Swedish residents who ate food containing more than 59g -- roughly two ounces -- of whole grains per day were 27% less likely to become prediabetic compared with residents who ate 30g or less, according to the research.
"This is of great importance because prediabetes is increasing," lead author Tina Wirström, a postdoctoral fellow at the Karolinska University Hospital in Sweden, told Reuters Health.
The American Diabetes Association estimates that one in four Americans older than 20 have prediabetes. Nearly a quarter of people with prediabetes eventually develop full-blown diabetes.
Previous research has linked whole grain rich diets with a decreased risk of diabetes, but the new study in the American Journal of Clinical Nutrition makes the connection to the disease precursor. Whole grains range from oatmeal to popcorn, from brown rice to whole wheat, as long as the entire kernel is consumed.
The study included 5,477 Stockholm residents from 35-56 years old without diabetes who kept food diaries of how much whole and refined grains they ate. Researchers measured blood glucose in study participants and followed up 10 years later.
Overall, one in 13 participants became prediabetic, while one in 33 became diabetic, the study found. When researchers took into account the daily amount of whole grains eaten, they found that eating more than 59g versus 30g of whole grain was associated with a 27% decreased risk of becoming prediabetic.
The association was stronger for men, and those with an increased genetic risk for developing diabetes did not see a difference, researchers found.
Nutrition experts noted that the study had little relevance to the U.S. since most Swedes get their grains from whole grain sources such as rye crisps. In contrast, Americans get most grains from refined wheat such as flour that contains fewer nutrients and fiber compared to rye.
"To say it has any relevance to the U.S. is really a stretch," said Joanne Slavin, nutrition professor at the University of Minnesota, who was not involved in the current study.
U.S. food manufacturers can label food that contains eight grams per serving as whole grain, whereas Sweden requires that a food product contain at least half whole grains to receive the label.
Many foods with claims of whole grain goodness have extra calories, fats and carbohydrates in addition to the grains, which can be risk factors for diabetes, Slavin said.
Further, fewer than 3% of Americans get the recommended 48g per day of whole grains. The average American eats 15g of whole grain daily.
"In the U.S., we can't even get 20g, much less 60g of whole grains in people (on a daily basis)," said Roger Clemens, pharmacology professor at the University of Southern California and an adviser for the most recent dietary guidelines from the U.S. Department of Agriculture.
"You can't get 60g of whole grains into a supplement," added Clemens, who was not involved in the study.
Consumers may take away the wrong message from the study, said Slavin, who advocated Americans eat fewer grains overall and switch from refined grains to whole grains.
"Adding whole grains on top of what you're already consuming is a bad idea," said Slavin, since over-consumption of calories and carbohydrates are big risk factors for diabetes. "We have "whole grain brownies" now; it's terrible."
A similar Dutch study, headed by Dr. Coen Stehouwer, internal medicine professor at the Maastricht University Medical Center, found losing weight was the most important factor for people with prediabetes to revert back to normal blood sugar level.
"Prediabetes is a reversible state," said Stehouwer, who was not involved in the Swedish study.