Higher protein intake, and particularly higher leucine intake, is associated with attenuated loss of lean body mass (LBM) over time in older individuals. Dietary leucine is thought to be a key mediator of anabolism. This study aimed to assess this relationship over 6 years among younger and older adult Danes. Dietary leucine intake was assessed at baseline and after 6 years in men and women, aged 35–65 years, participating in the Danish cohort of the WHO-MONICA (Multinational MONItoring of trends and determinants in CArdiovascular disease) study (n 368). Changes in LBM over the 6 years were measured by bioelectrical impedance using equations developed for this Danish population. The association between leucine and LBM changes was examined using multivariate linear regression and ANCOVA analyses adjusted for potential confounders. After adjustment for baseline LBM, sex, age, energy intake and physical activity, leucine intake was associated with LBM change in those older than 65 years (n 79), with no effect seen in those younger than 65 years. Older participants in the highest quartile of leucine intake (7·1 g/d) experienced LBM maintenance, whereas lower intakes were associated with LBM loss over 6 years (for trend: β=0·434, P=0·03). Sensitivity analysis indicated no effect modification of sex or the presence of CVD. Greater leucine intake in conjunction with adequate total protein intake was associated with long-term LBM retention in a healthy older Danish population. This study corroborates findings from laboratory investigations in relation to protein and leucine intakes and LBM change. A more diverse and larger sample is needed for confirmation of these results.