Low-carb diets are not similar to each other, according to a new Boston study. Your health may depend on what you have with it. The study found that people who ate low-carb diets rich in animal protein and fat were risking their lives more than people who ate higher amounts of carbohydrates. But people who ate low-carb diets with more plant-based protein and fat lived longer than people who ate high-carb diets, as reported in the current issue of Annals of Internal Medicine.

Low-carb diets like the popular Atkins diet have been compared against low-fat diets for weight loss, but Fung's research is different in that she compares death rates between people who ate high-carb diets and low-carb diets, combined with either animal protein and fat or vegetables such as legumes or nuts. The researchers say that eating less carbs means eating more of something else. That something else had better be plant-based sources of protein and fat rather than animal-based sources.

The study was based on results from 85,000 plus women enrolled in the Nurses' Health Study and 44,000 plus men in the Health Professionals' Follow-up Study. They replied to periodic questionnaires on the food they ate, how much exercise they did, their weight and whether they smoked, among other variables.

Twenty years later for the men and 26 years later, for the women, more than 20,000 people had died. To see if there was a connection between diet and death, the participants were divided into 10 groups based on carb intake and looked at deaths caused by cardiovascular disease or cancer.

Though observational study could not arrive at cause and effect, it did find a relationship between people on low-carb, high animal-source diets and mortality, especially from cancer, compared with high-carb diets. Mortality of people eating low-carb, high-plant-source diets was lower than people eating high-carb diets, especially from cardiovascular disease. Even after taking into account body mass index, smoking, and other factors, the association remained.

Though the researchers agreed that randomized clinical trials are important for medical research, they also argued that observational studies complemented clinical trials. This is because they are more likely to gather information over the long term. The participants, in the large randomized clinical trial called the Women's Health Initiative, could not stay on low-fat diets, so they ended up eating diets so similar to women in the comparison group. Therefore, they cannot be conclusive results. Thus all points towards the fact that plant-based diets are healthier and that they increase the life span.