Researchers from the Harvard School of Public Health found no evidence of an association between low-carbohydrate diets and increased cardiovascular risk, even when these diets were high in saturated animal fats.
Low-carb eating even seemed to be protective against heart disease when vegetables were the main sources of fat and protein in the diet.
The study, which appears tomorrow in the New England Journal of Medicine, included almost 83,000 female nurses in the Nurses' Health Study who provided detailed information about their eating patterns once per year for more than 20 years. The nurses were not asked to follow any particular diets.
A clear message from the research was that extreme diets, which severely restrict either fats or carbohydrates, are not the best choices for cardiovascular disease prevention, researcher Thomas L. Halton, Sc.D., tells WebMD.
"Neither a very low-fat diet or a very low-carbohydrate diet proved to be ideal," he says. "There were pros and cons to both of these diets."
Low-fat diets are by definition low in saturated fats, which is good for the heart, Halton says. But they also tend to be higher in refined carbohydrates like sugar and white flour, which spike blood sugar levels.
"Americans tend to pick the wrong carbohydrates," he says. "So the benefits of eating lower amounts of saturated fat and cholesterol are offset to some degree by the poor quality of the carbohydrates they eat."
The most protective diet, in terms of heart disease risk, was a low-carbohydrate that was also low in saturated fats and cholesterol where vegetables were the main sources of fats and protein. "The vegetable-based low-carbohydrate diet combined the best features of low-fat and low-carbohydrate eating," Halton says. Following this diet was associated with a 30 percent reduction in heart disease risk over 20 years.
"The quality of fat and carbohydrate is more important than the quantity," says study researcher Frank Hu, M.D., Ph.D. "A heart-healthy diet should embrace healthy types of fat and carbohydrates."
Hu was talking about carbohydrates that are slow to convert to sugar, or so-called low-glycemic-load foods.
Most fruits, vegetables, whole grains, and nuts have low glycemic loads. Refined white flour and sugar, as well as white rice and potatoes, have high glycemic loads.
Women in the study whose diets had the highest glycemic loads had a 90 percent increased risk of developing heart disease during the 20 years of follow-up, compared with women whose diets had the lowest glycemic loads.
"This is just one study, but the findings suggest that eating a high-glycemic-load diet may be even more harmful than eating a diet that is high in saturated fat and cholesterol," Halton says.
Frank Sacks, M.D., also studies diet and heart disease risk at the Harvard School of Public Health, but he was not involved with the study by Halton and colleagues. His research also suggests that following a strictly low-fat diet is less protective against heart disease than following a diet that includes fat from vegetable sources like olive and canola oil.
He is currently assessing the cardiovascular risks and benefits of some of the most widely promoted commercial diets, including Atkins, the South Beach Diet, and the Zone.
"One problem with very restrictive diets is that people don't stay on them very long," he says. "It doesn't matter how good they are or how protective they are if people don't follow them."
SOURCES: Halton, T.L. The New England Journal of Medicine, Nov. 9, 2006; Vol, 355: pp. 1991-2002. Thomas L. Halton, Sc.D., department of nutrition, Harvard School of Public Health, Boston. Frank M. Sacks, M.D., professor of cardiovascular disease prevention, Harvard School of Public Health, Boston.
By Salynn Boyles
Reviewed by Louise Chang, M.D