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Your social network has a big effect on your weight, according to a study just published in the New England Journal of Medicine.

The study was performed on data collected over a 30 year period on over 12,000 people.

Some key findings of the study:

• Obesity spreads in a social network in a similar way to a contagious disease
• Your close friends effect your weight more than your family
• Geographic distance is less important than social distance
• Friends and family of the same sex effect your weight more than opposite sex friends and family
• Spouses living together effected each other's weight less than close friends of the same sex living far away
• Since close friends effect weight more than close family, this study contradicts the idea that obesity is mainly hereditary
• Not only friends, but friends of friends have a significant effect on your weight

The bad news is that if you have obese friends you are likely to gain weight yourself.

The good news is that you and your friends can help each other lose weight.

The study helps explain why group meeting based diet and exercise plans such as Weight Watchers are more effective than the go-it-alone approach. In a group plan, you are attending meetings with your friends or making new friends at the meetings.

The fact that close friends who live far away has a greater effect than close neighbors that you don't know well is also very important. This means that it is not local restaurants or foods that are to blame for high obesity rates in certain cities or regions.

The study ruled out the possibility that the effects were simply caused by obese people forming friendships with other obese people. This was done by observing people who were not obese when they became friends. When one friend became obese the other gained weight later. For example, say two people who were not obese met in high school and became close friends. If one of the friends became obese 5 years later, the other might gain weight several years later.

The study doesn't provide any conclusive evidence as to exactly why all this is true. However, one possibility is that having overweight friends makes you accept being overweight yourself.

Here is a link to a summary of the study of the spread of obesity in a social network at The New England Journal of Medicine web site.

Here is a link to an animation showing how obesity spread in the study group. (Very cool)