Two or three fast-food meals among the typical 21 per week don’t sound like much but, over time, they add up.

After 15 years, healthy, young adults who ate fast-food meals two or more times a week gained approximately 10 more pounds and had twice the increase in insulin resistance compared with people who ate fast food less than once per week, researchers have found. (Insulin resistance is a condition that can precede diabetes and heart disease.)

The study appeared in the Jan. 1 issue of the Lancet. And last week health experts had much more to say about body weight and dieting. One study found that several popular diets seem to work, but many people have trouble sticking to them. Another study showed that children begin to experience small declines in their quality of life as soon as their weight exceeds average — a decline that continues the more weight they gain.

The Lancet study is the latest to indict fast food as a significant factor in the high rates of obesity in the United States. It found that independent of what else the subjects ate or how much they exercised, eating fast food regularly made a big difference in health. Those who ate fast food frequently — twice or more per week — gained about 36 pounds over 15 years compared with 26 pounds in the group that mostly avoided fast food.

Researchers aren’t sure why fast food seems so bad for health, but possible explanations abound. Among them, that the meals are packed with calories, are deficient in fiber and other nutrients, and are high in trans fat, saturated fat, sugar and starch; that people overeat fast food or eat the food too quickly; and that the large soft drinks that often accompany the meals cause health problems.

“The value meals, which are heavily marketed, pack an unusual number of calories — 50 percent to 100 percent of a person’s daily energy needs,” such as 1,000 or more calories per meal, Pereira said. A previous study in children showed that kids who consume a fast-food meal don’t compensate for the extra calories by eating less the remainder of the day. And, he noted, studies on soft-drink consumption show that the more calories a person consumes from liquids, the more likely he or she is to overeat throughout the day.

Other diets

In other dieting research, scientists from Tufts-New England Medical Center in Boston assessed four popular diets — Weight Watchers, Atkins, Zone and Ornish — to see how they worked and how well people adhered to them. The study included 160 overweight adults who were assigned to one of the four diets. The volunteers were asked to adhere strictly to the diet for two months and then to select their own level of adherence for the remaining months.

All four diets resulted in modest weight loss for at least some of the participants after one year. In each group, about 25 percent of the initial participants had lost more than 5 percent of their body weight at the end of a year. However, none of the diets could boast of good adherence rates; in other words, many people went off the diets. Discontinuation rates were highest in the Atkins and Ornish diets, suggesting that individuals found those diets too extreme, the authors said.

People are likely to do best when they can select the diet that’s right for them, said Dr. Robert H. Eckel, of the University of Colorado at Denver, in an editorial accompanying the study.