The traditional American diet is full of red meat - steaks and hamburgers and processed meat. That's where many Americans get their protein. As this diet becomes more popular in other countries, heart disease becomes more common, too. Now, new research shows that when people depend less on animal protein and replace it with plant protein, they can expect to live longer, healthier lives.

The study was led by Dr. Mingyang Song, a researcher at Massachusetts General Hospital and the Harvard T.H. Chan School of Public Health. "People should not only care about how much protein they consume, but also pay attention to what kind of food they consume to gather the protein," Song said.

Protein is important for body structure, physiology and metabolism. It helps build and repair bones and muscles. But the type of protein is also important.

The study, published in JAMA Internal Medicine, looked at data from two large studies that lasted 20-plus years. Song and his colleagues focused on how much protein 131,342 U.S. adults consumed in their daily diets and what the source of that protein was.

The data came from the national Nurses' Health Study that lasted 32 years and Health Professionals Follow-up Study at Harvard that went on for 26 years. The participants provided detailed information about their diets in frequent questionnaires.

Song said the findings could change the advice doctors and government agencies provide to their patients and to the general public. Co-author Andrew Chan, also of Massachusetts General Hospital and Harvard Medical School, said the study showed that “Individuals that consumed the highest amounts of animal based proteins had a somewhat higher risk of mortality, in particular, cardiovascular mortality.”

The researchers found that a 10 percent increase in animal protein was linked to an 8 percent increase in the risk of death due to cardiovascular disease. They found this association was stronger among people with unhealthy lifestyle factors - people who smoked or drank heavily, who were obese and/or were physically inactive. Chan added, “Some of the association between unhealthy lifestyle and animal protein may be driven primarily by the fact that these people tended to consume more red and processed meat than fish or chicken.” However, those with a healthier lifestyle did not see an increase in their risk of dying.

Still, the study showed that a 10 percent increase in plant protein (ie: nuts, seeds, beans and tofu) was linked to a 10 percent decrease in mortality, and a 12 percent decrease in death from cardiovascular disease. “Clearly, over the long term, a diet which is comprised of primarily plant-based proteins tends to be more favorable for longevity than someone who consumes most of their protein from animal sources,” Song said.