Starting the day with a bowl of whole-grain cereal or a piece of whole-grain toast can reduce the long-term risk of developing type 2 diabetes.
The inverse relationship between total daily whole grain intake and T2DM has been observed in multiple epidemiology studies, suggesting a diet high in whole grains may have a protective effect.1,2 How various whole-grain foods contribute to this total beneficial effect is less clear. Several cohort studies and a meta-analysis designed to examine the effect of individual whole-grain foods such as breakfast cereal, brown rice, whole grain bread, and wheat bran, all showed positive effects of these select foods on preventing T2DM.3,4,5
In a recently published article in the British Medical Journal, Hu, Yang et al. present findings from their study that explored the relationship between total and individual daily intake of select whole-grain foods and the risk of developing T2DM.5 A total of 194,784 men and women from three prospective cohort studies: Nurses’ Health Study (1984-2014), Nurses’ Health Study II (1991-2017), and the Health Professionals Follow-Up Study (1986-2016), were followed for an average of 24 years. Before study enrollment and every two years during the study, subjects completed a questionnaire to assess medical history, lifestyle factors, and diet. Any self-report of T2DM during follow-up was confirmed with a supplementary questionnaire. Subjects diagnosed with T2DM, cardiovascular disease, or cancer were excluded from the studies. A semiquantitative food frequency questionnaire (FFQ) was administered at the start of the study and every four years to capture food consumption information during the previous year. Total whole-grain consumption was used to divide subjects into five equal groups. Whole-grain foods were categorized as either commonly consumed, such as cold breakfast cereal, dark bread, and popcorn, or less commonly consumed such as oatmeal, added bran, wheat germ, and brown rice. Hazard ratios and 95% confidence intervals were calculated to measure the association between total whole grain and individual whole-grain food intake and the risk of developing T2DM.6.
During follow-up, approximately 10% of subjects were diagnosed with T2DM. At baseline, average age varied by cohort and ranged from 36 to 53 years. The lowest total whole grain intake group within each study was Group I, with an average daily intake ranging from 0.1 to 0.3 servings. The highest intake group was Group 5, with an average daily intake ranging from 1.9 to 2.8 servings per day. At baseline, subjects in Group 5 were more likely to be white, older, non-smoker, leaner, and more physically active. After adjusting for body mass index, lifestyle, and dietary risk factors, subjects in Group 5 had a lower risk of developing T2DM. Findings from a pooled analysis showed that patients in Group 5 had a 29% lower rate of developing T2DM (95% CI 0.67 to 0.74, P<0.001) compared to patients in Group I. For the more commonly consumed individual whole-grain foods, subjects who consumed ≥one daily serving of whole-grain breakfast cereal and dark bread were found to have a 19% and 21% reduced risk of developing T2DM, respectively, compared to those who consumed <1 serving a month. The reverse was seen with popcorn; when daily intake was greater than one serving, there was a significantly increased risk of T2DM. Hu Yang et al. proposed this may be due to added ingredients such as butter, sugar, and salt, and the popcorn manufacturing process. An analysis comparing the less commonly consumed individual whole grains between subjects who consumed ≥ two servings a week and those who consumed <1 serving a month found a risk reduction of T2DM by 21% for oatmeal, 12% for brown rice, 15% for added bran and 12% for wheat germ.6 Subgroup analysis found the relationship between high total whole grain intake and T2DM risk reduction was most robust in subjects with a lower body mass index.6
Study limitations such as confounding variables and limited generalizability need to be considered when evaluating study results. However, these findings are informative and provide further support that total and select individual whole grain foods may reduce the risk of developing T2DM.
- Increasing total daily whole-grain intake may reduce the long-term risk of type 2 diabetes.
- The beneficial effects of whole grains on reducing T2DM risk likely vary depending on the type of whole grain.
- Cold breakfast cereal, dark bread, oatmeal, added bran, wheat germ, and brown rice have all shown benefits on reducing the risk of T2DM; popcorn intake greater than one serving a day had the opposite effect.
- Aune, Dagfinn et al. “Whole grain and refined grain consumption and the risk of type 2 diabetes: a systematic review and dose-response meta-analysis of cohort studies.” European journal of epidemiology vol. 28,11 (2013): 845-58. doi:10.1007/s10654-013-9852-5
- de Munter, Jeroen S L et al. “Whole grain, bran, and germ intake and risk of type 2 diabetes: a prospective cohort study and systematic review.” PLoS medicine, vol. 4,8 (2007): e261. doi:10.1371/journal.pmed.0040261
- Slavin, Joanne. “Whole grains and human health.” Nutrition research reviews vol. 17,1 (2004): 99-110. doi:10.1079/NRR200374
- Cho, Susan S, et al. “Consumption of cereal fiber, mixtures of whole grains and bran, and whole grains and risk reduction in type 2 diabetes, obesity, and cardiovascular disease.” The American journal of clinical nutrition, vol. 98,2 (2013): 594-619. doi:10.3945/ajcn.113.067629
- Sun, Qi et al. “White rice, brown rice, and risk of type 2 diabetes in US men and women.” Archives of internal medicine vol. 170,11 (2010): 961-9. doi:10.1001/archinternmed.2010.109
- Hu, Yang, et al. “Intake of whole-grain foods and risk of type 2 diabetes: results from three prospective cohort studies”. BMJ vol. 370, (2020). doi:10.1136/bmj.m2206
Louise Brown, PharmD Candidate, University of Colorado Skaggs School of Pharmacy and Pharmaceutical Sciences