San Francisco ? Youngsters who skimp on milk and other dairy food to avoid calories actually appear to substantially increase their risk of becoming overweight, a study found.

Several reports in recent years have shown health benefits of dairy products, despite their fat content. The latest research shows an unusually striking effect on weight as children go through their teens.

Pediatricians say too much weight is now the most common medical condition of childhood. The problem has doubled over the past two decades, and about 15 percent are now considered overweight or obese.

While the overall cause is too much food and too little exercise, many studies are attempting to tease apart the precise changes in habits that are driving this health hazard. Several were reported Thursday at a meeting in San Francisco of the American Heart Assn.

Lynn Moore, an epidemiologist at Boston University School of Medicine, found that just two servings of dairy food a day are linked to a substantial reduction in adolescent fatness.

Childhood dairy intake has been failing for the last 20 years, in part as youngsters’ preferences have switched from milk to soft drinks. During this time, soda consumption has risen by 300 percent.

Another factor, though, has been fat phobia. Youngsters “consume less and less as they get older,” Moore said. “Adolescent girls in particular are concerned about eating dairy because they think it will make them fat.”

However, her research, based on the Framingham Children’s Study, found just the opposite is true.