Displacement of high glycemic-index carbohydrates and high and monounsaturated fat content may explain the positive effects….
Effie Viguiliouk and colleagues from St. Michael’s Hospital in Toronto, Ontario conducted a systematic review and meta-analysis of randomized controlled trials to assess the effect of tree nuts on glycemic control in patients with type 2 diabetes. Tree nuts that were being considered included almonds, Brazil nuts, cashews, hazelnuts, macadamia nuts, pecans, pine nuts, pistachios and walnuts. Based on the findings of 12 randomized trials involving 450 middle-aged adults, diets with median amount of 56g of tree nuts per day significantly reduced HbA1c by 0.07% and fasting glucose by 0.15mmol/L after an eight week period. But analysis of the effects of tree nuts on fasting insulin and homeostasis model assessment of insulin resistance (HOMA-IR) indicated insignificant effects.
"The ability of tree nuts to improve glycemic control may relate to a carbohydrate displacement mechanism by which tree nuts reduce the glycemic load of the diet by displacing high glycemic-index carbohydrates," the authors suggested. In addition, high amounts of magnesium and monounsaturated fat (MUFA) content in nuts might be factors in reducing the amount of glucose in the blood. Magnesium plays an important role in insulin-mediated glucose uptake. Low magnesium levels could result in impaired tyrosine-kinase activity of the insulin receptors and lead to defective insulin action. Similarly, MUFA might involve in the process of glucose transporting across skeletal muscle, as well as in buffering beta cell hyperactivity and insulin resistance. Thus, high amount of MUFA could significantly reduce HbA1c level.
Even though the results of the system review and meta-analysis on randomized controlled trials showed positive effects of tree nuts on glycemic control, there were several factors that needed to be considered. For instance, the sample size of these trials was relatively small, and the studies were poorly conducted. Also, the short follow-up period (median duration = 8 weeks) of these trials might not be long enough to alter the glucose levels in the patients’ body. "In our meta-analysis, all 5 trials with a crossover design contained a washout period ranging from 1–8 weeks. Since HbA1c reflects mean glycemia for the previous 3 months, it is not certain whether this is long enough to diminish any potential carry-over effects." Last, most of the subgroup analyses were underpowered; therefore, effect of other factors on glycemic control could not be evaluated.
- A systematic review of randomized controlled trials indicated that consumption of diets rich in tree nuts for an average of 8 weeks could reduce HbA1c levels in patients with type 2 diabetes.
- Limitations of the evaluated trials included small sample size, short duration of follow-up and poor quality.
- Larger, longer, and higher quality trials should be conduct in the future to help guide the development of nutrition recommendations.
Viguiliouk E, Kendall C, 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(7): e103376.