We investigated the effect of weight loss, independent of change in diet composition, on HDL and apoAI metabolism in men with metabolic syndrome (MetS). Subjects (19 men with MetS [NCEP-ATPIII)]) were first fed an isoenergetic Mediterranean-style diet for 5 weeks (all foods provided). Participants then underwent a 20-week free-living period during which they were counseled to restrict energy intake, after which they were again fed an isoenergetic Mediterranean-style diet for 5 weeks. At the end of the two controlled diets, participants received a single bolus of [5,5,5-2H3] L-leucine and fasting blood samples were collected over a 96-hr period. ApoAI kinetic was assessed using multicompartmental modeling of the tracer enrichment data. Participants achieved a 9.1 ± 2.8% reduction in body weight (P<0.001). Weight loss resulted in an increase in plasma HDL-cholesterol (C) concentrations of 6.0% (P=0.059) and HDL3-C of 7.9% (P=0.045), attributable to a reduction in apoAI fractional catabolic rate (-7.8%, P=0.046) with no change in apoAI production rate (2.2%, P=0.58). These data indicate that weight loss, independent of variation in diet composition, increases plasma HDL primarily by delaying the catabolism of apoAI. ClinicalTrial.gov registration number: NCT00988650.