Is Obesity Infectious?

We all know that many viral deceases are contagious and spread by physical contact. Similarly behavior can be influenced by family and friends. In the same way, is it possible that obesity is acquired from our social contacts?

Recent research at Harvard University suggests so. A research article published by Hill, Rand, Nowak and Christakis recently in PLOS computational biology journal suggests that obesity is spread similar to an infectious disease transmission among friends and social contacts.

This research not only identified the inter-relation between obesity and social contacts but also quantified.

Models of infection

In order to understand the models, individuals are classified as susceptible and infected.

In the SIR model, an individual is susceptible to start with, gets infected and then becomes immune to infection and so after the first infection, the individual gets into recovery stage. Hence the name SIR is given, which is derived from the first letters of the words - Susceptible, Infected and Recovered.

However in some deceases and social behaviour patterns, an individual does not become immune and so becomes susceptible once again after a recovery. This model is SIS model -meaning Susceptible, Infected and Susceptible.

In cases of social contacts, such behaviour can be 'infected' spontaneously or automatically as the present study has shown. To explain this phenomenon, the authors have extended the SIS model and called it SISa, the suffix standing for automatic.

Using the model, the research quantified the social influence on obesity. In fact the model calculates that for each obese social contact, the rate of obesity increases by 0.5 percentage points.

This is an interesting study and probably adds one more fact to prove the mind over body phenomenon. However one important point the study highlights is, while the number of obese contacts influence the change of state from non-obese to obese, the reverse is not true. This observation shows that it is not a simple mind body influence. Or if it indeed is, the influence is only on the negative side. Actually the authors concluded from the study that while gaining weight is contagious, losing weight is not.

This study used data points from earlier obesity studies. As the earlier studies did not focus on information of social network data, the authors suggest that some of the social data might be incomplete. While this might have caused some under sampling, the authors are confident that the results would not have changed qualitatively.