The study of thousands of women over two decades found that those who got lots of their carbohydrates from refined sugars and highly processed foods nearly doubled their risk of heart disease.
At the same time, those who ate a low-carb diet but got more of their protein and fat from vegetables rather than animal sources cut their heart disease risk by 30 percent on average, compared with those who ate more animal fats.
The findings came from researchers at Harvard University's schools of medicine and public health who reviewed records of 82,802 women in the ongoing Nurses' Health Study over 20 years. The women were not dieting to lose weight. In fact, on average they were slightly overweight and increased their body-mass index roughly 10 percent during the study.
Conventional wisdom says risk of heart disease should increase for those eating the lowest-carb, highest-fat diet, said lead author Thomas Halton.
"It didn't, which was a little eye-opening," he said.
Halton said that may be because the women eating the fewest carbs were compared directly to the group eating the highest-carb, lowest-fat diet.
"Neither diet is ideal," he said. "You need to take the best of both."
The findings, reported in Thursday's New England Journal of Medicine, came from an analysis of food questionnaires the nurses filled out every two to four years starting in 1980. The nurses also reported their use of aspirin, vitamins and hormones for menopause symptoms, and on any history of smoking and heart problems.
The researchers calculated the percentage of calories coming from carbohydrates and animal and vegetable fats and proteins, then divided the nurses into ten groups, from the lowest to the highest calorie percentage from carbs.
The lowest-carb group ate carbohydrate amounts similar to the maintenance program of the Atkins diet, less extreme than the early phase of the diet, said dietitian Geri Brewster, former nutrition director at the Atkins Center for Complementary Medicine in Manhattan.
Still, she said most women in this study ate fewer carbohydrates than traditional diets recommend. While she thinks the Atkins diet allows too much animal fat, Brewster said reducing carbohydrates works because it forces the body to convert stored fat into an energy source and can curb appetite.
American Dietetic Association spokeswoman Susan Moores, a dietitian in St. Paul, Minn., said that because the study only included women, many going through menopause and taking hormones, it is unclear how it applies to men.
For Moores, the key finding was that women reduced heart disease risk by eating more protein and fat from vegetable sources.
"That was the biggest, "Aha!"' she said.
Dr. Robert Eckel, immediate past president of the American Heart Association, said the study was well done, but noted that the nurses' recall of what they ate likely isn't perfect.
Eckel, an endocrinologist at University of Colorado School of Medicine, said many studies have shown heart disease risk is cut by eating less fat and more whole grains, fresh fruit and vegetables - the approach of the government's food pyramid. He said medical guidelines won't be changed by the new study, although it raises questions about the role of refined sugar.