Eating whole grains such as popcorn, oats and quinoa is linked to increased longevity, and may decrease risk for deaths from cardiovascular disease over a 25-year period, but not cancer deaths, a new study finds.

The new research is one of many large studies that tie a diet high in whole grains to increased longevity, including deaths due to cardiovascular disease. "I think it's quite conclusive that if you eat whole grains, you almost always benefit," said the new study's senior researcher, Dr. Qi Sun, an assistant professor of nutrition at the Harvard School of Public Health in Boston.

In the study, the researchers looked at two large studies, including about 74,000 women who were taking part in the Nurses' Health Study, and nearly 44,000 men participating in the Health Professionals Follow-Up Study. The participants recorded their whole-grain intake on food surveys given every two to four years. After 24 to 26 years, 26,920 people in the studies had died.

The new study has three main findings, Sun said. First, the researchers found that people who ate at least 28 grams of whole grains a day had a 5 percent lower risk of dying over the study period, and a 9 percent lower risk of dying from cardiovascular-disease-related death, than people who ate little or no whole grains during the course of the study.

The researchers also found that eating the part of the whole grain called the bran had the largest effect on reducing mortality and deaths from cardiovascular disease. The bran is the tough skin that covers the kernel of a whole grain, and holds antioxidants, B vitamins and fiber. Processing whole grains into refined grains typically removes much of the grains' bran, according to the Harvard School of Public Health.

People in the study who replaced one serving a day of refined grains with whole grains reduced their risk of dying over the study period by 8 percent, and people who replaced one daily serving of red meat with whole grains reduced their risk of dying over the study period by 20 percent, the researchers also found.

The researchers accounted for other factors that could have affected the study participants' risk of dying over the study period, such as age, smoking, body mass index, exercise and general diet. This was especially important because the men and women, who ate more whole grains also tended to get more exercise, eat other healthy foods, smoke less and drink less alcohol than people who ate fewer whole grains.

"If you are really looking into whole-grain consumption with other diseases, stroke, heart disease and colorectal cancer, whole grains are consistently associated with [a] lower risk of those diseases," Sun said.

However, a higher intake of whole grains did not appear to reduce the risk of dying from cancer, the researchers found. It's possible that the two studies didn't have enough cases of specific kinds of cancer to show whether eating whole grains is helpful for cancer-related longevity, Sun said. Experts said the results of these studies add to previous evidence of the healthful effects of whole grains.

"It's showed, as quite a few other studies have shown in several other settings, that the consumption of whole-grain foods is associated with reduced total mortality and mortality from cardiovascular disease, but not particularly strongly with mortality from cancer," said David Jacobs, a professor of epidemiology at the University of Minnesota School who was not involved in the study.

However, it's difficult to parse apart the health effects of the bran from the effects of the other parts of a whole grain, such as the germ, Jacobs said. "It's a very difficult thing in nutritional epidemiology to separate things like that out and to make particular statements about foods or parts of foods," he added.

But Jacobs and Sun agreed that whole grains are part of a healthy diet. It's likely that whole grains are nutritious because they have a lower glycemic index, meaning they lead to a smaller rise and fall in blood-sugar levels after a meal than refined grains do. Whole grains are also rich in healthy compounds, minerals and vitamins, Sun said.

Along with eating a healthy diet and exercising regularly, "Half of the grains that a person eats every day should come from whole grains," Sun said.