BackgroundThere is increasing recognition of the potential for physically active travel modes to protect cardiovascular health, but existing research has rarely considered whether any benefits differ by pre-existing risk factors. This study aimed to investigate whether any benefits of active commuting differ according to objectively defined adiposity status, using the example of developing diabetes. MethodsData come from the nationally representative UK Household Longitudinal Study, with nurse visits in 2010–2012 providing objective baseline data on body-mass index (BMI), body fat, and waist circumference for 6007 working aged adults (16–84 years, mean 44·4, SD 11·9), followed up through 2017. Commuting was categorised as private transport, public transport, and walking or cycling. The outcome was self-reported development of diabetes, excluding participants with diabetes at baseline. Cox regression assessed associations between active commuting and development of diabetes. Models controlled for age, sex, ethnicity, social class, long-term limiting illness, work and leisure physical activity, vegetable consumption, household income, and current smoking. Analyses were stratified into normal and overweight or obese at baseline. FindingsAt baseline 4480 participants (74·6%) commuted using private transport, 601 (10·0%) used public transport, and 926 (15·4%) walked or cycled. 193 participants (3·2%) reported developing diabetes during the study period (154 private transport users [3·4%], 20 public transport users [3·3%], and 19 walkers or cyclists [2·1%]). In fully adjusted analyses stratified by BMI-defined overweight status at baseline (≥25 kg/m2), there was a reduced risk of developing diabetes for walkers and cyclists compared with private transport users (17 [2·9%] vs 144 [4·7%], hazard ratio 0·22, 95% CI 0.07–0.69) although this was not evident among those of normal weight at baseline. Public transport use was not associated with diabetes in any group, and results were similar when sex-specific percentage body fat and waist circumference thresholds were used to define adiposity status. InterpretationThese findings suggest that physically active forms of travel may be most beneficial for those at higher risk of developing diabetes. Practitioners and policy makers should consider efforts to encourage active travel particularly among those at increased risk of disease, and further research should seek to examine whether these associations differ between overweight and obese people. FundingUnderstanding Society Biomarker Data Project Fellowship