Fast-food meals are causing consumers to gain weight, according to a 15-year study on the health effects of the diet pattern.

Researchers in the U.S. followed 3,000 people aged 18 to 30 who were given medical checkups and asked about their diet, physical activity and lifestyle.

They found people who ate fast food more than twice a week gained an extra 4.5 kilograms compared to those who ate the high-calorie meals less than once a week.

Nutritionists say consumers would benefit from healthier options at fast-food eateries.

The frequent fast-food consumers also showed a doubling in insulin resistance, a disorder linked to diabetes.

Nutritionists have long said that fast food tends to be poor in nutrients, but few long-term studies have traced the effects of the dietary pattern, the researchers said.

Without such data, the fast-food industry claimed its products can be part of a healthy diet, said Dr. David Ludwig, director of the obesity program at Children's Hospital Boston and the senior author of the study.

After using statistical techniques to account for other factors, the researchers found "fast-food habits have strong, positive, and independent associations with weight gain and insulin resistance in young black and white adults," Ludwig said in a release.

The huge portions and high caloric density at most fast-food restaurants likely account for the obesity link, said Arne Astrup, an obesity expert at the Royal Veterinary and Agricultural University in Copenhagen, Denmark.

Since fast food can be high in calories, people may eat a lot without feeling full, Astrup said.

In a commentary accompanying the study in this week's issue of The Lancet, Astrup said consumers should ask for normal-sized portions, leaner meat in burgers, whole-grain bread, lower-fat fries, reduced-sugar soft drinks and more vegetables.

He said he hopes the trend toward offering healthier choices, such as fruit for dessert at some major fast-food chains, continues.