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Bravata says those findings show that much more research is needed to evaluate the long-term safety of lower-carbohydrate and very low-carbohydrate diets as well as their effect on older people who may have other health issues aside from obesity.

Researchers found that among obese people who participated in the studies, successful weight loss was linked to limiting calorie intake and longer diet duration, but not with limiting the amount of carbohydrates they ate.

But Bravata says they found no evidence that these high-protein, low-carbohydrate diets were unsafe over the short term. The studies showed the diets had no significant effect on cholesterol, blood sugar and insulin, and blood pressure levels.

And Bravata says they were also not able to evaluate the role exercise or ethnicity may have played in the success or failure of the low-carb diets in promoting weight loss.

In an editorial, George A. Bray, MD, of Louisiana State University in Baton Rouge, says this study shows once again that "a calorie is a calorie," and low-carbohydrate diets induce weight loss by reducing calorie intake.

The first low-carbohydrate diet was introduced by William Banting in 1863. Since then, Bray says, the diets have become a persistent theme and "cash cow" for authors and publishers of diet books for the last 140 years because they produce quick weight loss, "something prized by dieters and diet promoters alike."

But the quick weight loss induced by low-carbohydrate diets is largely due to excessive urination. After seven to 14 days of this, Bray says the rapid phase of weight loss slows.

Bray says the more important point is that, "Diets do not cure obesity. If they did, Banting's diet would have eliminated overweight and obesity and made the need for new diet revolutions unnecessary."

He says the question of whether a unique diet exists that will produce lasting weight loss has yet to be evaluated, and long-term studies to examine this are needed to address this issue.

"Given the increasing prevalence of obesity, studies such as these deserve the highest priority," Bray concludes.

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