By Roy Strom

CHICAGO (Reuters) - Some Americans are losing weight resulting in more people of "normal weight," according to a new study, but it is not clear if the trend will last.

The Gallup-Healthways Well-Being Index found that for the first time in three years there are more normal weight Americans than those in the overweight category.

But the majority of Americans are at an unhealthy weight, the study said. Obese and overweight Americans combined make up more than 60 percent of the population, according to the study.

"Although the majority of Americans are still overweight or obese, it is an encouraging sign that obesity rates are trending downward in the U.S.," the study said.

The study found the percent of normal weight Americans in the third quarter of 2011 to be 36.6 percent, while the number of overweight people came in at 35.8 percent. Obese Americans make up 25.8 percent of the population.

Obesity has become a major problem in the U.S. over the last decade.

In 2000, no U.S. state had an adult obesity rate higher than 30 percent, according to Centers for Disease Control data. In 2010, there were 12 states at that level, and another eight poised to join them with adult obesity rates of 28 percent or higher.

The study said it was not clear what caused the drop in overweight and obese Americans, but said it could be due to the tough economy, with cash-strapped Americans choosing to eat in rather than eating at high-calorie restaurants.

Another reason could be increased public awareness campaigns such as Michelle Obama's "Let's Move!" anti-obesity campaign, and business initiatives, that prompted more health-conscious decisions.

A downward trend in obesity rates could also mean a drop in U.S. health care costs, the study said. The Centers for Disease Control estimated medical costs associated with obesity were $147 billion in 2008.

The study used self-reported data of height and weight to determine a score of body mass index. A score of 30 or higher was classified as "obese," while 25.0 to 29.9 was "overweight." A score from 18.5 to 24.9 is classified as "normal weight." Scores below 18.5 were "underweight."

The study conducted telephone interviews with a random sample of 90,070 people aged 18 and older from July to September of this year, and had a margin of error of plus or minus one percent.

(Editing by Greg McCune)