The analysis was conducted for the American Society for Nutrition. In a position statement, the group said evidence suggests foods with cereal fiber or mixtures of whole grains and bran are "modestly associated" with a reduced disease risk.
The strongest evidence for benefit came from cereal fiber, researchers said. That would include breakfast cereals as well as breads and brown rice with a high fiber content listed on the label, according to Teresa Fung, a nutrition researcher at Simmons College in Boston.
"Cereal fiber may be one of the protective ingredients of whole grains that contribute to lower disease risk," Lu Qi, one of the study's authors from the Harvard School of Public Health in Boston, told Reuters Health in an email.
His team's review included 28 studies that looked at the risk of developing diabetes among people who ate different amounts of whole grain and bran, 33 studies on the risk of cardiovascular disease and 19 on obesity.
Qi and his colleagues found that overall, people who ate the most cereal fiber or whole grains and bran had an 18 to 40 percent lower risk of diabetes than those who ate the least.
Likewise, people with diets high in cereal fiber had a 22 to 43 percent lower risk of stroke across the studies and were 14 to 26 percent less likely to die of cardiovascular disease.
Fiber-rich grains were also tied to a lower body weight, but the effect was small. Two studies found people who ate the most of those grains lost about one more pound than other participants, the researchers wrote in the American Journal of Clinical Nutrition.
The benefits of whole grains in general - without bran and outside of cereal fiber - were less clear, they said.
The analysis was funded by grants from the Kellogg Company as well as other food and nutrition companies.
Because none of the studies randomly assigned people to eat different amounts of whole grains, including cereal fiber, they can't prove it was the fiber, itself, that prevented diabetes and heart disease.
But a large enough long-term study to prove cause and effect would be difficult, the study team wrote.
"It may simply be, people (who eat cereal fiber) are full longer, and therefore they don't eat so much, and they're leaner," Fung, who wasn't involved in the new study, told Reuters Health.
"Another possibility is people who eat a lot of cereal fiber, they don't just eat a lot of cereal fiber. They're also more healthy" in other ways, she said.
SOURCE: http://bit.ly/14MVLs2 American Journal of Clinical Nutrition, online June 26, 2013.Copyright © 2015, The Baltimore Sun