But Paul Ernsberger, an obesity researcher at Case Western Reserve University School of Medicine in Cleveland, said that there are many reasons why people become fat and that friendship may be a marker for social forces the researchers were not able to measure.
He said body weight varies by social class and religious denomination, so it was possible that friends who got fat in the study were members of a larger group, such as Baptists, with a greater tendency for becoming overweight.
Discrimination might also explain the friendships between fat people, he said.
"Thin people may be excluding overweight and obese people from their social networks," he said.
"What they are showing is birds of a feather flocking."
However, Christakis said, the dynamic of spreading obesity was more complex because friends weren't getting fat at the same time.
Christoffel, the Northwestern researcher, said the report suggests diet and exercise plans focused on obese individuals would not be as effective as interventions aimed at networks of overweight relatives and friends.
Obesity treatment programs should move away from their emphasis on individual willpower, she said.
"The truth is almost no one can do it own their own," she said.