Health experts say about a third of Americans are obese and at risk of developing type 2 diabetes, heart disease and many types of cancer, USA Today reported.
Harvard scientists studying the obesity epidemic have applied an infectious-disease mathematical model to data from recent obesity studies.
"We find that having four obese friends doubled people's chance of becoming obese compared to people with no obese friends," researcher Alison Hill, the study's lead author, says.
The more obese people you come in contact with, the greater the chance of your becoming obese, Harvard researcher David Rand says.
Researchers say they aren't sure why this is so, but it may be that if you have many acquaintances with unhealthy eating habits, you wind up with similar eating habits, Rand says.
Other lifestyle factors such as diet and physical activity have a huge impact on weight, Rand says, but the lesson of this research is "it's in your best interest to help your friends lose weight."