People who go on low-carbohydrate diets typically lose weight, but it might not be because the diet is better than others, according to a new study.

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Low-carb dieters lose weight mostly because of their restricted caloric intake and longer diet duration, according to researchers from Stanford University Medical Center and collaborators at Yale University.

The sweeping review of literature on this popular diet also found there are no short-term adverse effects of the diet, but also that there is insufficient evidence on the diet's long-term effects and impact on people over 53.

"Low-carbohydrate diets have been extremely popular as of late, and the lay press has suggested they're a safe and effective means of weight loss," said lead researcher Dr. Dena Bravata, social science research associate at Stanford's Center for Primary Care and Outcomes Research. "While these diets are effective in the short term, weight loss results from reduced calories, not carbohydrate restriction."

The study appears in Wednesday's issue of the Journal of the American Medical Association.

Books on low-carbohydrate/high-protein diets -- such as the popular Atkins diet -- have sold in the millions, and proponents say these diets cause rapid weight loss without adverse side effects.

But numerous medical associations and physicians have expressed concern that these diets are too high in fat and can lead to kidney and liver problems and other health risks.

Despite their popularity -- and the concern of some in the medical community -- Bravata said little evidence exists on the efficacy and safety of low-carbohydrate diets.

The research team collected literature on low-carbohydrate diets published between 1966 and 2003. They reviewed a total of 107 diet studies, which involved 3,268 people from around the world. Although the studies varied, they had two things in common: None of the studies had participants with a mean age over 53, and none of them lasted longer than 90 days.

"Information on older adults and long-term results are scarce at best, and this should be kept in mind when looking at our findings," Bravata said.

The researchers found that people on diets of 60 or fewer grams of carbohydrates a day did lose weight. But the weight loss was associated with restriction of caloric intake and longer diet duration, not with reduced carbohydrate intake.

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

The researchers found no significant adverse effects on cholesterol, glucose, insulin and blood-pressure levels among participants on the diets. But, Bravata stressed, the adverse effects may not have shown up within the short period of the studies. She also said losing weight typically leads to an improvement in some of these levels, so this could have had an impact on the numbers.

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