FEBRUARY 2007

LOW-CARBOHYDRATE eating plans, such as those popularized by the South Beach and Zone diets, don’t appear to raise the dieter’s risk of heart disease—despite higher intake of fat and protein. And those who replace carbs with vegetable protein and fat—rather than with protein and fat from animal sources—may actually reduce coronary heart disease risk.

That’s the conclusion of Thomas L. Halton, ScD, of the Harvard School of Public Health, and colleagues. They analyzed data from the long-running Nurses’ Health Study on 82,802 women over 20 years, including 1,994 new cases of heart disease. Their findings were published in the New England Journal of Medicine.

Using data from questionnaires on intake of more than 120 foods, the scientists divided subjects into 11 groups based on carbohydrate, protein and fat consumption as a percentage of total calories. Additional scores rated consumption of fat and protein calories from vegetables versus animal sources.

“Our findings suggest that diets lower in carbohydrates and higher in protein and fat are not associated with increased risk of coronary heart disease in women,” Halton concluded. Women whose diets were highest in vegetable protein and fat were about 30% less likely to develop heart disease than those who ate the most carbohydrates or those consuming the most animal protein and fat.

The researchers also found a significant association between heart-disease risk and both high dietary glycemic load and high overall glycemic index. The glycemic index measures the effect of food on blood glucose; generally, refined carbohydrates have a higher index than unprocessed grains or fruits and vegetables. Glycemic load multiplies a food’s glycemic index by the amount of carbohydrate in a serving, to reflect the overall glycemic effect of a diet.

“We found the direct association between glycemic load and coronary heart disease was much stronger than the association between carbohydrate and coronary heart disease,” the researchers wrote, “probably because glycemic load reflects both the quantity and quality of carbohydrates.”

One possible explanation why diets that replaced carbs with animal protein and fat were not associated with higher heart-disease risk, Halton said, “is that the adverse effects of animal products might be counterbalanced by reducing refined carbohydrates. The quality of fat and carbohydrate is more important than quantity. A hearthealthy diet should embrace healthy types of fat and carbohydrates.”

The study did not address other concerns about low-carb, high-protein diets such as possible effects on bone health and kidney function or low fiber consumption.

The news for carb-cutters was less good, however, when the study looked at the main motivation for such diets—weight loss. Adherence to a low-carb diet had no long-term effect on weight, according to the findings. In an accompanying editorial, MaryAnn McLaughlin, MD, MPH, of Mount Sinai Medical Center, did note that the average Body Mass Index (BMI) increase of the low-carb group—2.5 units over 20 years—may be lower than that typically seen by women as they age.