fat-free mass

Abstract

Context: An important goal during weight loss is to maximize fat loss while preserving metabolically active fat-free mass (FFM). Massive weight loss typically results in substantial loss of FFM potentially slowing metabolic rate.

Objective: Our objective was to determine whether a weight loss program consisting of diet restriction and vigorous exercise helped to preserve FFM and maintain resting metabolic rate (RMR).

Participants and intervention: We measured body composition by dual-energy x-ray absorptiometry, RMR by indirect calorimetry, and total energy expenditure by doubly labeled water at baseline (n = 16), wk 6 (n = 11), and wk 30 (n = 16).

Results: At baseline, participants were severely obese (× ± SD; body mass index 49.4 ± 9.4 kg/m(2)) with 49 ± 5% body fat. At wk 30, more than one third of initial body weight was lost (-38 ± 9%) and consisted of 17 ± 8% from FFM and 83 ± 8% from fat. RMR declined out of proportion to the decrease in body mass, demonstrating a substantial metabolic adaptation (-244 ± 231 and -504 ± 171 kcal/d at wk 6 and 30, respectively, P < 0.01). Energy expenditure attributed to physical activity increased by 10.2 ± 5.1 kcal/kg.d at wk 6 and 6.0 ± 4.1 kcal/kg.d at wk 30 (P < 0.001 vs. zero).

Conclusions: Despite relative preservation of FFM, exercise did not prevent dramatic slowing of resting metabolism out of proportion to weight loss. This metabolic adaptation may persist during weight maintenance and predispose to weight regain unless high levels of physical activity or caloric restriction are maintained.

Fig. 1.

A, Loss of FFM and FM at wk 6 (n = 11) and wk 30 (n = 16) of the weight-loss competition. The FFM did not decrease significantly from baseline to wk 6 (P > 0.05). All other decreases were significant at P < 0.01. B, The progression of weight loss over the 30-wk competition. The numbers below each data point indicate the number of participants who had their body weight measured at that time and comprise the measurement. At week 13.4, 5 participants were weighed prior to all leaving the ranch; however, only 4 remained in contention and were considered finalists.

Fig. 2.

RMR adjusted for sex, age, and FM (adjusted RMR, kilocalories per day) at baseline (●) and wk 30 (○) of the weight-loss competition (n = 16). The regression line was derived from RMR measurements at baseline in all 16 participants. The deviation from the regression line at wk 30 suggests that RMR per kilogram of FFM was reduced, indicative of metabolic adaptation.

Fig. 3.

A, TEE partitioned into resting (RMR) and nonresting (NREE) components. The NREE includes the energy expended in physical movement and diet-induced thermogenesis. At wk 6, the NREE is significantly increased (P = <0.001) and the RMR decreased (P = 0.04) from baseline. At wk 30, the NREE is not different from the baseline (P = 0.82) and the RMR is significantly decreased (P < 0.001). B, The increase in estimated physical activity from baseline was10.2 ± 5.1 kcal/kg·d at wk 6 and 6.0 ± 4.1 kcal/kg·d at wk 30 (P < 0.001 vs. zero). The decrease from wk 6 to wk 30 was not significant (P = 0.16).