After decades of controversy, the verdict is in: Low-carbohydrate, higher-fat diets do not increase women’s risk of coronary heart disease (CHD). But the source of fat matters: It should be vegetable instead of animal.

Investigators at the Harvard School of Public Health evaluated data on 82,802 women who completed extensive food diaries over a 20-year span as part of the Nurses’ Health Study. After adjusting for age, BMI, smoking, exercise, and other factors, the researchers found no evidence that diets lower in carbohydrates and higher in protein and fat increased the risk of MI or other manifestations of CHD. Moreover, when vegetable instead of animal sources of fat and protein were consumed, low-carb diets moderately reduced the risk of heart disease (N Engl J Med. 2006;355:1991-2002).

Protective carbohydrates included whole-wheat bread, brown rice, and legumes; the salubrious fats came mainly from nuts and canola or olive oil-based salad dressing. “Vegetable fats improve insulin sensitivity and lower cholesterol; good carbs contain fiber, minerals, phytochemicals, and antioxidants. Animal fats don’t,” explains lead investigator Frank Hu, MD, MPH, associate professor of nutrition and epidemiology at Harvard.

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The study also found that women who ate foods with high dietary glycemic loads increased their risk of CHD. “White bread, bagels, cake, mashed potatoes, and soft drinks rapidly increase blood sugar and lead to insulin resistance, diabetes, and heart disease,” says Dr. Hu.

Few study participants followed the popular Atkins low-carb, high-fat program long-term, so the findings cannot be used as a test of the Atkins diet, the researchers said. Nevertheless, Dr. Hu stresses that even though reducing carbohydrate intake is a good idea for most patients, especially for diabetics, the Atkins plan and similar diets can’t be recommended. The study findings “don’t give people a green light to eat steak and bacon every day,” he says, adding that it’s also important to keep an eye on calories.

“If patients want to follow low-carb diets, clinicians should advise a healthy, plant-based version.” Dr. Hu also encourages his patients to use full-fat salad dressing rather than fat-free or low-fat versions.