Little Evidence to Support Low-Carbohydrate Diets

From this week's MEDSCAPE Medical News

Little Evidence to Support Low-Carbohydrate Diets

News Author: Laurie Barclay, MD
Clinical Reviewer: Gary Vogin, MD
CME Editor: Bernard M. Sklar, MD, MS

April 8, 2003 — Low-carbohydrate diets have been all the rage lately, but a systematic review published in the April 9 issue of The Journal of the American Medical Association reveals insufficient evidence to support or refute the concept thus far. The bottom line appears to be that a calorie is a calorie. The editorialist reviews the issue of adult weight loss.

"Recently, low-carbohydrate diets have resurged in popularity as a means of rapid weight loss, yet their long-term efficacy and safety remain poorly understood," write Dena M. Bravata, MD, MS, from the Center for Primary Care and Outcomes Research in Stamford, California, and colleagues. "Our results demonstrated the marked discordance between the knowledge needed to guide dietary choices and the information that is available in the medical literature."

Although millions of copies of three books on low-carbohydrate diets have sold in the U.S. over the past five years, the American Dietetic Association, the American Heart Association, and other professional organizations have warned the public against potentially serious medical consequences of these diets.

The investigators searched the literature for studies of low-carbohydrate diets published between 1966 and February 2003. They identified 107 articles reporting data on 3,268 participants, of whom 663 patients received lower-carbohydrate diets (60 grams of carbohydrates per day [g/d] or less), and 71 patients received the lowest-carbohydrate diets containing 20 g/d or less of carbohydrates, which is the recommended threshold for some of the most popular diets.

None of the studies evaluated diets of 60 g/d or less of carbohydrates in subjects with a mean age older than 53 years, and only five studies evaluated these diets for more than 90 days. weight loss in obese patients was associated with longer diet duration (P = .002) and restriction of calorie intake (P = .03), but not with reduced carbohydrate content. Low-carbohydrate diets had no significant adverse effects on serum lipid levels, fasting serum glucose levels, fasting serum insulin levels, or blood pressure.

"Our quantitative synthesis...on the efficacy and safety of low-carbohydrate diets suggests that there is insufficient evidence to make recommendations for or against the use of these diets," the authors write. "We found insufficient evidence to conclude that lower-carbohydrate content is independently associated with greater weight loss compared with higher-carbohydrate content.... Given the limited evidence in this review, when lower-carbohydrate diets result in weight loss, it also is likely due to the restriction of calorie intake and longer duration rather than carbohydrate intake."

The investigators recommend additional research evaluating the long-term effects and consequences of low-carbohydrate diets in both older and younger subjects with and without diabetes, high cholesterol, and hypertension.

None of the authors has financial or other conflicts of interest concerning low-carbohydrate diets or diet projects. No manufacturer or vendor of dietary goods or services funded this study.

In an accompanying editorial, George A. Bray, MD, from Louisiana State University in Baton Rouge, notes that the worldwide epidemic of obesity will be followed by a worldwide epidemic of diabetes.

"The broader issue of whether a unique diet exists that will produce long-term weight loss has yet to be evaluated," he writes. "Although the truth of 'a calorie is a calorie' has been reaffirmed by [this review], the question of whether patients can adhere more easily to one type of diet or another remains to be answered."

JAMA. 2003;289:1837-1850, 1853-1855

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