Americans are losing weight, but only by a very thin margin, according to the results of a Gallup-Healthways Well-Being Index.

The index used height and weight information provided by respondents to calculate body mass index (BMI) scores.

Researchers found a slight decrease in 11 of 13 demographic groups this year.

Blacks, adults aged 45 to 64, and those whose annual income is less than $36,000 all saw decreases but still held the top three spots for obesity levels. 

Americans whose annual income is $90,000 or more and those aged 18 to 29 had two of the three lowest obesity levels.

Asians saw an increase in weight (8.2 percent in 2010 to 11.5 percent in 2011) but still had the lowest obesity rate of all groups.

Women, as well as people in the 65 and older crowd, both have an obesity rate of 24.6 percent. The percentage for the latter group remained the same as it was in 2010.

For the first time in more than three years, more Americans are a normal weight (36.6%) than are overweight (35.8%), Gallup said on its Web site.

Gallup researchers are not sure why weight levels saw a decrease, but they suggested a growing awareness of obesity-related health problems, rising healthcare costs, and health initiatives such as First Lady Michelle Obama's Let's Move! campaign.

Still, 61.6 percent of Americans are either overweight or obese, the study noted.

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. and among demographic subgroups, Gallup said on its Web site. Still, future monitoring is needed to determine whether this is a temporary trend.