SUMMARIES FOR PATIENTS

Weight Loss and Results of Low-Carbohydrate Diets

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The summary below is from the full report titled "The Effects of Low-Carbohydrate versus Conventional Weight Loss Diets in Severely Obese Adults: One-Year Follow-up of a Randomized Trial." It is in the 18 May 2004 issue of Annals of Internal Medicine (volume 140, pages 778-785). The authors are L. Stern, N. Iqbal, P. Seshadri, K.L. Chicano, D.A. Daily, J. McGrory, M. Williams, E.J. Gracely, and F.F. Samaha.

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

Low-carbohydrate diets are popular among people who want to lose weight. Many people believe that more weight can be lost with low-carbohydrate diets than with diets that restrict calories and fat. A concern is that low-carbohydrate diets might have an unfavorable effect on metabolic factors such as cholesterol level and other types of fats in the blood that are linked to heart disease. However, low-carbohydrate diets may be beneficial for other metabolic factors, such as blood sugar, that are linked to diabetes. In 2003, these researchers reported that adults assigned to a low-carbohydrate diet lost more weight after 6 months than did adults on a low-fat diet. The low-carbohydrate diet group also had greater improvements in metabolic measures than did the low-fat diet group. Other studies suggest that people on low-carbohydrate diets regain weight by 1 year.

Why did the researchers do this particular study?

To determine the effect of a low-carbohydrate diet on weight and metabolic factors after 1 year.

Who was studied?

132 severely overweight adults. To participate in the study, people had to have a body mass index (BMI) of at least 35 kg/m². Body mass index is a measure of the appropriateness of a person's weight. It is calculated by dividing weight in kilograms by the square of height in meters. A BMI calculator is available from the National Heart, Lung, and Blood Institute (http://www.nhlbisupport.com/bmi). Normal BMI is 18.5 kg/m² to 24.9 kg/m². People with BMIs of 25 kg/m² to 29.9 kg/m² are overweight, and people with BMIs of 30 kg/m² or more are obese.

How was the study done?

The researchers assigned patients to either a low-carbohydrate diet or a conventional low-fat weight loss diet. They instructed the low-carbohydrate group to eat no more than 30 grams of carbohydrate per day. The conventional diet group received instructions to decrease calorie intake by 500 calories per day and have less than 30% of total calories from fat. The researchers collected information about weight at the start of the diet, after 6 months, and after 1 year. They collected blood specimens to test the levels of metabolic factors.

What did the researchers find?

After 1 year, patients in the low-carbohydrate group had more favorable levels of 2 types of blood fats (triglycerides and high-density lipoprotein cholesterol) than did patients on the conventional diet. However, weight loss and the other metabolic factors were similar in the 2 groups.

What were the limitations of the study?

Because the study included only severely overweight people, results might not apply to people with less severe weight problems. In addition, many people dropped out of the study by 1 year.

What are the implications of the study?

After 1 year, the low-carbohydrate diet provided no weight loss advantage over the conventional, low-fat diet. However, a low-carbohydrate diet may modestly improve some, but not all, metabolic factors. It is unknown whether these improvements will influence the future development of diabetes and cardiovascular disease.