Americans will keep growing fatter until 42 percent of the nation is considered obese, and having fat friends is part of the problem, researchers said.

The prediction by a team of researchers at Harvard University contradicts other experts who say the nation's obesity rate has peaked at 34 percent of the U.S. population.

The finding is from the same group, led by Nicholas Christakis, that reported in 2007 that if someone's friend becomes obese, that person's chances of becoming obese increase by more than half.

They now think this same phenomenon is driving the obesity epidemic, which will climb slowly but steadily for the next 40 years.

Alison Hill, a graduate student at Harvard and the Harvard-Massachusetts Institute of Technology Division of Health Sciences and Technology, said the study is based on the idea that obesity can spread like an infectious disease and people can catch it from their friends.

For the study, she and colleagues applied a mathematical model to four decades of data from the long-running Framingham study -- a study of the health and habits of nearly an entire town in Massachusetts.

"We looked at the probability of becoming obese and what that was influenced by," Hill said in a telephone interview.

"We found there is some baseline risk of becoming obese based on the friends you have," Hill said.

Hill said that based on their calculations and looking at the influence of social interactions on obesity in the Framingham study, they think the U.S. obesity rates will top out at 42 percent of the population.

Over the long-running study, the rate of weight gain caused by social interaction -- a person's contact with friends who are obese -- has grown quite rapidly since 1971, Hill said.

"It looks like obesity is becoming more infectious," said Hill. The findings are reported in the Public Library of Science journal PLoS Computational Biology.

Multiple Social Contacts
In the study, the team found that an American adult has a 2 percent chance of becoming obese in any given year, and each obese social contact increases the risk of becoming obese by 0.5 percent per year.

And while having fat friends increases the odds that you will become fat, befriending thin people does not appear to help you lose weight.

"We didn't find having more healthy-weight friends made it more likely to help people lose weight. It fits in with this idea of thinking about it as an infectious disease. You don't really catch healthiness," she said.

Hill said the findings are the best-case scenario based on current assumptions, but changes in public policy could make a difference.

U.S. government researchers in January said 68 percent of U.S. adults are considered overweight, having a body mass index or BMI of 25 or higher, and a third are obese, having a body mass index of 30 or higher.

BMI is equal to weight in kilograms divided by height in meters squared. A person 5 feet 5 inches tall is classified as overweight at 150 pounds (68 kg) and obese at 180 pounds.

Being overweight or obese raises the risk of heart disease, diabetes, some cancers, arthritis and other conditions.

Obesity-related diseases account for nearly 10 percent of medical spending in the United States or an estimated $147 billion a year.