Objective We aimed to investigate the relation between informal caregiving and body mass index (BMI) longitudinally. Study designThe data were drawn from wave 2 (2002) to wave 5 (2014) of the German Ageing Survey. This is a representative sample of the community-dwelling population aged 40 years and above in Germany. Methods Self-rated BMI was used. Individuals were asked whether they provide informal care on a regular basis. Adjusting for employment status, age, marital status, morbidity and depressive symptoms, fixed effects regressions were used. Results The fixed effects regressions showed that the onset of informal caregiving was not associated with changes in BMI in the total sample and in women, whereas the onset of informal caregiving was associated with increasing BMI in men (β = 0.15, P < 0.05). In addition, an increase in BMI was positively associated with ageing, an increase in morbidity and a decrease in frequency of sports activities in the total sample and in both sexes. Conclusions Our findings stress the longitudinal relation between informal caregiving and BMI in men. Consequently, it might be helpful to generate weight management strategies specifically designed for male informal caregivers.