The aim of the present study was to examine to what extend different social network mechanisms are factors explaining the spread of obesity and obesity associated co-morbidities. Based on our theoretical framework we derive testable hypotheses regarding an indirect and direct impact of social networks on EGO’s BMI and insulin resistance. To test our hypotheses we undertook a clinical and social survey including a sample of 1397 probands. Collected data include anthropometric and biochemical measures as well as health attitudes, behavioural and socio-economic variables and social network data. We used nonparametric and parametric regression models to analyse whether EGO’s BMI and insulin resistance are determined by EGO’s social network characteristics controlling for EGO’s individual characteristics. We found significant PSM and GPS treatment effects for high sport activities, a frequent diet behaviour (p=0.000) of EGO’s social peer group. Since our regression analyses results that obesity is the main determinant of the HOMA-index this established a significant indirect network effect on insulin resistance. We also found significant direct social network effects on EGO’s insulin resistance, i.e. controlling for EGO’s obesity status frequent diet behaviour (p=0.033) and sport activities (p=0.041) of EGO’s peer group decreases EGO’s HOMA index. Network phenomena appear not only to be relevant for the spread of obesity, but also for the spread of associated co-morbidities.