Effectiveness and Safety of Low-Carbohydrate Diets

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The summary below is from the full report titled "A Low-Carbohydrate, Ketogenic Diet versus a Low-Fat Diet To Treat Obesity and Hyperlipidemia. A Randomized, Controlled Trial." It is in the 18 May 2004 issue of Annals of Internal Medicine (volume 140, pages 769-777). The authors are W.S. Yancy Jr., M.K. Olsen, J.R. Guyton, R.P. Bakst, and E.C. Westman.

What is the problem and what is known about it so far?

For many years, low-fat diets have been the standard recommendation for weight reduction. Recently, low-carbohydrate diets have gained popularity despite the relative lack of experimental data about their long-term effectiveness or safety.

Why did the researchers do this particular study?

To compare the effects of a low-fat diet with those of a low-carbohydrate diet with regard to weight loss and levels of fat in the blood in a large group of people over a prolonged period.

Who was studied?

120 relatively healthy, mildly to moderately obese people with elevated levels of fat (high levels of cholesterol, low-density lipoprotein cholesterol, or triglycerides) in the blood were recruited from the community.

What did the researchers do?

Participants were randomly assigned to a low-carbohydrate diet or a low-fat diet. Both groups were given diet instructions and exercise recommendations. Twice a month, the participants attended group meetings at which they received additional dietary instructions and psychological support while the researchers collected measurements of their physical condition. All participants bought and prepared their own food according to the instructions they had received. The low-carbohydrate diet group also received vitamins and other nutritional supplements. The low-fat diet group was instructed to eat 500 to 1000 calories less than they needed to maintain their usual weight. The researchers measured each person's weight; they also determined how closely each person had followed the diet by listening to what they said they ate and by examining food records they had kept. Because a low-carbohydrate diet changes chemicals in the urine, urine was tested to assess adherence to the diet. In addition, levels of fat in the blood were periodically measured.

What did the researchers find?

76% of persons in the low-carbohydrate diet group and 57% of persons in the low-fat diet group completed the study. During the 24 weeks of the study, average body weight dropped by 12 kilograms in the low-carbohydrate diet group compared with 6.5 kilograms in the low-fat diet group. The low-carbohydrate diet group had more beneficial changes in blood triglyceride levels and high-density lipoprotein cholesterol (good cholesterol) levels than did the low-fat diet group. However, 2 persons in the low-carbohydrate group dropped out of the study because of a rise in the low-density lipoprotein cholesterol (bad cholesterol) level. Also, the low-carbohydrate group reported more adverse effects (such as constipation, headache, bad breath, muscle cramps, diarrhea, general weakness and rash) than the low-fat group.

What were the limitations of the study?

Nutritional supplements given only to the low-carbohydrate group may have affected the study results. The effects of the low-carbohydrate diet beyond 24 weeks cannot be determined from this study.

What are the implications of the study?

Low-carbohydrate diets seem to be effective and relatively safe. However, people on this diet must be monitored for harmful elevations of bad cholesterol.

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Melanie R. Polk

Annals Online, 24 May 2004 [Full text]