Earlier studies had linked whole grains to a decreased risk of type 2 diabetes and heart disease, the researchers say.
“Reading the ingredients of food labels, consumers will know whether the food contains any whole grain contents,” said senior author Dr. Qi Sun of the nutrition department at the Harvard School of Public Health in Boston.
Common types of whole grains include whole wheat flour, brown rice, whole oats, whole cornmeal, and popcorn, Sun told Reuters Health by email.
Almost all dietary guidelines recommend whole grains, with the total amount varying by age. The U.S. Department of Agriculture (USDA) advises that at least half the grains consumed each day should come from whole grains, Sun said.
(The USDA’s recommendations are here: http://1.usa.gov/1BB4cat.)
The new findings, reported in JAMA Internal Medicine, are from women in the Nurses’ Health Study from 1984 to 2010 and men in the Health Professionals Follow-Up Study between 1986 and 2010. In both studies, health records and dietary questionnaires were updated periodically.
Men and women who reported consuming more whole grains were more likely to be physically active than the other participants, to consume less alcohol, to have healthier diets overall - and to have a history of high cholesterol. They were also less likely to be current smokers.
Of the more than 188,000 study participants who started out without heart disease or cancer, almost 27,000 had died by 2010.
Those who reported eating the most whole grains were almost 10 percent less likely to die during the course of the study than those who reported eating the least, even when the authors accounted for age, smoking, body mass index, physical activity and other dietary habits.
With every additional 28 grams (about 1 ounce) of whole grains per day, the risk of death went down by five percent and the risk of heart disease death by nine percent, the authors estimate.
One serving of old fashioned oats, or half a cup uncooked, contains 40 grams of whole grains.
While there was a slightly larger drop in the risk of dying from heart disease with more whole grains, there was no change in risk for cancer death.
But this kind “observational” study can’t prove that whole grains directly influence mortality, according to David M. Klurfeld, national program leader in human nutrition for the United States Department of Agriculture in Beltsville, Maryland.
“Most studies have not been able to single out an independent effect of whole grains, including this study which did not even report the amount of fiber being consumed by the participants,” Klurfeld told Reuters Health by email.
Fiber supplements may have a similar association with mortality and heart disease, but have been difficult to study, he said.
Whole grains don’t lead to rapid changes in blood sugar and insulin levels, while refined grains or other carbohydrates, such as sugar, do, Sun said, which may explain why whole grains are associated with lower diabetes and cardiovascular disease risk. They also contain beneficial nutrients like magnesium, vitamins and lignans, and they may jointly lead to lower risk of developing heart disease and diabetes.
While we don’t know what precise “dose” of whole grains is most beneficial, “the current evidence from our investigation and other studies suggests the more the better,” Sun said.
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