Low-Carbohydrate Diet vs. Low-Fat Diet: No Differences After 1 Year

Ads touting the benefits of a low-carbohydrate diet are everywhere—on restaurant menus, and in supermarkets, bookstores, and even bakeries. But, does a low-carbohydrate diet really work? A one-year study comparing the outcome of a low-carbohydrate diet and a conventional weight loss diet has enhanced our understanding of the benefits and drawbacks of a low-carbohydrate diet (Ann Intern Med 2004; 14:778).

Linda Stern, MD, and colleagues at the Philadelphia Veterans Affairs Medical Center randomized 132 obese adults with a body mass index of 35 kg/m2 or greater to one of two groups—a low-carbohydrate diet or conventional low-fat diet. In the first group, participants restricted their daily carbohydrate intake to <30 g. The low-fat diet group restricted their caloric intake by 500 kcal/day with fewer than 30% of calories from fat. The researchers then measured changes in weight, lipid levels, glycemic control, and insulin sensitivity.

Results at one year

After one year, the mean weight change for persons on the low-carbohydrate diet was –11.2 lb compared with –6.8 lb for persons on the conventional low-fat diet. In addition, those who were on the low-carbohydrate diet had greater decreases in triglyceride levels and smaller decreases in high-density lipoprotein (HDL) cholesterol levels. Among a subgroup with diabetes (54 subjects), hemoglobin A1c levels were more improved among those on the low-carbohydrate diet. These factorable metabolic responses to the low-carbohydrate diet remained significant after adjustment for weight loss differences.

Caloric intake

Persons in the low-carbohydrate diet group decreased their caloric intake more than did the conventional diet group, although this was not statistically significant. The low-carbohydrate group reduced their carbohydrate intake by 52%, reduced their fiber intake by 42%, increased total fat intake by 31%, increased dietary cholesterol intake by 32%, and reduced sodium intake by 21% compared to baseline levels.

Study participants on the low-carbohydrate diet maintained most of their 6-month weight loss, whereas those on the conventional diet continued to lose weight throughout the year. The difference in weight loss was not significant between the two groups. Those on the low-carbohydrate diet who dropped out lost less weight than those who completed the study. In comparison, weight loss was not significantly different from those on the conventional diet, whether they dropped out or completed the study.

Serum lipid levels

Changes in total and LDL cholesterol were not significantly different between the two groups, but triglyceride levels did decrease more in the low-carbohydrate group than in the conventional diet group. The HDL cholesterol concentration decreased more in the conventional diet group than in the low-carbohydrate group at one year. The difference in mean HDL cholesterol response between diet groups remained significant after adjustments for both baseline variables and weight loss, suggesting that there were direct diet-related effects on HDL cholesterol.

Dr. Stern reported there was no significant difference in overall weight loss between the two groups, and in contrast to findings in an earlier study (N Engl J Med 2003;348:2082), persons on the low-carbohydrate diet maintained most of their initial weight loss, whereas those on the conventional diet continued to lose weight. The authors also reported that despite speculation that a low-carbohydrate diet would increase weight loss by promoting the metabolism of adipose tissue, their data seem to show that weight loss differences may be explained by lower caloric intake on a low-carbohydrate diet. If this is true, it may be due to the simplicity of a low-carbohydrate diet or to greater effects on satiety. They point out that those on the low-carbohydrate diet who dropped out of the study were less likely to lose weight, whereas those assigned to the conventional diet lost a similar amount of weight whether or not they remained in the study.

Adverse reactions

Three participants in the low-carbohydrate group had adverse reactions. One was hospitalized with noncardiac chest pain in the third month of the study. Two others died, including one who died of complications of hyperosmolar coma 5 months into the study. Another person had severe ischemic cardiomyopathy and died suddenly 10 months after enrolling in the study. Laboratory studies 14 days before this person’s death showed no electrolyte abnormalities. Among the other participants, changes in serum creatinine concentrations did not differ significantly between the two groups; however, blood urea nitrogen levels increased more in the low-carbohydrate diet group.

The authors noted several important limitations of their study: the overall weight loss was modest and the dropout rate was high. Most of the participants did not meet their dietary targets (i.e., <30 g of carbohydrate in the low-carbohydrate group and reducing 500 kcal per day in the conventional diet).

Though weight loss was similar in the groups, the authors found that the low-carbohydrate diet had a direct and more favorable effect on triglyceride levels, HDL cholesterol level, and glycemic control among the small subgroup of diabetic patients. According to the authors, future studies will be needed to evaluate the long-term effects of a low-carbohydrate diet on the development of diabetes and cardiovascular outcomes.