Background Although social networks’ influence on obesity has been increasingly recognised, it remains unclear if different dimensions of social support, for example, emotional or practical support, received from one’s closest relationship are associated with weight outcomes over mid-life and old age.

Methods Using linear mixed models we examined whether person-level body mass index (BMI) and waist to hip ratio (WHR) trajectories vary according to levels of emotional, practical and negative aspects of social support in a large UK-based cohort of healthy civil servant workers (n=5460) with objectively measured anthropometry data on five occasions over two decades (1989–1990 to 2012–2013).

Results We found that gender modified the associations, with more consistent patterns found in men. In men, high negative aspects of support compared with low were consistently associated with steeper increase in BMI (0.024, 95% CI 0.001 to 0.047 kg/m²) and WHR (0.00020, 95% CI −0.00001 to 0.00040) after adjustment for demographic and socioeconomic covariates, mental health, health behaviours and long-standing illness. We found that low emotional support, compared with high, was associated with steeper BMI gain in men (0.024, 95% CI 0.0001 to 0.047 kg/m²).

Conclusions Low levels of negative aspects of the relationships with the closest person and high levels of emotional support may be protective against weight gain over time, particularly in men. If replicated in other studies, these results would suggest that the quality of social support in close relationships has been an overlooked risk factor for weight gain in an ageing population.