Diabetes mellitus is a worldwide pandemic that affected 382 million people in 2013, and is expected to increase to 592 million people by 2035.¹ Type 2 diabetes mellitus (T2DM) and atherosclerotic cardiovascular disease (ASCVD) share a number of common risk factors, and patients with T2DM are at 2- to 4-fold higher risk for an ASCVD event compared to those without T2DM.² Lifestyle therapies, particularly dietary modification and regular physical activity, are central to the management of cardiometabolic risk in those with T2DM, or at risk for its development.³ The Diabetes Prevention Program showed that a lifestyle intervention including targets of 7% body weight loss and ≥ 150 min per week physical activity lowered risk of new-onset T2DM by 58% compared with usual care, and this finding has been confirmed in several subsequent trials in many countries.^4,5 However, beyond weight loss and exercise, diet quality has important effects on cardiometabolic risk factors, and measures of diet quality have been associated with risks for incident ASCVD and T2DM in large, prospective cohort studies.⁶

The recently released Dietary Guidelines for Americans (2015-2020) recommend dietary patterns that have been associated with low risk for the development of T2DM and ASCVD in observational studies and have been shown to improve well-established cardiometabolic risk factors in dietary intervention studies.⁷ These dietary patterns can take a number of forms, such as a healthy US-style diet, a Mediterranean diet, the Dietary Approaches to Stop Hypertension diet and others, but these dietary patterns share an emphasis on consumption of whole grains, nuts, seeds, legumes, nontropical oils, fruits, and vegetables, seafood, lean meats, and low-fat or fat-free dairy products.^7,8 In addition, the Dietary Guidelines for Americans emphasize that dietary patterns high in solid fats and shortenings, refined grains, added sugars, high-fat meats, and high-fat dairy products have been associated with increased risks for both T2DM and ASCVD.7,8 Recommendations for individuals with T2DM are similar to those for the general population, emphasizing healthful eating patterns, with particular attention to macronutrient distribution, especially the types of carbohydrates and fats consumed.⁹

In practice, applications of these dietary recommendations focus on reducing calories from saturated and trans fats, refined starches and added sugars. Foods high in these dietary components may be substituted with alternatives that are higher in unsaturated fats, carbohydrates from whole grains and other unrefined sources such as fruits, vegetables, nuts, seeds and legumes, and lean protein foods.⁸

Macronutrients and ASCVD Risk

Recently, Li et al.¹⁰ published an analysis from two large prospective cohorts in which they modeled the predicted effects of substituting calories from saturated fatty acids with calories from other types of fatty acids and carbohydrates from different sources on coronary heart disease risk. The study involved participants in the Nurses' Health Study (1980-2010) and the Health Professionals Follow-Up Study (1986-2010) who were free from diabetes and CVD at baseline. Diet was assessed using a semi-quantitative food frequency questionnaire every 4 years. During the 24-30 years of follow-up there were 7,667 documented cases of coronary heart disease. The main results are shown in Figure 1.

The modeling indicated that replacing calories from SFA with those from refined starches and added sugars was not associated with a significant difference in coronary heart disease risk. Thus, saturated fatty acids and refined starches/added sugars have similar associations with coronary heart disease incidence, illustrating why the Dietary Guidelines for Americans have identified these as targets for reduction in the average American diet. In contrast, replacing 5% of energy from SFA with either polyunsaturated fatty acids (PUFA) or monounsaturated fatty acids (MUFA) was associated with reductions in coronary heart disease risk of 25% and 15%, respectively. Replacing 5% of energy from saturated fatty acids with carbohydrates from whole grains was associated with 9% lower risk for coronary heart disease.

Dietary Carbohydrates and T2DM Risk

Foods high in refined starches and added sugars tend to have high glycemic index, since the glucose liberated from digestion of refined starches and disaccharides, such as sucrose, is rapidly absorbed into the circulation. A high glycemic index creates additional demand on the pancreatic beta-cells to produce sufficient insulin to maintain normoglycemia. In contradistinction, when glucose is absorbed more slowly, a larger proportion is disposed of through insulin-independent mechanisms.^8,11

Bhupathiraju and colleagues¹² evaluated the relationships between dietary glycemic index and glycemic load (average dietary glycemic index multiplied by the quantity of carbohydrate consumed) with incident T2DM in three large cohorts. Participants included 74,248 women from the Nurses' Health Study (1984–2008), 90,411 women from the Nurses' Health Study II (1991–2009), and 40,498 men from the Health Professionals Follow-Up Study (1986–2008) who were free of diabetes, CVD, and cancer at baseline. Diet was assessed using a semi-quantitative food frequency questionnaire every four years. During 3,800,618 person-years of follow-up, 15,027 cases of incident T2DM were documented.

In pooled analyses, compared to a reference group that had a diet with low glycemic index and high intake of cereal fiber (categorized by tertiles), those who had a high dietary glycemic index and a low cereal fiber intake had 59% higher risk for T2DM in adjusted models. Similar results were obtained for glycemic load, with the high glycemic load, low cereal fiber intake category showing a 47% higher risk for T2DM development.

AlEssa and colleagues¹³ completed a similar analysis from the Nurses' Health Study cohort to assess relationships between dietary carbohydrate quantity and quality with risk for T2DM. As shown in Table 1, higher starch intake and lower total dietary fiber intake were associated with higher T2DM risk. A high starch, low dietary fiber (tertiles) combination was associated with 43% higher incidence than the low starch, high total fiber reference group in an adjusted model.

Results from observational studies, such as the prospective cohort studies described herein, commonly represent the primary evidence base from which dietary recommendations are formulated. Observational studies are limited by their lack of randomization, which makes it difficult to rule out bias and residual confounding as potential explanations for the relationships detected between dietary exposures and disease.¹⁴ Limited data from randomized, controlled trials are available from which to assess the effects of dietary interventions on clinical outcomes. Therefore, results from observational studies along with data from intervention trials evaluating effects on disease risk factors are often the best available evidence from which to draw inferences and make dietary recommendations.

Our group and others have shown in dietary intervention studies that increasing intakes of some dietary fibers enhances whole body insulin sensitivity, while increasing intake of sugar-sweetened products impairs insulin sensitivity.^8,15,16 Such results provide biologic plausibility for the associations between lower glycemic index/load and higher fiber intake with reduced cardiometabolic disease risk, and for dietary patterns high in refined starches and added sugars with greater cardiometabolic disease risk.

Conclusions

The findings summarized above illustrate the rationale for the recently released Dietary Guidelines for Americans, which suggest that the average American diet contains excessive quantities of saturated fats, refined starches, and added sugars. The cardiometabolic risk profile will generally be improved when foods high in these dietary components are substituted with alternatives that are higher in unsaturated fats such as nuts, seeds and nontropical oils; carbohydrates from whole grains and other unrefined sources such as fruits, vegetables, and legumes; and lean protein foods such as seafood and low-fat dairy products.⁶

References

1. Guariguata L, Whiting DR, Hambleton I, et al. Global estimates of diabetes prevalence for 2013 and projections for 2035. Diab Res Clin Prac 2014;103:137-149.
2. Mozaffarian D, Benjamin EJ, Go AS, et al. Heart disease and stroke statistics—2016 update: a report from the American Heart Association. Circulation 2016;133:e38-e360.
3. Fox CS, Golden SH, Anderson C, et al. Update on prevention of cardiovascular disease in adults with type 2 diabetes mellitus in light of recent evidence: a scientific statement from the American Heart Association and the American Diabetes Association. Diabetes Care 2015;38:1777-803.
4. Diabetes Prevention Program Research Group; Knowler WC, Barrett-Connor E, Fowler SE, et al. Reduction in the incidence of type 2 diabetes with lifestyle intervention or metformin. N Engl J Med 2002;346:393-403.
5. Balk EM, Earley A, Raman G, et al. Combined diet and physical activity promotion programs to prevent type 2 diabetes among persons at increased risk: a systematic review for the Community Preventive Services Task Force. Ann Intern Med 2015;163:437-51.
6. Mozaffarian D, Appel LJ, Van Horn L. Components of a cardioprotective diet: new insights. Circulation 2011;123:2870-91.
7. U.S. Department of Agriculture and U.S. Department of Health and Human Services. 2015-2020 Dietary Guidelines for Americans. 8th Edition. December 2015. Available at http://health.gov/dietaryguidelines/2015/guidelines/.
8. Maki KC, Phillips AK. Dietary substitutions for refined carbohydrates that show promise for reducing risk of type 2 diabetes in men and women. J Nutr 2015;145:159S-163S.
9. Evert AB, Boucher JL, Cypress M, et al. Nutrition therapy recommendations for the management of adults with diabetes. Diabetes Care 2013;36:3821-42.
10. Li Y, Hruby A, Bernstein AM, et al. Saturated fats compared with unsaturated fats and sources of carbohydrates in relation to risk of coronary heart disease: a prospective cohort study. J Am Coll Cardiol 2015;66:1538-48.
11. Willet W, Manson J, Liu S. Glycemic index, glycemic load, and risk of type 2 diabetes. Am J Clin Nutr 2002;76:274S-280S.
12. Bhupathiraju SN, Tobias DK, Malik VS, et al. Glycemic index, glycemic load, and risk of type 2 diabetes: results from 3 large US cohorts and an updated meta-analysis. Am J Clin Nutr 2014;100:218-32.
13. AlEssa HB, Bhupathiraju SN, Malik VS, et al. Carbohydrate quality and quantity and risk of type 2 diabetes in US women. Am J Clin Nutr 2015;102:1543-53.
14. Maki KC, Slavin JL, Rains TM, et al. Limitations of observational evidence: implications for evidence-based dietary recommendations. Adv Nutr 2014;5:7-15.
15. Maki KC, Davidson MH, Witchger MS, et al. Effects of high-fiber oat and wheat cereals on postprandial glucose and lipid responses in healthy men. Int J Vitam Nutr Res 2007;77:347-56.
16. Maki KC, Nieman KN, Schild AL, et al. Sugar-sweetened product consumption alters glucose homeostasis compared with dairy product consumption in men and women at risk of type 2 diabetes mellitus. J Nutr 2015b;145:459-66.

Keywords: Biological Products, Cardiovascular Diseases, Coronary Disease, Diabetes Mellitus, Type 2, Diet, Mediterranean, Epidemiologic Studies, Food Habits, Glucose, Glycemic Index, Hypertension, Insulin, Insulin Resistance, Life Style, Neoplasms, Nutrition Policy, Pandemics, Prospective Studies, Risk Factors, Weight Loss, Primary Prevention

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