ROCHESTER, MINN. — Contrary to conclusions reached by Dietary Guidelines Advisory Committees both in 2015 and 2020, scientific data do not suggest an association between higher intake of refined grains and an increased risk of type 2 diabetes (T2D), said Glenn A. Gaesser, a professor in the College of Health Solutions, University of Arizona, Phoenix.
The findings, which challenge recommendations in the Dietary Guidelines to reduce intake of refined grains, will be published in a commentary in the August issue of Mayo Clinic Proceedings.
Dr. Gaesser, who is on the scientific advisory committee of the Grain Foods Foundation, based his conclusions on data in 10 publications looking at 11 adult cohorts that included a total of 398,994 individuals — 268,419 women and 130,575 men. Of the 11 cohorts, 9 showed no statistically significant association between higher intake of T2D and refined grains intake, 1 showed a higher risk and 1 showed a lower risk. The one study showing a higher risk was the smallest cohort in the collection, accounting for 0.3% of participants in the studies.
“These findings refute the commonly held belief that refined and non-whole grains can directly lead to T2D when consumed,” the GFF said. “Eating refined staple grain foods such as breads, cereals, and pasta were not associated with T2D risk, and total grain intake was consistently associated with lower risk of T2D. Even in studies that included indulgent grain foods such as cakes, cookies, sweet rolls, and muffins in the refined grain food category, no association with T2D risk was observed.”
In a YouTube video about the commentary featuring Dr. Gaesser, he concluded, “The take home message here is individuals should be consuming more whole grains, that is for sure, and the studies support that quite strongly, but it also suggests we probably should relax our recommendations on reducing refined grain intake, because there is no compelling evidence to suggest a reduction in refined grain intake will lead to a reduced diabetes risk.”
The results were not for staple grain foods items alone, such as bread, pasta, cereal or rice.
“Whether refined grains were defined to include staple grain foods only or a combination of staple and indulgent grain foods did not appear to influence the results,” he said.
By way of background, Dr. Gaesser said the last two Dietary Guidelines Advisory Committees (DGACs) had based recommendations to cut consumption of refined grains on “moderate” evidence suggesting such reductions were associated with a lower risk of T2D. The moderate evidence was generated using dietary pattern research.
“’Healthy’ dietary patterns are characterized by higher consumption of fruits, vegetables, whole grains, low-fat or non-fat dairy products, seafood, legumes, and nuts,” Dr. Gaesser said. “In contrast, ‘unhealthy’ (Western) dietary patterns are characterized by higher intakes of red and processed meat, sugar-sweetened foods and beverages, french fries, high-fat dairy products, and refined grains. A substantial body of research shows that a healthy dietary pattern is associated with a lower risk of T2D and that an unhealthy dietary pattern is associated with increased risk of T2D.”
Because dietary pattern research clusters many foods together, Dr. Gaesser said the risk the DGAC identified may not be “attributable to refined grains per se but rather to the other foods within the unhealthy dietary pattern.” In fact, he noted that T2D risk has been independently associated with intake of red and processed meat and sugar-sweetened beverages.
To conclude refined grain intake is associated with T2D, studies in which refined grains are considered as a separate food category, independent of a dietary pattern, should be considered. It was toward that end that Dr. Gaesser looked at studies analyzing refined grains as a distinct food category.
Ideally, when trying to assess the risk of grains intake and T2D, Dr. Gaesser said further distinctions should be made between so-called staple grain foods such as bread, cereal, pasta and rice and indulgent grain foods like cookies and cakes. Such distinctions were not made in the 10 studies he analyzed, making the lack of association between refined grains and T2D surprising, Dr. Gaesser said.
The studies he assessed included in the refined grains categories cakes, cookies, sweets and desserts, sweet rolls, pancakes, waffles, muffins and pizza.
“These foods typically contain high amounts of sugar and fat and low amounts of fiber,” Dr. Gaesser said. “None of the cohort studies examined the separate associations between T2D risk and intake of either staple or indulgent refined grain foods or provided information as to the percentage contributions of the different refined grain foods to total refined grain intake.”
He continued, “Even with inclusion of indulgent grain-based foods, none of the studies showed an elevated risk of T2D associated with the highest intake of refined grains, and none of the meta-analyses indicated an association between refined grain intake and T2D.”
Relying on dietary patterns to establish an association with T2D likely caused the DGACs to reach its conclusions about refined grains, conclusions that now appear suspect.
“The higher T2D risk associated with the unhealthy (Western) dietary pattern is likely attributable to consumption of red and processed meat and sugar sweetened beverages rather than to refined grain foods per se,” Dr. Gaesser said.
The DGAC consistently has been recommending consuming at least three servings of whole grains per day, but Dr. Gaesser said less than 7% of the US population consumes the recommended minimum three servings per day of whole grains, and more than 70% of Americans consume less than one serving per day of whole grains. He cited data indicating refined grains intake is five times greater than whole grains intake (in the case of wheat flour, USDA data actually suggest refined grains intake is about 20 times greater than whole grains).
“Although research strongly supports the recommendation to increase consumption of whole grains for reducing risk of T2D, the optimal amount of whole-grain intake is not entirely clear,” Dr. Gaesser said. “Risk appears to be reduced significantly up to about three servings per day of whole grains. Americans currently consume less than one serving per day.”
In the Women’s Health Initiative Observational Study, one of the largest studies discussed in Dr. Gaesser’s commentary, the lower risk of diabetes for the highest intake of refined grains was similar to that for the highest intake of whole grains.
Dr. Gaesser said there is evidence that as intake of total grains increases, the risk of T2D decreases in a non-linear fashion up to 10 servings per day.
“This may be particularly relevant for US dietary recommendations, as this level of total grain consumption exceeds current grain intake among Americans,” Dr. Gaesser said.