Body Composition Changes after 6- and 12-months of Dietary Weight Loss: Insights from the DIETFITS Trial

Context While popular weight-loss medications have raised concerns about lean mass loss, the normative effects of non-pharmacologic weight loss on body composition remain unclear. Objective We assessed how the extent of weight loss from dietary changes affects lean mass and bone mineral content (BMC) in women and men. Methods We conducted a secondary analysis of the DIETFITS trial (NCT01826591), a randomized controlled study with 609 adults (aged 18-50, BMI 28-40kg/m²). Data from 323 participants (200 women, 123 men) who lost weight at 6-months and had dual-energy X-ray absorptiometry (DXA) assessments at baseline, 6 months and 12 months were analyzed. Participants were stratified as follows: <5% (n=102), 5-9% (n=121), and ≥10% (n=100). The primary outcome was the percentage of lean mass lost relative to total weight loss (%LML). Results Baseline mean BMI (33kg/m2) was similar across weight-loss groups and sexes. Mean percentage weight loss was greatest at 6 month (2.9%, 7.3%, 14.6% in those losing <5%, 5-9%, and ≥10% respectively) and was similar between sexes. %LML varied widely with weight loss < 5% but plateaued with greater loss. Men had higher %LML than women when weight loss was ≥5% (median [IQR], 33 [24,41] vs 28 [17, 36], p<0.001). All groups, except for women losing ≥10%, significantly regained weight and fat mass at 12 months, while BMC continued to decline. Conclusions Contrary to our hypothesis, %LML plateaus with greater weight loss. Moreover, men experience greater %LML than women. The decline in BMC despite weight regain warrants further investigation.