By Tim Seyfert

As hordes of fad diets, exercise trends and nutritional supplements flood the U.S. fitness market, one might be quick to label the United States a nation of health nuts.

Not quite, according to recent studies.

Despite the booming fitness industry, it seems more than half of American adults still can’t seem to keep those extra pounds away.

Federal officials recently declared obesity a national epidemic, with more than 60 percent of American adults overweight and 30 percent considered obese, according to the Journal of the American Medical Association. The prevalence of obesity has also more than tripled among teen-agers since the early 1980s.

So what are Americans doing wrong?

The journal devoted its April 9 edition to the growing epidemic, examining popular diets and other supposed slim-down solutions. The findings concluded that while there are indeed many gimmicks that promise to shed pounds, there’s still no substitute for exercising and cutting calories when it comes to long-term success.

In one research review from Stanford University, Dr. Dena Bravata found that results from trendy low-carbohydrate diets, such as the Atkins diet, resulted primarily from reducing calories and not just cutting carbohydrates. The group found no evidence suggesting that low-carb diets are “independently associated” with greater weight loss compared to high-carb diets.

“The greatest predictors of weight loss appear to be caloric intake and diet duration,” Bravata wrote. “The findings suggest that if you want to lose weight, you should eat fewer calories and do so over a long time period.”

Inactivity is another factor contributing to nationwide obesity. A study out of Harvard University found that among more than 50,000 women, each two hours of increased television watching was associated with a 23 percent increase in obesity. Researchers wrote that watching television boosts the risk of weight gain because it displaces physical activity and encourages unhealthy eating habits.

On the other hand, watching fewer than 10 hours of television per week and performing some kind of low-impact exercise daily reduced the risk of obesity by around 30 percent.

“It’s simple: If you want to lose weight, you have to get active. There’s no way around it,” said Greg Fischer, a trainer at 24 Hour Fitness in Mountain View. “It doesn’t take much (exercise), but it does take something.”

Looking at weight-loss drugs, participants in a Duke University study took the epilepsy medication zonisamide daily for 16 weeks and lost an average of 13 pounds, compared to 2 pounds in patients given placebos. Both groups were also encouraged to exercise and reduce their caloric intake.

Despite zonisamide’s slimming effects, fitness experts warn that medication to treat obesity is almost always used as a last resort and reserved mostly for severe cases.

Portola Valley physical trainer Brian Bettendorf said people should also be wary of over-the-counter “miracle formulas.”

“These ‘fat trappers,’ ‘exercise in a bottle,’ diet pill manufacturers don’t care about your health,” said Bettendorf, co-founder of Pacific THERx, a physical training and therapy clinic.

Bettendorf, who also teaches kinesiology at San Jose State University, said that dieters are just going to have to accept that there’s no quick fix for losing weight.

“If people can increase their cumulative physical activity and cut back on their food portion sizes, the epidemic of obesity and its related health problems will be drastically reduced,” he said.

For more information, logon to the Journal of the American Medical Association’s Web site, www.jama.ama-assn.org.