London ? A new study gives scientific clout to a conclusion many already see as obvious: Eating a lot of fast food makes you fat and increases the chance of developing diabetes.

A study published this week in the Lancet medical journal found those who frequently ate fast food gained 10 pounds more than those who did so less often, and were more than twice as likely to develop an insulin disorder linked to diabetes.

“Fast food is commonly recognized to have very poor nutritional quality,” said Dr. David Ludwig, director of the obesity program at Children’s Hospital Boston and the senior author of the study. “But there have been very few studies, essentially no long-term studies that have documented the effects of this dietary pattern on the key chronic diseases of Western civilization: obesity, Type 2 diabetes, heart disease.

“In the absence of such data, the fast-food industry continues to claim that fast food can be part of a healthful diet,” he said.

Ludwig’s U.S.-based team followed 3,000 young people enrolled in a study of cardiac health over 15 years, giving them checkups and asking questions about diet, physical activity and other factors.

Even after the scientists used statistical techniques to cancel out the effects of other factors, those who said they visited fast-food outlets twice a week or more gained 10 pounds more over the course of the study than those who ate fast food less than once a week.

They also had more than double the chance of developing insulin resistance, considered a predictor of Type 2 diabetes, the form of the disease linked to obesity.

Dr. Rudolph Leibel, an obesity expert at Columbia University in New York, said while the study was sound and its conclusions likely true, it was important not to demonize fast food as the sole cause of the obesity epidemic in wealthy nations.

Fast-food restaurants, he said, are responding to a real societal need — the inability of many families in which both parents work to find time to cook for themselves.