|May 17, 2004 Two randomized trials reported in the May 18 issue of the Annals of Internal Medicine further support the efficacy of a low-carbohydrate diet. A six-month trial showed higher participation and greater weight loss, and a 12-month study showed similar weight loss but better lipid profile than with a conventional diet. The editorialist uses findings from both studies to advise physicians and patients.
"This diet can be quite powerful. We found that the low-carb diet was more effective for weight loss," lead author Will Yancy, MD, from Duke University Medical Center and the Veterans Affairs Medical Center in Durham, North Carolina, says in a news release. "We also found cholesterol levels seemed to improve more on a low-carb diet compared to a low-fat diet."
In this study, which is the first randomized controlled trial of an Atkins-style diet, 120 study participants were assigned to a low-carbohydrate, high-protein diet or a low-fat, low-cholesterol, low-calorie diet.
The low-carbohydrate group (<20 g/day) was allowed daily unlimited calories, animal foods (meat, fowl, fish, and shellfish), and eggs, as well as 4 oz of hard cheese, two cups of salad vegetables (lettuce, spinach, or celery) and one cup of low-carbohydrate vegetables (broccoli, cauliflower, or squash). They also received daily nutritional supplements including a multivitamin, essential oils, an Atkins diet formulation, and chromium picolinate.
The low-fat, low-cholesterol, low-calorie group received less than 30% of daily caloric intake from fat, less than 10% of calories from saturated fat, and less than 300 mg cholesterol daily. They were advised to restrict daily calories by 500 to 1,000 calories less than their maintenance diet.
At study entry, all subjects were between 18 and 65 years of age and in generally good health, with a body mass index (BMI) between 30 and 60, and a total cholesterol level greater than 200 mg/dL. None had dieted or used weight loss medications in the previous six months. All subjects were encouraged to exercise 30 minutes at least three times per week and had regular group meetings at an outpatient research clinic for six months.
The study was completed by 76% of participants in the low-carbohydrate diet group and by 57% of participants in the low-fat diet group (P = .02). At six months, weight loss was -12.9% in the low-carbohydrate diet group and -6.7% in the low-fat diet group (P < .001).
In both groups, loss of fat mass (change, -9.4 kg with the low-carbohydrate diet vs. -4.8 kg with the low-fat diet) was greater than loss of fat-free mass (change, -3.3 kg vs. -2.4 kg, respectively). Compared with the low-fat diet group, the low-carbohydrate diet group had greater decreases in serum triglyceride levels (change, -0.84 mmol/L vs. -0.31 mmol/L [-74.2 mg/dL vs. -27.9 mg/dL]; P = .004) and greater increases in high-density lipoprotein [HDL] cholesterol levels (0.14 mmol/L vs. -0.04 mmol/L [5.5 mg/dL vs. -1.6 mg/dL]; P < .001).
Changes in low-density lipoprotein (LDL) cholesterol level were not significantly different between groups (0.04 mmol/L [1.6 mg/dL] with the low-carbohydrate diet and -0.19 mmol/L [-7.4 mg/dL with the low-fat diet; P = .2). However, participants in the low-carbohydrate diet group had more minor adverse effects, such as constipation and headaches, than did patients in the low-fat diet group.
Study limitations include inability to distinguish effects of the low-carbohydrate diet and those of the nutritional supplements provided only to that group, and use of healthy participants followed for only 24 weeks, limiting generalizability of the study results.
The authors are currently testing whether a low-carbohydrate diet can improve glycemic control in diabetes. However, they warn that patients with medical conditions such as diabetes and hypertension or who use diuretics should not begin a low-carbohydrate diet without close medical supervision, because the diet affects hydration and blood glucose levels. Nor do they recommend an Atkins-type diet for individuals attempting to lose weight for the first time.
"Over six months the diet appears relatively safe, but we need to study the safety for longer durations," Dr. Yancy says, noting potential long-term health risks including elevations in LDL cholesterol, bone loss, or kidney stones.
The Robert C. Atkins Foundation funded this research. The study authors have no financial interest in Atkins Nutritionals, Inc. In the second study, by Linda Stern, MD, from the Philadelphia Veterans Affairs Medical Center in Pennsylvania, and colleagues, 132 obese adults were randomized to receive counseling to either restrict carbohydrate intake to less than 30 g per day (low-carbohydrate diet) or to restrict caloric intake by 500 calories per day with less than 30% of calories from fat (conventional diet). At baseline, BMI was at least 35 kg/m2, and 83% of participants had diabetes or metabolic syndrome.
By one year, mean weight change was -5.1 ± 8.7 kg in the low-carbohydrate diet group and -3.1 ± 8.4 kg in the conventional diet group (difference, -1.9 kg; 95% confidence interval [CI], -4.9 to 1.0 kg; P = .20). The low-carbohydrate diet group fared better in terms of greater decrease in triglyceride levels (P = .044) and less decrease in HDL cholesterol levels (P = .025).
In the subgroup of 54 persons with diabetes, hemoglobin A1c levels improved more with the low-carbohydrate diet, but the difference was not statistically significant in sensitivity analyses. Both groups had similar changes in other lipids and in insulin sensitivity.
Study limitations include high dropout rate of 34% and suboptimal dietary adherence.
"Despite modest overall weight loss in both diet groups, assignment to the low-carbohydrate group had a direct and more favorable effect on triglyceride level, HDL cholesterol level, and glycemic control in the smaller subgroup of patients with diabetes," the authors write. "These findings give further evidence that restriction of carbohydrates in obese persons, who may be overconsuming carbohydrates at baseline, may have favorable metabolic effects. Caution is still needed, however, in recommending a low-carbohydrate diet, as important concerns remain."
The Veterans Affairs Healthcare Network supported this study. The authors report no potential financial conflicts of interest.
"We can no longer dismiss very-low-carbohydrate diets," Walter C. Willett, MD, DrPH, from the Harvard School of Public Health in Boston, Massachusetts, writes in an accompanying editorial.
"We can encourage overweight patients to experiment with various methods for weight control, including reduced-carbohydrate diets, as long as they emphasize healthy sources of fat and protein and incorporate regular physical activity," he concludes. "Patients should focus on finding ways to eat that they can maintain indefinitely rather than seeking diets that promote rapid weight loss. For many patients, the roll will have little role."
Ann Intern Med. 2004;140:769-777, 778-785, 836-837