We examined the mechanism of the effect of weight loss through dieting on LDL and HDL metabolism in abdominally obese men. LDL apoB-100 and HDL apoA-I kinetics were studied using a primed-constant infusion of 1-[13C]-leucine in a controlled, dietary intervention trial of 16 weeks duration in middle-aged, obese men with the metabolic syndrome. Isotopic enrichment in apoB and apoA-I was measured by gas chromatography-mass spectrometry and fractional turnover rates estimated using multi-compartmental modelling.