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Everyone knows whole grains are better for you than sticky, processed white flour or rice. But how much better?

A new study from the Harvard School of Public Health shows that each average daily serving of whole grains lowers a person’s risk of dying from heart disease by 9 percent and lowers the overall risk of dying from anything by 5 percent over a quarter-century. That’s even taking into account that whole grain lovers tend to do other healthy stuff, too.

The benefits probably come from the bran, that fibrous coating that processing takes off of whole wheat and brown rice, the researchers write in the American Medical Association’s journal JAMA Internal Medicine.

For the report, Hongyu Wu of the Harvard School of Public Health and colleagues used two giant, long-term, detailed health studies. One was the Nurses' Health Study, looking specifically at more than 74,000 women between 1984 and 2010, and the other was Health Professionals Follow-Up Study, which followed nearly 44,000 men from 1986 to 2010. Everyone whose health records they examined was healthy and free of cancer or heart disease at the beginning.

“Eating whole grains as a regular part of a healthy diet for many years is associated with a lower risk of death from cardiovascular-related diseases."

These are detailed studies that provide a gold mine of information to researchers. The volunteers get regular medical exams and agree to have their medical records open for scrutiny. They fill out regular questionnaires on what they eat, how often, when plus whether they exercise, smoke and reveal other personal habits.

Over the 24 to 26 years of the two studies, 29,920 of the volunteers died. Wu’s team determined that, if they took into account age, weight, whether people smoked and other factors, those who ate whole grains more regularly were less likely to die, period, during that time and much less likely to die of heart disease.

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People who swapped whole grains for red meat benefited especially, lowering their risk of dying from heart disease by 20 percent. But eating whole grains didn’t really affect the risk of dying from cancer.

"These findings further support current dietary guidelines that recommend increasing whole grain consumption to facilitate primary and secondary prevention of chronic disease and also provide promising evidence that suggests a diet enriched with whole grains may confer benefits toward extended life expectancy," Wu’s team wrote.

They even looked at what kinds of whole grains people ate. Whole grains preserve the bran—the outer coating of a grain, as well as the germ, which is the inner part. It was definitely eating bran that seemed to keep people healthier, Wu’s team said.

Whole grains are richer than white bread and white rice in a range of beneficial nutrients, from the fiber to magnesium, vitamin E and plant-based compounds called phytochemicals. They help the body regulate blood sugar, fats and cholesterol, keep the blood vessels healthier, help prevent DNA damage and reduce inflammation in the body.

Whole grains include whole wheat, brown rice and whole oats.

Many studies have found that people who eat more whole grains, versus pasty processed carbs, have a lower risk of diabetes and heart disease.

“Eating whole grains as a regular part of a healthy diet for many years is associated with a lower risk of death from cardiovascular-related diseases,” says NBC’s Diet & Nutrition editor Madelyn Fernstrom. “This dietary strategy appears specific to heart diseases, since no association was seen for deaths by other causes, including cancer.”

Many people argue that those who choose to eat whole grains also tend towards other healthier behaviors, so it’s hard to credit the whole grains specifically. Wu’s team tried to take this into account.

"Replacing one serving of refined grains or total red meat with one serving of whole grains daily was associated with lower cardiovascular disease mortality."

“We controlled for age and ethnicity, as well as … body mass index; smoking status; alcohol intake; physical activity; multivitamin use; aspirin use; a family history of heart disease, cancer, or diabetes; a history of hypertension, high cholesterol, or diabetes at baseline,” they wrote.

They also figured out how much food people ate overall and what other healthful foods, such as fruits and vegetables, they ate.

“We estimated the association of substituting whole grains for refined grains, red meat, and potato,” they added.

“Replacing one serving of refined grains or total red meat with one serving of whole grains daily was associated with lower cardiovascular disease mortality: 8 percent for replacing refined grains and 20 percent for replacing red meat,” they wrote.

The federal government’s food guidelines advise Americans to try to eat more whole grains.