In a large study of nearly 200,000 US adults, those eating whole grains most frequently had a 29% lower risk of developing type 2 diabetes across the approximately 30-year study period than those rarely or never eating whole grains, even after adjusting for other lifestyle factors. When looking at specific whole grain foods, common foods like whole grain breakfast cereal, oatmeal, whole grain bread, and brown rice were all linked with a lower risk of type 2 diabetes. The exception was popcorn, which was linked with a higher risk of type 2 diabetes when eaten more than once per day, perhaps due to its association with butter sauces or sugary flavorings.
BMJ. 2020 Jul 8;370:m2206. doi: 10.1136/bmj.m2206. (Hu Y et al.)

While fruits and vegetables are known as sources of healthy antioxidants and phenolic compounds, research increasingly shows that whole grains contain them too. In this review, researchers analyzed the total phenolic contents, phenolic acid profile and antioxidant activity of several whole grains, including wheat, corn, rice, barley, sorghum, rye, oat and millet. The review shows that whole grains contain a number of phytochemicals (including antioxidants) and significantly exhibit antioxidant activity. Researchers conclude that the consumption of whole grains is considered to have significant health benefits including prevention of chronic diseases such as cardiovascular disease, diabetes, and cancer because of the contribution of phenolic compounds. 
Critical Reviews in Food Science and Nutrition. 2014 July 30. [epub ahead of print] (Van Hung P et al.)

An increasing body of research shows that resistant starch, a newly-recognized type of dietary fiber found in grains, cold potatoes, legumes and other foods, has many health benefits. Now researchers at the University of Toronto have found that certain whole grain varieties with naturally-high levels of resistant starch may be especially good at making us feel full longer. In the study, 17 male volunteers consumed five different test soups, at one week intervals, after which scientists recorded their glycemic response and their food intake at various intervals over the next few hours. Eating whole grain corn soup with 66% resistant starch content reduced subsequent food intake by 15% compared to eating a high-glycemic control soup.
American Journal of Clinical Nutrition, February 17, 2010 doi:10.3945/ajcn.2009.28443

Children and youth with type 1 diabetes must be especially careful to eat well, but, like other children, have strong likes and dislikes. Researchers at the University of Massachusetts Medical School worked with 128 young people, ages 7 to 17, at a diabetes camp, to gauge acceptability of a range of whole grains and legumes. Whole grain cornbread was the favorite (85% tried it and liked it, with another 11% willing to try it) followed by whole wheat bread (72% tried/liked and 3% more were willing to try). Those living in an urban setting or frequently consuming fast food were less willing to try whole grain foods.
Diabetes Education. May-June 2009; 35(3): 422-7. Epub Mar 16, 2009.

At the University of Nebraska, researchers examined data from the 1999-2002 National Health and Nutrition Examination Survey (NHANES) to learn whether popcorn consumption was associated with different dietary intake patterns. They found that, on average, those who regularly ate popcorn consumed 250% more whole grain overall (2.5 vs 0.7 servings per day), and about 22% more fiber (18.1g vs 14.9g per day). The popcorn-eaters also consumed fewer meat servings and more carbohydrates.
Journal of the American Dietetic Association. May 2008; 108(5): 853-6.