Low-carb diets defy traditional weight loss advice but do not seem be linked to a higher risk of heart disease in women, according to a long-term study published Thursday.

The Atkins and other low-carb diets have long claimed to prevent obesity and heart disease, but nutrition experts questioned the safety of encouraging people to eat animal proteins that are high in saturated fats and cholesterol.

Low-carb diets, which encourage people to eat more proteins, are popular for weight loss.

Now researchers from the Harvard School of Public Health have weighed in after analyzing data collected over 20 years from more than 82,000 women participating in the Nurses' Health Study.

In Thursday's issue of the weekly New England Journal of Medicine, they conclude the answer lies in the middle.

"This study doesn't mean that you should load your plate with steak and bacon," said the study's senior author, Dr. Frank Hu, a professor of nutrition and epidemiology at Harvard.

There was no increase or decrease in the risk of heart disease among low-carb eaters in the study.

Healthy fats, carbohydrates

As for why, it could be that the side-effects of animal protein are countered by eating fewer refined carbohydrates, Hu said.

"The quality of fat and carbohydrate is more important than quantity," he said. "A heart-healthy diet should embrace healthy types of fat and carbohydrates."

Women who chose fat and protein from vegetable sources were the exception, showing a 30 per cent lower risk of heart disease, the team found.

Two of the study's authors reported receiving grants from food companies or growers.