Tree Nuts and Type 2 Diabetes: Is the Science Conclusive?

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Studies on the health benefits of tree nut consumption go back decades, and link many varieties with reduced risk for diabetes and improved glycemic control. Yet the topic is not without controversy, since nutritional studies are prone to methodologic flaws. So, will tree nuts added to the daily diet really help our patients with type 2 diabetes (T2DM) or those at risk for the disease? Two recent analyses, both published in the same week, say yes. Yet both studies were partially sponsored by the International Tree Nut Council, raising concerns about conflicts of interest.

Surveys Say: Go Nuts!

Both studies undertook systematic reviews and meta-analyses of RCTs published up to April 2014. The authors of both reviews noted significant heterogeneity between studies, the majority of which were limited by poor quality and short duration (median follow-up about 8 weeks).

In the first study, published in BMJ Open, Blanco-Mejia and colleagues¹ looked at RCTs about nut consumption and metabolic syndrome. They included 47 trials, which covered 2111 participants who were either healthy or had metabolic syndrome or T2DM. Compared with controls, results suggested that nut consumption:

  • “Modestly” lowered triglyceride levels by about 0.06 mmol/L  (95% CI, −0.09 to −0.03 mmol/L)

  • “Modestly” lowered fasting blood glucose by about 0.08 mmol/L (95% CI, −0.16 to −0.01 mmol/L)

  • Had little effect on waist circumference, HDL, or blood pressure

“These data support recommendations to consume tree nuts alone or as part of heart healthy dietary patterns such as the Mediterranean, Portfolio, Vegetarian, and DASH dietary patterns as a means for improving metabolic control,” the authors wrote.

In the second study, published in PLoS One, Viguiliuk and colleagues² looked at the effect of nut consumption on glycemic control in people with T2DM. The analysis included 12 trials with 450 total participants. Compared with controls, nut consumption:

  • Lowered HbA1c by 20.07% (95% CI, 20.10 to 20.03%; P=.0003)

  • Lowered fasting glucose by 20.15 mmol/L (95% CI,  20.27 to 20.02 mmol/L; P=.03)

  • Had little effect on fasting insulin and HOMA

“Pooled analyses show that tree nuts improve glycemic control in individuals with type 2 diabetes, supporting their inclusion in a healthy diet,” the authors wrote.

The authors of both papers pointed out that results could be explained by an overall shift to a healthier eating pattern in which nuts displace foods high in saturated fat, or carbohydrates with a high glycemic index.
 

The Mediterranean Diet and Tree Nuts

The ample body of research on the Mediterranean diet has linked this eating pattern to many health benefits, including decreased risk for metabolic syndrome, diabetes, and heart disease. The recent, large PREDIMED trial, an RCT conducted by researchers in Spain and at Tufts University, has looked at nut consumption within the context of the Mediterranean diet. Participants aged 55 to 80 who had high cardiovascular risk were randomized to Mediterranean diets supplemented with nuts (about 3 times per week) or extra virgin olive oil, versus a low-fat diet as control. Among the many subanalyses, results have suggested:

  • After about 1 year, those on the olive oil and nut diets had lower prevalence and incidence of metabolic syndrome³

  • After about 1 year, increased adiponectin/leptin ratios were seen (a measure of atherogenesis) for low-fat (P=.043), olive oil (P=.001), and nut diets (P<.001), and increased adiponectin/HOMA-IR ratios seen (a measure of insulin resistance) for low-fat (P=.061), olive oil (P=.027), and nut diets (P=.069).⁴

  • After about 4.8 years, among 7216 participants, nut consumption was linked to a significantly reduced risk of all-cause mortality (P <.05), equating to about 39% lower risk of mortality (hazard ratio (HR) 0.61; 95% CI, 0.45 to 0.83), compared with controls.⁵

Commenting independently about the PREDIMED results that linked nut consumption to reduced risk for all-cause mortality, Rohrmann and Faeh,⁶ writing in BMC Med and who declared no competing interests, stated:

“The current analysis of the PREDIMED data speaks in favor of an additional protective effect of nut consumption in the framework of a healthy diet.” They based their statement on past PREDIMED analyses, and the fact that most research evidence supporting the benefits of nut consumption on cardiovascular and chronic diseases comes from the US, where nut consumption and adherence to the Mediterranean diet is low.

Nut consumption, note Rohrmann and Faeh, enhance overall diet quality, adding polyunsaturated fatty acids, minerals (calcium, magnesium, postassium) and vitamins as well as phytochemicals, such as phenolic acids, polyphenols and phytosterols.

Guidelines: Pro Nuts

The American Diabetes Association (ADA) 2013 guidelines⁷ mention a possible benefit of plant sterols and stanols in blocking intestinal absorption of cholesterol, and link them to lower LDL in people with T2DM. The guidelines note that Mediterranean-style, plant-based (vegan or vegetarian), low-fat and lower-calorie eating patterns may be effective in management of diabetes.   

In its 2013 clinical practice guidelines, the Canadian Diabetes Association (CDA) mentions that consumption of nuts in conjunction with healthy eating patterns has been linked to reduced risk of cardiovascular disease. The CDA recommends nut consumption as part of a dietary pattern that replaces saturated fats with healthier options, such as the Mediterranean and DASH diets.⁸

Conclusion

At the end of the day, the evidence supports nuts. As part of a healthy diet, eating tree nuts-as long as they’re not covered in salt or chocolate-is a good alternative to French fries or a bag of potato chips. Replacing high-glycemic-index, high-saturated-fat, empty-calorie foods with healthier choices, and educating your patients on how to do it, at the very least “does not harm.” 

What are your thoughts on encouraging nut consumption among your patients with T2DM? Would they swap an almond for a French fry? Leave a comment for your colleagues, below.

References:

• Blanco Mejia S, Kendall CWC, Viguiliouk E, et al. Effect of tree nuts on metabolic syndrome criteria: a systematic review and meta-analysis of randomised controlled trials. BMJ Open 2014;4:e004660. doi:10.1136/bmjopen-2013- 00466
• Viguiliouk E, Kendall CWC, Blanco Mejia S, et al. Effect of tree nuts on glycemic control in diabetes: a systematic review and meta-analysis of randomized controlled dietary trials. PLoS One. 2014;9:e103376. doi:10.1371/journal.pone.010337
• Mayneris-Perxachs J, Sala-Vila A, Chisaguano M, et al; PREDIMED Study Investigators. Effects of 1-year intervention with a Mediterranean diet on plasma fatty acid composition and metabolic syndrome in a population at high cardiovascular risk. PLoS One. 2014;9:e85202. doi:10.1371/journal.pone.0085202. eCollection 2014.
• Lasa A, Miranda J, Bulló M, et al. Comparative effect of two Mediterranean diets versus a low-fat diet on glycaemic control in individuals with type 2 diabetes. Eur J Clin Nutr. 2014;68:767-772. doi:10.1038/ejcn.2014.1. Epub 2014 Feb 12.
• Guasch-Ferré M, Bulló M, Martínez-González MÁ, et al; PREDIMED Study Group. Frequency of nut consumption and mortality risk in the PREDIMED nutrition intervention trial. BMC Med. 2013;11:164. doi:10.1186/1741-7015-11-164.
• Rohrmann S, Faeh D. Should we go nuts about nuts? BMC Med. 2013;11:165. doi:10.1186/1741-7015-11-165.
• American Diabetes Association. Clinical Practice Recommendations. Diabetes Care. 2013;36:S1-S110. http://care.diabetesjournals.org/content/36/Supplement_1.
• Evert AB, Boucher JL, Cypress M, et al. Nutrition therapy recommendations for the management of adults with diabetes. Diabetes Care. 2013;36:3821–3842. doi:10.2337/dc13-2042