Processed Versus Intact Whole Grains and Glycemic Control

Many foods are marked with the whole grains stamp of approval, but are these products equivalent in terms of glycemic control for people with diabetes?

Whole grains have a well-established role in diabetes prevention and management. The 2019 ADA Consensus Report recommends minimizing refined grains and choosing whole foods over highly processed foods to the extent possible. It mentions whole intact grains as a food that can help increase fiber intake, which may help modestly lower A1c. The definition of a whole grain includes the intact grain and more refined grain if the components of the grain are found in similar relative proportions, allowing a wide variety of products to carry this label. Whole grain oats have been identified as a type of food that can blunt the blood glucose response after a meal, resulting in a lower glycemic response than refined grains. A study published in Diabetes care earlier this year found that whole-grain bread made with more intact and coarsely ground whole grains reduced postprandial glucose in adults with type 2 diabetes compared to whole-grain bread made with finely milled whole grains. Several mechanisms have been proposed regarding these findings. The first is that the shear between rollers during milling leads to the breakage of cell walls, increasing the bioaccessibility of starch by digestive enzymes. The second is that the breakdown product of intact grains can pass into the colon for digestion by the microbiome into short-chain fatty acids without influencing blood glucose levels.

This study’s objective is to determine whether the structural integrity of whole grains as a result of differences in milling is a determinant of overall glycemic control. A randomized crossover design was utilized in this study and included 28 free-living adults between 18 and 80 with type 2 diabetes. Participants were assigned either a less-processed grain group or a finely milled grain group. Each intervention group had a two-week duration followed by a washout of at least two weeks. Participants were instructed to replace the grains in their regular diet with grains provided by the study. No advice was given regarding changing the number of grains consumed, and no further dietary or other lifestyle advice was given. Participants also had CGM systems fitted to their upper arms on the first day of the study and removed on day 14.

Additionally, serum alkylresorcinols were measured as a marker of adherence to the whole-grain intervention. The primary outcome was a change in blood glucose control following meals and over the day. The study estimated the sample size based on a power calculation with an α of 0.05 and a power of 0.80 to detect within-group differences in primary outcome variables. IAUC measured a 20% difference in mean postprandial glycemia.

Baseline characteristics were similar among the study population, including 14 (45%) females, average age of 63, an average HbA1c of 7.5%, and a duration of diabetes average of 11.4 years. Most participants were on metformin with or without a gliclazide, nine were on oral agents plus insulin, and three were managing with diet alone. The intake of all macronutrients and dietary fiber was similar between the groups, as was the increase in alkylresorcinols. Most whole grains were consumed during breakfast, followed by lunch and dinner. The mean iAUC of the three hours following all the breakfast meals was 9% (95% CI 3-15) lower during the two weeks of consuming less-processed whole grains than when finely milled whole grains were consumed. In addition, glycemic variability reduced when participants consumed the less-processed whole grains compared with the finely ground whole grains as measured by MAGE (-0.36 [95% CI -0.65 to -0.08]) and the SD of the mean glucose value (-0.16 mmol/L [95% CI -0.25 to -0.06]). Lastly, body weight increased during the finely milled whole-grain intervention and decreased when less-processed whole grains were consumed (0.81 kg [95% CI 0.62-1.05]). No differences were observed between blood lipids, insulin, inflammatory markers, or blood pressure interventions.

This study’s findings are similar to a study published earlier this year, demonstrating that consumption of less processed whole-grain foods improves glycemic control in adults with type 2 diabetes compared to an equivalent amount of finely milled whole grains. In addition, consumption of the less-processed whole-grain foods was also associated with a mean reduction of body weight. In contrast, there appeared to be a modest increase during the two-week period where finely milled whole-grain foods were provided. Given the importance of glycemic control, this study adds to the growing body of evidence that consuming intact grains over finely milled grains can lead to long-term health benefits. 

Additionally, this study’s results indicate it would be beneficial to redefine a “whole grain” to not group intact grains and finely milled grains. Several limitations include the short duration, which does not observe potential changes in HbA1c or the lipid profile. Additionally, this study did not detect any changes in subgroup analyses, including medication type, sex, individual patient glycemic control, or specific whole-grain foods. Future studies could include a larger population and have a longer duration of follow-up to determine the long-term benefits.

Practice Pearls:

• Compared to an equivalent amount of finely milled whole grains over two weeks, less-processed whole-grain foods improved glycemic control.
• Consumption of intact grains was associated with a mean reduction in body weight compared to a modest increase in weight in the finely milled whole-grain group.
• A study with a longer duration follow-up is needed to determine the impact on HbA1c and the lipid profile.

Åberg, Sebastian et al. “Whole-Grain Processing and Glycemic Control in Type 2 Diabetes: A Randomized Crossover Trial.” Diabetes care, May 18. 2020,

Jessica Rogers, PharmD candidate, University of Colorado Skaggs School of Pharmacy and Pharmaceutical Sciences