A sweeping analysis of studies dating back to the 1960s suggests people on low-carbohydrate regimens such as the Atkins diet lose weight merely because of the reduction of calories, not some miraculous metabolic process.
The analysis also found insufficient evidence to recommend such diets, especially for longer than three months or for people older than 50.
"There's nothing magical about carbohydrates," said Dena Bravata, a Stanford University social science researcher and lead author of the study. "Low-carb diets are effective in the short run, but it's because people on them are consuming less calories."
The review, the first of its kind, appears in today's issue of the Journal of the American Medical Association. It analyzed 107 articles containing data on 3,268 participants.
Advocates of low-carb, high-protein diets have long claimed they promote rapid weight loss because they force the body to metabolize fat for energy instead of carbohydrates. Cardiologist Robert Atkins, the best-known advocate of the philosophy, has sold millions of books promoting the view.
Most medical organizations, such as the American Dietetic Association and the American Heart Association, dismiss such regimens as dangerous. They say they could lead to kidney and liver problems because of their high-fat content.
The Atkins diet, which was pioneered in the 1970s but fell out of favor, has had a recent resurgence in popularity, thanks partly to a supportive article in the New York Times Magazine and a handful of short-term studies that found people lost more weight on it than on the standard low-fat, high-carb approach recommended by most health organizations.
Moreover, said those studies, they did so without driving up cholesterol, triglycerides, blood pressure and other signs of heart disease risk.
But Bravata's analysis of all existing studies found little evidence exists on the efficacy and safety of low-carb diets. Only five of the studies evaluated diets lasting longer than 90 days, and Bravata said adverse effects may not have shown up within the short period of the studies. She noted that losing weight typically leads to an improvement in some heart disease risk levels.
In a written statement, Atkins applauded the study for not finding evidence to support claims his diet might have serious medical consequences but criticized it for "likely" not including results from studies just out and soon to be published. The analysis covered every low-carb study done between 1966 and February 2003.
"We think this review raises questions of why so many opponents are unwilling to recognize emerging science and continue with biased claims about the safety of their approach," said Dr. Stuart Trager, a Philadelphia-based surgeon and Atkins consultant. "In this epidemic of obesity, we need to give people more tools to lower their weight, not ideological beliefs."
Based on the average 2,000-calorie diet, a person should consume about 300 grams of carbohydrates daily, according to U.S. Recommended Daily Allowances figures. The threshold for some of today's popular low-carb diets, such as Atkins, is 25 grams. Carbohydrate-rich foods include but are hardly limited to bread, pasta and rice.
Emphasizing that there is insufficient evidence that the Atkins diet is safe or unsafe, Bravata acknowledged that people having success on it might continue it as long as they do so under monitoring by a doctor. She said that for some people, Atkins is clearly "an effective strategy."
And in an accompanying editorial, Dr. George Bray of Louisiana State University in Baton Rouge wrote, "Although the truth of 'a calorie is a calorie' has been reaffirmed by the systematic review by Bravata, the question of whether patients can adhere more easily to one type of diet than another remains to be answered."
But Rebecca Reeves, a dietician and director of Baylor College of Medicine's Behavior Medicine Research Center, said there has to be "a safer way than eliminating food groups like fruit, vegetables and grains" carbohydrates limited by the Atkins diet.
She said she'll welcome results of a National Institutes of Health study about to begin that will compare health benefits of people on low-carb diets and standard approaches over a five-year period.
The JAMA issue contained numerous studies on obesity. One found that watching too much television increased women's risk of obesity and diabetes after accounting for exercise, diet, smoking and age, each two-hour-per-day increment of TV watching was associated with a 23 percent increase in obesity risk and a 14 percent increase in diabetes risk.