Diabetes

Walnut Consumption and Type 2 Diabetes Risk: The Importance of Associations

Abstract and Commentary

By Anne Cook, MD, FACP, Associate Program Director, AnMed Health Family Medicine Residency, Anderson, SC. Dr. Cook reports no financial relationships relevant to this field of study.

Synopsis: This large, prospective, cohort trial followed women for 10 years using validated food questionnaires to assess if the consumption of walnuts, other tree nuts, and peanuts was associated with the incidence of the development of type 2 diabetes. The results showed that women who consumed walnuts had a lower incidence of developing type 2 diabetes and that this reduction appeared dose-related though somewhat attenuated when adjusted for body mass index (BMI). The previously described inverse associations between diabetes and total nut or tree nut consumption were essentially negated when adjusted for BMI.1

Source: Pan A, et al. Walnut consumption is associated with lower risk of type 2 diabetes in women. J Nutr 2013 jn.112.172171.

Given the epidemic of type 2 diabetes confronting the United States and the world, extensive research is focusing on primary prevention. Diet and lifestyle interventions are known to be key components in the prevention of type 2 diabetes with obesity the prime driver of the epidemic. However, evidence in the literature suggests that the composition of the diet, specifically the type of fat consumed, may play a role in the development of diabetes. There is evidence that higher intake of polyunsaturated fatty acids (PUFAs) and monounsaturated fatty acids (MUFAs) and lower intake of saturated fat and trans fat is associated with a decreased risk of type 2 diabetes.2

Prior reports from the Nurses’ Health Study (NHS) have shown inverse associations between nut consumption in women and the risk of developing type 2 diabetes but they did not address the issue of specific types of nuts.3 The current study examined the association between walnuts and the development of type 2 diabetes and compared walnut consumption to consumption of other tree nuts and total nut consumption.

This was a prospective cohort study done as part of the NHS and the Nurses’ Health Study II (NHS II) assessing the consumption of walnuts and other nuts and the statistical relationship between consumption of these nuts and the development of type 2 diabetes during the 10-year follow-up period. Because of the known association between diabetes and obesity, analysis also was performed to assess any modulation of effect based on BMI.

The study population previously has been described but briefly consisted of registered nurses with 121,700 participants from 11 states initially enrolled in the NHS in 1976 and an additional 116,671 younger nurses from 14 states beginning in 1989 for the NHS II. This study looked at women aged 52-77 years in the NHS study and aged 35-52 years in the NHS II study each over 10-year periods (NHS 1998-2008 and NHS II 1999-2009). Walnut consumption was included as a variable in the surveys beginning in 1998 (NHS) or 1999 (NHS II), though nut consumption in general had been a part of the surveys beginning in 1986. Exclusion criteria included known diabetes, heart disease, stroke, or cancer at baseline as well as missing information on nut variables, implausible energy intake, or missing information on body weight. This left 58,063 women in the NHS and 79,893 women in the NHS II. The cumulative follow-up of both cohorts exceeds 90% of the potential person-times. Standardized and validated food consumption questionnaires were administered every 4 years. A serving size of 1 ounce or 28 g was used. The diagnosis of diabetes was established by self-reporting of patients confirmed by supplemental questionnaires asking for specific ADA diagnostic criteria. This method had been previously validated within the NHS study.

There were other covariates assessed and included in the multivariate analysis using time-dependent Cox proportional hazard models including weight, cigarette smoking, physical activity, menopausal status, hormone status, and dietary issues including consumption of alcohol, whole grains, fruits, vegetables, fish, red meat, coffee, and sugar-sweetened beverages. Family history, race, and use of multivitamins were also included in the analysis.

Walnut consumption was inversely associated with the development of type 2 diabetes. Pooled hazard ratios (HRs) (95% confidence intervals [CI]) for participants consuming 1-3 servings/month, 1 serving/week, and > 2 servings/week of walnuts were 0.93 (0.88-0.99), 0.81 (0.70-0.94), and 0.67 (0.54-0.82), respectively, when compared to women who never or rarely consumed walnuts (P-trend < 0.0001). Adjustment for updated BMI did slightly attenuate the association, and the HRs (95% CI) were 0.96 (0.90-1.02), 0.87 (0.75-1.01), and 0.76 (0.62-0.94), respectively (P-trend = 0.002). The authors did not find any association between walnut consumption and obesity status, physical activity, dietary quality, or family history of diabetes.

When total nut consumption (peanuts, walnuts, and other tree nuts), peanut consumption, and other tree nut consumption (excluding walnuts) were analyzed, there also was an inverse association with the development of diabetes, but this was attenuated to null when adjusted for BMI in all three groups.

Commentary

This study offered convincing evidence for the negative association between walnut consumption and type 2 diabetes at the amounts of intake studied. It is a little puzzling that when walnut consumption was part of the total nut analysis, the results did not support the inverse relationship when adjusted for BMI. It is unclear in the “total nut” or “total tree nut” groups what amount of the nut consumption included walnuts.

The plausibility of the hypothesis that walnuts may confer special health benefits comes from their somewhat unique fatty acid composition. Walnuts are higher in PUFAs (47% by weight) and specifically the omega-3 fatty acid, alpha-linolenic acid (9% by weight). There is substantial evidence for the positive influence of walnut consumption on lipid profiles, markers of endothelial cell function, and markers of the metabolic syndrome.4,5

Although the data from this study and others support the inclusion of walnuts as part of a healthful diet, what does this mean in practice? As with all studies that examine the components of diet, it is very difficult to remove other potential confounders. In this study, for instance, it was noted, “women who ate more walnuts also consumed more fish, whole grains, fruit and vegetables, and total energy.” They also “were older and tended to weigh less, exercise more, and smoke less than women with infrequent consumption.” Additionally, study participants were divided into groups consuming 1-3 servings per month, 1 serving/week, and > 2 servings/week. A serving was defined as 28 g or about 7 walnuts (14 halves).6 A 28-g serving of walnuts has approximately 185 calories. Although the study showed benefit independent of BMI, these intakes were certainly very modest and there must be some concern about the results of this study encouraging people to increase caloric intake excessively. However, there have been studies showing that people eating unrestricted diets containing nuts had lower BMIs.4 One weakness was there was not any subanalysis to remove the confounding variables mentioned above.

It should always be cautioned against to attribute cause and effect when looking at cohort dietary studies but it seems reasonable to encourage patients to include modest consumption of walnuts as part of a healthy lifestyle. Walnuts appear to offer greater benefits than do other nuts. Patients should realize that walnuts and all nuts are very calorie dense and that the information we have is limited to the “doses” discussed. For many people, a serving of 7 walnuts two or three times a week would be considered medicinal consumption, not dietary intake!

References

1. Pan A, et al. Walnut consumption is associated with lower risk of type 2 diabetes in women. J Nutr 2013 jn.112.172171.

3. Jiang R, et al. Nut and peanut butter consumption and risk of type 2 diabetes in women. JAMA 2002;288:2554-2560.

2. Risérus U, et al. Dietary fats and prevention of type 2 diabetes. Prog Lipid Res 2009;48:44-51.

3. USDA ARS, Nutrient Data Laboratory. USDA National Nutrient Database for Standard Reference. Available at: www.nal.usda.gov/fnic/foodcomp/search. Accessed April 20, 2013.

4. Banel DK, Hu FB. Effects of walnut consumption on blood lipids and other cardiovascular risk factors: A meta-analysis and systematic review. Am J Clin Nutr 2009;90:56-63.

5. Risérus U. Fatty acids and insulin sensitivity. Curr Opin Clin Nutr Metab Care 2008;11:100-105.

6. Sabaté J. Nut consumption and body weight. Am J Clin Nutr 2003;78(suppl):647S-650S.