Advocates of low-carbohydrate diets, such as the popular Atkins diet, claim that those diets may help prevent obesity and coronary heart disease. A new Harvard study suggests that's true only if other dietary components aren't high in animal fat.

The long-term safety of high-carb diets has been debated for years, particularly because they encourage the consumption of animal products, which are high in saturated fats and cholesterol and could potentially increase the risk of heart disease.

Prevailing dietary recommendations have advocated the opposite approach -- recommending diets that are low in fat and high in carbohydrates as the best way to manage weight and reduce the risk of cardiovascular disease.

In the first study to look at the long-term effects of low-carbohydrate diets, researchers from the Harvard School of Public Health found no evidence of an association between low-carb diets and an increased risk of heart disease in women.

Their findings did suggest, however, an association between low-carb diets high in vegetable sources of fat and protein and a low risk of heart disease.

"This study suggests that neither a low-fat dietary pattern nor a typical low-carbohydrate dietary pattern is ideal with regards to risk of heart disease; both have similar risks. However, if a diet moderately lower in carbohydrates is followed, with a focus on vegetable sources of fat and protein, there may be a benefit for heart disease," said Tom Halton, a former doctoral student in the Department of Nutrition at HSPH.

The study appears in the November 9, 2006, issue of The New England Journal of Medicine.

The researchers looked at data collected over a 20-year period from 82,802 women in the Nurses' Health Study, a long-term study that began in 1976. Study participants were divided into 10 categories according to their overall diet score, which was measured by calculating fat, protein and carbohydrate intake as a percentage of energy.

The scores ranged from 0 (the lowest fat and protein intake and highest carbohydrate intake) to 30 (the highest fat and protein intake and lowest carbohydrate intake). A higher score meant a person followed a low-carbohydrate diet more closely; that score was called the "low-carbohydrate-diet score."

Halton and his colleagues also created two additional low-carbohydrate-diet scores. The first calculated percentages of energy from carbohydrate, animal protein and animal fat. The second calculated percentages of energy from carbohydrate, vegetable protein and vegetable fat.

The researchers documented 1,994 cases of coronary heart disease over the study period.

The results showed that a low-carbohydrate score was not associated with risk of heart disease in women. There was no evidence that the relationship was modified as a result of physical activity levels, body-mass index, or the presence or absence of hypertension, diabetes, or hypercholesterolemia.

Total amounts of fat or carbohydrate did not appear to have an appreciable relationship with risk of heart disease. However, types of fat and carbohydrates do make a difference. Vegetable fat was associated a lower risk of risk of heart disease, whereas higher dietary glycemic load (reflecting the amount of refined carbohydrates that can rapidly elevate blood sugar levels) -- typical of a high-carb diet -- was strongly associated with increased risk. The authors found that, when vegetable sources of fat and protein were chosen instead of animal sources, the low-carbohydrate-diet score was associated with a 30% lower risk of heart disease.

"This study doesn't mean that you should load your plate with steak and bacon," said Hu. "One likely explanation that we did not see increased risk of heart disease with low-carbohydrate diets 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 heart-healthy diet should embrace healthy types of fat and carbohydrates."

The study was supported by grants from the National Institutes of Health.