Diets rich in whole grains are associated with reduced incidence of type 2 diabetes, coronary heart disease, and colorectal cancer
Randomized controlled trials of increasing whole-grain intakes have demonstrated their potential to improve glycemic control, body weight, the lipid profile, and other cardiometabolic risk factors in adults with diabetes (2).
CONCLUSIONS
Consuming whole-grain foods over 2 weeks improved measures of glycemia in free-living adults with type 2 diabetes compared with an equivalent amount of whole-grain foods that were finely milled. Dietary advice should promote the consumption of minimally processed whole grains.
https://diabetesjournals.org/care/article/43/8/1717/35554/Whole-Grain-Processing-and-Glycemic-Control-in
Whole grains and risk of T2D: Dose–Response Meta-analysis

Each additional 50 grams WG consumption per day was associated with a 23% reduced risk of T2D.

60 grams WG intake per day would give the highest benefit to prevent T2D ...

In conclusion, a high intake of WG was associated with a lower risk of T2D. Randomized controlled trials are needed to confirm our results.
Consumption of whole grains and risk of type 2 diabetes: A comprehensive systematic review and dose–response meta‐analysis of prospective cohort studies

https://onlinelibrary.wiley.com/doi/full/10.1002/fsn3.2811

Cereal grains and legumes in the prevention of coronary heart disease and stroke: a review of the literature

Abstract
A number of reviewers have examined studies investigating the relationship between coronary heart disease and stroke prior to 2000. Since then, several key studies have been published. Five studies have examined the relationship between wholegrain consumption, coronary heart disease (CHD) and cardiovascular (CVD) disease and found protection for either or both diseases.
The researchers concluded that a relationship between wholegrain intake and CHD is seen with at least a 20% and perhaps a 40% reduction in risk for those who eat wholegrain food habitually vs those who eat them rarely.
Notwithstanding the fact that fibre is an important component of wholegrains, many studies have not shown an independent effect of fibre alone on CHD events. Thus in terms of CHD prevention, fibre is best obtained from wholegrain sources.
Wholegrain products have strong antioxidant activity and contain phytoestrogens, but there is insufficient evidence to determine whether this is beneficial in CHD prevention...
Promotion of carbohydrate foods should be focused on wholegrain cereals because these have proven to be associated with health benefits.
Recommendation: Carbohydrate-rich foods should be wholegrain and if they are not, then the lowest GI product available should be consumed. Glycemic index is largely irrelevant for foods that contain small amounts of carbohydrate per serve (such as most vegetables).
https://pubmed.ncbi.nlm.nih.gov/16670693/
Association of whole grains intake and the risk of digestive tract cancer: a systematic review and meta-analysis
Open Access
Published: 03 June 2020
Abstract
Background
Several epidemiological studies have investigated the association between whole grains intake and digestive tract cancer risk; however, the results are still controversial. The purpose of this meta-analysis was to assess the association.
Methods
Studies published before March 2020 were searched in database and other sources. The risk ratio (RR) with the 95% confidence interval (CI) were pooled using fix or random-effects models.
Results
This meta-analysis included 34 articles reporting 35 studies, 18 studies of colorectal cancer, 11 studies of gastric cancer and 6 studies of esophagus cancer, involving 2,663,278 participants and 28,921 cases.

Comparing the highest-intake participants with the lowest-intake participants for whole grains, we found that the intake of whole grains were inversely related to colorectal cancer (RR = 0.89, 95% CI: 0.84–0.93, P < 0.001), gastric cancer (RR = 0.64, 95% CI: 0.53–0.79, P < 0.001), esophagus cancer (RR = 0.54, 95% CI: 0.44–0.67, P < 0.001), respectively.

Conclusion
This meta-analysis provides further evidence that whole grains intake was associated with a reduced risk of digestive tract cancer. Our result supports the dietary guidelines that increase whole grains intake to reduce the risk of digestive tract cancer.

https://nutritionj.biomedcentral.com/articles/10.1186/s12937-020-00556-6