Six-Month Study Shows Low-Carb Diet Is More Effective Than Low-Fat Diet

News Author: Laurie Barclay, MD
CME Author: Bernard M. Sklar, MD, MS

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April 29, 2003 — Women eating ad lib on a low-carbohydrate diet lost more weight than those on a calorie-restricted low-fat diet even though the calorie load was similar, according to the results of a randomized trial published in the April issue of the Journal of Clinical Endocrinology and Metabolism. There were no adverse effects from either diet during the six-month study period.

"Untested alternative weight loss diets, such as very low carbohydrate diets, have unsubstantiated efficacy and the potential to adversely affect cardiovascular risk factors," write Bonnie J. Brehm, PhD, and colleagues from the University of Cincinnati and Children's Hospital Medical Center in Ohio.

Of 53 healthy obese female volunteers (mean body mass index, 33.6 ± 0.3 kg/m2), who were randomized to six months of either an ad libitum very low carbohydrate diet or a calorie-restricted diet with 30% of the calories as fat, 42 women (79%) completed the trial.

Both groups reduced caloric intake by comparable amounts at three and six months. Compared with the low-fat diet group, the low-carbohydrate diet group lost more weight (8.5 ± 1.0 vs. 3.9 ± 1.0 kg; P < .001) and more body fat (4.8 ± 0.67 vs. 2.0 ± 0.75 kg; P < .01).

Mean levels of blood pressure, lipids, fasting glucose, and insulin were within normal ranges in both groups at baseline and improved during the study, but there were no differences between the two groups at three or six months. Beta-hydroxybutyrate increased significantly in the low-carbohydrate group at three months (P = .001).

The authors warn that these results should not be extrapolated to subjects with cardiovascular risk factors at baseline, and that longer studies are needed. This study suggested some recidivism during the last three months, and low calcium and fiber intake could prove to be problematic over longer periods.

"Based on these data, a very low carbohydrate diet is more effective than a low fat diet for short-term weight loss and, over 6 months, is not associated with deleterious effects on important cardiovascular risk factors in healthy women," the authors write. "There are important, interesting, and poorly understood effects of severe carbohydrate restriction that warrant further investigation as we seek effective therapeutic strategies to manage the epidemic of obesity."

J Clin Endocrinol Metab. 2003;88:1617-1623

Clinical Context

The incidence of obesity in the U.S. has risen continuously over the last several decades and is now estimated to be about 30%. Despite great efforts by obese patients to lose weight, there are currently no proven, effective approaches for long-term weight loss for most overweight individuals.

Mainstream strategies supported by the majority of physicians and dietitians emphasize restriction of fat intake; these strategies lead to only modest weight loss and poor long-term compliance.

There has been a recent resurgence in popularity of low-carbohydrate diets as a means of rapid weight loss. The very low carbohydrate, high-protein diet, promoted by Robert C. Atkins, MD, and others, is one of the most popular alternative weight loss approaches.

Dr. Atkins maintained that diets higher in protein and lower in carbohydrates promote the metabolism of adipose tissue and result in rapid weight loss without significant long-term adverse events. According to Dr. Atkins and others, severe restriction of dietary carbohydrate (<10% of daily caloric intake) results in ketosis, lipid oxidation, satiety, and increased energy expenditure, factors that should promote negative energy balance and weight loss.

But many have cautioned against low-carbohydrate diets. There has been concern that low-carbohydrate diets can cause accumulation of ketones and can result in abnormal metabolism of insulin and impaired liver and kidney function. Such diets may also lead to salt and water depletion, causing postural hypotension, fatigue, constipation, and kidney stones. Excessive consumption of animal proteins and fats could lead to hyperlipidemia; higher dietary protein loads could impair renal function.

Also, as studies that severely restrict carbohydrate intake have generally been of short duration, the long-term weight loss benefits of severe restriction of dietary carbohydrate have not been proven.

To try to clarify these issues, the present study attempted to evaluate the effects of a very low carbohydrate diet on weight loss and cardiovascular risk factors; it compares the results of six months of a very low carbohydrate diet with a low-fat diet conforming to the guideline currently recommended by the American Heart Association and other expert panels.

Study Highlights

53 healthy obese female volunteers with mean body mass index of 33.6 (± 0.3) kg/m2 were randomized to six months of either an ad libitum very low carbohydrate diet or a calorie-restricted diet with 30% of the calories as fat; 42 women (79%) completed the trial.

Both groups reduced caloric intake by comparable amounts at three and six months. Compared with the low-fat diet group, the low-carbohydrate diet group lost more weight (8.5 ± 1.0 vs. 3.9 ± 1.0 kg; P < .001) and more body fat (4.8 ± 0.67 vs. 2.0 ± 0.75 kg; P < .01).

Mean levels of blood pressure, lipids, fasting glucose, and insulin were within normal ranges in both groups at baseline and improved during the study, but there were no differences between the two groups at three or six months. Beta-hydroxybutyrate increased significantly in the low-carbohydrate group at three months (P = .001).

Pearls for Practice

Patients on an ad libitum very low carbohydrate diet lost significantly more weight and more body fat than those on a calorie-restricted diet with 30% of the calories as fat.

Mean levels of blood pressure, lipids, fasting glucose, and insulin were within normal ranges in both groups at baseline and improved during the study; there were no differences between the two groups at three or six months. Beta-hydroxybutyrate levels increased significantly in the low-carbohydrate group at three months (indicating ketosis), but they returned to normal at six months.