Abbreviations and Acronyms: DGAC (Dietary Guidelines Advisory Committee), T2D (type 2 diabetes)Grain foods are considered part of a healthy diet, and current US dietary guidelines recommend that at least one-half of grain consumption come from whole grains.1U.S. Department of Agriculture and U.S. Department of Health and Human Services
Dietary Guidelines for Americans, 2020-2025. Health benefits of whole grains are well established, and several meta-analyses have demonstrated inverse associations between whole grain intake and risk of several chronic diseases, including type 2 diabetes (T2D).2Aune D. Norat T. Romundstad P. Vatten L.J. Whole grain and refined grain consumption and the risk of type 2 diabetes: a systematic review and dose-response meta-analysis of cohort studies.,3Schwingshackl L. Hoffmann G. Lampousi A.M. et al.Food groups and risk of type 2 diabetes mellitus: a systematic review and meta-analysis of prospective studies. Less than 7% of the US population consumes the recommended minimum 3 servings per day of whole grains, and more than 70% of Americans consume less than 1 serving per day of whole grains.4O'Neil C.E. Nicklas T.A. Zanovec M. Cho S. Whole-grain consumption is associated with diet quality and nutrient intake in adults: the National Health and Nutrition Examination Survey, 1999-2004. In contrast, refined grain consumption in the United States is approximately 5 times greater than whole grains, averaging ∼84% of total grain intake.5Ahluwalia N. Herrick K.A. Terry A.L. Hughes J.P. Contribution of whole grains to total grains intake among adults aged 20 and over: United States, 2013-2016. Because of this imbalance, the 2015 Dietary Guidelines Advisory Committee (DGAC) recommended that Americans should “replace most refined grains with whole grains,”6Dietary Guidelines Advisory Committee
2015. Scientific Report of the 2015 Dietary Guidelines Advisory Committee: Advisory Report to the Secretary of Health and Human Services and the Secretary of Agriculture. and the 2020 DGAC emphasized that “a shift toward higher proportion of total grains as whole grains and a reduction in refined grains is needed.”7Dietary Guidelines Advisory Committee
2020. Scientific Report of the 2020 Dietary Guidelines Advisory Committee: Advisory Report to the Secretary of Agriculture and the Secretary of Health and Human Services. Specifically with regard to T2D, both DGACs concluded that the strength of scientific evidence was “moderate” that a reduced intake of refined grains was associated with lower risk of T2D.The conclusions of the 2015 and 2020 DGACs with regard to refined grains and risk of T2D were based predominantly on 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. 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.8Alhazmi A. Stojanovski E. McEvoy M. Garg M.L. The association between dietary patterns and type 2 diabetes: a systematic review and meta-analysis of cohort studies.,9Jannasch F. Kroger J. Schulze M.B. Dietary patterns and type 2 diabetes: a systematic literature review and meta-analysis of prospective studies. However, dietary pattern research does not provide for an assessment of the risks associated with each particular food group within each dietary pattern. It is plausible that the risk associated with refined grain intake is not attributable to refined grains per se but rather to the other foods within the unhealthy dietary pattern.10Perspective: Refined grains and health: genuine risk, or guilt by association?. For example, risk of T2D has been shown to be associated with consumption of red and processed meat3Schwingshackl L. Hoffmann G. Lampousi A.M. et al.Food groups and risk of type 2 diabetes mellitus: a systematic review and meta-analysis of prospective studies.,11Micha R. Wallace S.K. Mozaffarian D. Red and processed meat consumption and risk of incident coronary heart disease, stroke, and diabetes mellitus: a systematic review and meta-analysis.,12Neuenschwander M. Ballon A. Weber K.S. et al.Role of diet in type 2 diabetes incidence: umbrella review of meta-analyses of prospective observational studies. and sugar-sweetened beverages.3Schwingshackl L. Hoffmann G. Lampousi A.M. et al.Food groups and risk of type 2 diabetes mellitus: a systematic review and meta-analysis of prospective studies.,12Neuenschwander M. Ballon A. Weber K.S. et al.Role of diet in type 2 diabetes incidence: umbrella review of meta-analyses of prospective observational studies. To establish the T2D risk associated with refined grain intake, it is necessary to include studies in which refined grains are considered as a separate food category and not within a dietary pattern. Moreover, interpretation of the published data on health risks associated with refined grain intake is complicated by how refined grain foods are defined in research studies. Refined grains can include both staple grain foods (eg, bread, cereal, pasta, rice) and indulgent grain foods (eg, cakes, cookies, and pastries).10Perspective: Refined grains and health: genuine risk, or guilt by association?.

The purpose of this commentary is to summarize published research on the association between refined grain intake and risk of T2D. PubMed, the Institute for Scientific Information’s Web of Science, and the Cochrane Database of Systematic Reviews were searched to identify relevant meta-analyses and prospective cohort studies that published relative risks (RRs) or hazard ratios (HRs) for T2D associated with refined grain intake. Only studies that analyzed refined grains as a distinct food category were considered relevant to this commentary. All RRs or HRs presented in text and in figures represent results for fully adjusted statistical models, which typically included age, sex, body mass index, smoking status, physical activity, family history of T2D, total energy intake, and intake of other foods and beverages.

Refined Grain Intake and T2D RiskTen publications (11 cohorts) provided data on T2D risk associated with intake of refined grains, including 4 cohorts of women,13Ericson U. Sonestedt E. Gullberg B. et al.High intakes of protein and processed meat associate with increased incidence of type 2 diabetes., 14Liu S. Manson J.E. Stampfer M.J. et al.A prospective study of whole-grain intake and risk of type 2 diabetes mellitus in US women., 15Meyer K.A. Kushi L.H. Jacobs Jr., D.R. Slavin J. Sellers T.A. Folsom A.R. Carbohydrates, dietary fiber, and incident type 2 diabetes in older women., 16Parker E.D. Liu S. Van Horn L. et al.The association of whole grain consumption with incident type 2 diabetes: the Women's Health Initiative Observational Study. 3 cohorts of men,13Ericson U. Sonestedt E. Gullberg B. et al.High intakes of protein and processed meat associate with increased incidence of type 2 diabetes.,17Fung T.T. Hu F.B. Pereira M.A. et al.Whole-grain intake and the risk of type 2 diabetes: a prospective study in men.,18Kochar J. Djousse L. Gaziano J.M. Breakfast cereals and risk of type 2 diabetes in the Physicians' Health Study I. and 4 cohorts of women and men combined.19Esmaillzadeh A. Mirmiran P. Azizi F. Whole-grain consumption and the metabolic syndrome: a favorable association in Tehranian adults., 20Montonen J. Knekt P. Jarvinen R. Aromaa A. Reunanen A. Whole-grain and fiber intake and the incidence of type 2 diabetes., 21Yu D. Zheng W. Cai H. et al.Long-term diet quality and risk of type 2 diabetes among urban chinese adults., 22Anjana R.M. Sudha V. Nair D.H. et al.Diabetes in Asian Indians: how much is preventable? Ten-year follow-up of the Chennai Urban Rural Epidemiology Study (CURES-142). These cohorts included a total of 268,419 women and 130,575 men. Of the 11 cohorts, 1 reported a higher risk of T2D when comparing the highest and lowest intakes of refined grains;22Anjana R.M. Sudha V. Nair D.H. et al.Diabetes in Asian Indians: how much is preventable? Ten-year follow-up of the Chennai Urban Rural Epidemiology Study (CURES-142). 1 reported a lower risk;16Parker E.D. Liu S. Van Horn L. et al.The association of whole grain consumption with incident type 2 diabetes: the Women's Health Initiative Observational Study. and 9 reported risk estimates, indicating no statistically significant associations (Figure 1). The single study reporting a higher T2D risk was the smallest cohort (1376 men and women from India), in which the major source of refined grains was white rice.22Anjana R.M. Sudha V. Nair D.H. et al.Diabetes in Asian Indians: how much is preventable? Ten-year follow-up of the Chennai Urban Rural Epidemiology Study (CURES-142). In the Women’s Health Initiative Observational Study,16Parker E.D. Liu S. Van Horn L. et al.The association of whole grain consumption with incident type 2 diabetes: the Women's Health Initiative Observational Study. the HR for the highest (≥6 servings per day) compared with the lowest (13Ericson U. Sonestedt E. Gullberg B. et al.High intakes of protein and processed meat associate with increased incidence of type 2 diabetes., 14Liu S. Manson J.E. Stampfer M.J. et al.A prospective study of whole-grain intake and risk of type 2 diabetes mellitus in US women., 15Meyer K.A. Kushi L.H. Jacobs Jr., D.R. Slavin J. Sellers T.A. Folsom A.R. Carbohydrates, dietary fiber, and incident type 2 diabetes in older women., 16Parker E.D. Liu S. Van Horn L. et al.The association of whole grain consumption with incident type 2 diabetes: the Women's Health Initiative Observational Study., 17Fung T.T. Hu F.B. Pereira M.A. et al.Whole-grain intake and the risk of type 2 diabetes: a prospective study in men., 18Kochar J. Djousse L. Gaziano J.M. Breakfast cereals and risk of type 2 diabetes in the Physicians' Health Study I.,20Montonen J. Knekt P. Jarvinen R. Aromaa A. Reunanen A. Whole-grain and fiber intake and the incidence of type 2 diabetes. were included in 2 published meta-analyses, each of which reported summary risk estimates that demonstrated no significant association between refined grain intake and T2D (Figure 1).2Aune D. Norat T. Romundstad P. Vatten L.J. Whole grain and refined grain consumption and the risk of type 2 diabetes: a systematic review and dose-response meta-analysis of cohort studies.,3Schwingshackl L. Hoffmann G. Lampousi A.M. et al.Food groups and risk of type 2 diabetes mellitus: a systematic review and meta-analysis of prospective studies.Figure 1Refined-grain intake and risk of type 2 diabetes in cohort studies, comparing highest and lowest (referent = 1.00) intakes. Vertical bars = 95% confidence intervals. ∗Risk estimate significantly higher for highest intake group, P<.05. ∗∗Risk estimate significantly lower for the highest intake group, P<.05. CURES, Chennai Urban Rural Epidemiological Study;22Anjana R.M. Sudha V. Nair D.H. et al.Diabetes in Asian Indians: how much is preventable? Ten-year follow-up of the Chennai Urban Rural Epidemiology Study (CURES-142). DR, dose-response meta-analysis (per 3 servings per day);2Aune D. Norat T. Romundstad P. Vatten L.J. Whole grain and refined grain consumption and the risk of type 2 diabetes: a systematic review and dose-response meta-analysis of cohort studies. FMCHES, Finnish Mobile Clinic Health Exam Survey;20Montonen J. Knekt P. Jarvinen R. Aromaa A. Reunanen A. Whole-grain and fiber intake and the incidence of type 2 diabetes. HPFS, Health Professionals Follow-up Study;17Fung T.T. Hu F.B. Pereira M.A. et al.Whole-grain intake and the risk of type 2 diabetes: a prospective study in men. IWHS, Iowa Women’s Health Study;15Meyer K.A. Kushi L.H. Jacobs Jr., D.R. Slavin J. Sellers T.A. Folsom A.R. Carbohydrates, dietary fiber, and incident type 2 diabetes in older women. MDC, Malmo Diet and Cancer study;13Ericson U. Sonestedt E. Gullberg B. et al.High intakes of protein and processed meat associate with increased incidence of type 2 diabetes. NHS, Nurses’ Health Study;14Liu S. Manson J.E. Stampfer M.J. et al.A prospective study of whole-grain intake and risk of type 2 diabetes mellitus in US women. PHS, Physicians’ Health Study;18Kochar J. Djousse L. Gaziano J.M. Breakfast cereals and risk of type 2 diabetes in the Physicians' Health Study I. SHWS, Shanghai Women’s Health Study;21Yu D. Zheng W. Cai H. et al.Long-term diet quality and risk of type 2 diabetes among urban chinese adults. SHMS, Shanghai Men’s Health Study;21Yu D. Zheng W. Cai H. et al.Long-term diet quality and risk of type 2 diabetes among urban chinese adults. TGLS, Tehran Lipid and Glucose Study;19Esmaillzadeh A. Mirmiran P. Azizi F. Whole-grain consumption and the metabolic syndrome: a favorable association in Tehranian adults. WHI, Women’s Health Initiative observational study.16Parker E.D. Liu S. Van Horn L. et al.The association of whole grain consumption with incident type 2 diabetes: the Women's Health Initiative Observational Study.The lack of association between intake of refined grain and T2D is surprising, considering the foods defined as refined grains. In addition to staple refined grain foods mentioned here, refined grains were defined to include cakes,14Liu S. Manson J.E. Stampfer M.J. et al.A prospective study of whole-grain intake and risk of type 2 diabetes mellitus in US women.,17Fung T.T. Hu F.B. Pereira M.A. et al.Whole-grain intake and the risk of type 2 diabetes: a prospective study in men.,20Montonen J. Knekt P. Jarvinen R. Aromaa A. Reunanen A. Whole-grain and fiber intake and the incidence of type 2 diabetes. cookies,20Montonen J. Knekt P. Jarvinen R. Aromaa A. Reunanen A. Whole-grain and fiber intake and the incidence of type 2 diabetes. sweets and desserts,14Liu S. Manson J.E. Stampfer M.J. et al.A prospective study of whole-grain intake and risk of type 2 diabetes mellitus in US women.,15Meyer K.A. Kushi L.H. Jacobs Jr., D.R. Slavin J. Sellers T.A. Folsom A.R. Carbohydrates, dietary fiber, and incident type 2 diabetes in older women. sweet rolls and bread,14Liu S. Manson J.E. Stampfer M.J. et al.A prospective study of whole-grain intake and risk of type 2 diabetes mellitus in US women.,17Fung T.T. Hu F.B. Pereira M.A. et al.Whole-grain intake and the risk of type 2 diabetes: a prospective study in men.,19Esmaillzadeh A. Mirmiran P. Azizi F. Whole-grain consumption and the metabolic syndrome: a favorable association in Tehranian adults.,20Montonen J. Knekt P. Jarvinen R. Aromaa A. Reunanen A. Whole-grain and fiber intake and the incidence of type 2 diabetes. pancakes,14Liu S. Manson J.E. Stampfer M.J. et al.A prospective study of whole-grain intake and risk of type 2 diabetes mellitus in US women.,15Meyer K.A. Kushi L.H. Jacobs Jr., D.R. Slavin J. Sellers T.A. Folsom A.R. Carbohydrates, dietary fiber, and incident type 2 diabetes in older women.,17Fung T.T. Hu F.B. Pereira M.A. et al.Whole-grain intake and the risk of type 2 diabetes: a prospective study in men.,20Montonen J. Knekt P. Jarvinen R. Aromaa A. Reunanen A. Whole-grain and fiber intake and the incidence of type 2 diabetes. waffles,14