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In the last few decades, the number of Americans who are obese has more than doubled. Particularly troubling is the number of children who are overweight or obese.
Joanne Ikeda is with UC Berkeley's Center on Weight and Health.
Joanne Ikeda/ UC Berkeley Nutrition Expert: "CHILDHOOD OBESITY IS A VERY SERIOUS PROBLEM. ABOUT 20 PERCENT OF CHILDREN ARE AFFECTED."
That extra weight puts kids at increased risk of everything from diabetes to high blood pressure and heart disease.
The latest Journal of the American Medical Association says prevention is the best way to tackle the problem.
The researchers say once a child or adolescent has a weight problem, it is difficult for them to lose that excess weight. That means more physical activity, less time in front of the TV or computer, and healthier meals.
The same findings apply to adults.
Another study in the journal compared people signed up for a commercial weight loss program against those trying to lose weight on their own.
They found that over two years, those in the weight loss program maintained a modest weight loss of about six pounds. Those trying to go it alone lost less than a pound.
It may be that the support system the weight loss program offered made it more effective than self help.
As for those low-carb diets, another study says there is not enough evidence to show they work.
Researchers at Stanford Medical Center examined more than 100 studies over almost 30 years. They found no evidence that low carb diets were any more effective in the long run than high carb diets.