Low-carb diets effective, but may raise cholesterol levels

A synthesis of data from five previous clinical trials suggests that both low-fat and low-carbohydrate diets appear to be effective for weight loss up to one year, but low-carbohydrate diets may be linked to higher overall and LDL or "bad" cholesterol levels, according to a study in the Archives of Internal Medicine.

As obesity levels increase, more American adults are dieting--at any one time, 45 percent of women and 30 percent of men are trying to lose weight, according to background information in the article. Those who succeed may reduce their risk of type 2 diabetes, control their hypertension and decrease their chances of cardiovascular disease and related death. Low-carbohydrate, high-protein diets have become a popular alternative to the generally recommended low-fat, calorie-restricted diet, the authors report. However, because these diets contain large amounts of protein and fat, concern remains about their effect on cholesterol levels and the cardiovascular system, they write.

Alain J. Nordmann, M.D., M.Sc., University Hospital Basel, Switzerland, and colleagues analyzed five previous clinical trials that compared low-fat to low-carbohydrate diets. A total of 447 individuals with an average age ranging from 42 to 49 years participated in the studies--222 on low-carbohydrate diets and 225 on low-fat diets.

After six months, those on low-carbohydrate diets were more likely to remain on the diet and had lost more weight than those on low-fat diets. However, after 12 months, blood pressure, completion rates and weight loss were the same for both groups. After six and 12 months, individuals on low-carbohydrate diets had increased total cholesterol levels and LDL levels. However, they also had lower triglyceride levels and higher HDL or "good" cholesterol levels.

"We believe there is still insufficient evidence to make recommendations for or against the use of low-carbohydrate diets to induce weight loss, especially for durations longer than six months," the authors write. "The differences in weight loss between low-carbohydrate and low-fat diets after 12 months were minor and not clinically relevant."

Because no trials have yet examined the risk of heart attack or death in people on low-carbohydrate diets, it's unclear whether the beneficial effects low-carbohydrate diets appear to have on HDL and triglyceride levels cancel out their apparent negative effects on overall and LDL cholesterol levels, the authors write. "In the absence of evidence that low-carbohydrate diets reduce cardiovascular morbidity and mortality, such diets currently cannot be recommended for prevention of cardiovascular disease," they conclude.