Abstract
BackgroundTree nut consumption has been associated with reduced diabetes risk, however, results from randomized trials on glycemic control have been inconsistent.ObjectiveTo provide better evidence for diabetes guidelines development, we conducted a systematic review and meta-analysis of randomized controlled trials to assess the effects of tree nuts on markers of glycemic control in individuals with diabetes.Data SourcesMEDLINE, EMBASE, CINAHL, and Cochrane databases through 6 April 2014.Study SelectionRandomized controlled trials ≥3 weeks conducted in individuals with diabetes that compare the effect of diets emphasizing tree nuts to isocaloric diets without tree nuts on HbA1c, fasting glucose, fasting insulin, and HOMA-IR.Data Extraction and SynthesisTwo independent reviewer’s extracted relevant data and assessed study quality and risk of bias. Data were pooled by the generic inverse variance method and expressed as mean differences (MD) with 95% CI’s. Heterogeneity was assessed (Cochran Q-statistic) and quantified (I2).ResultsTwelve trials (n = 450) were included. Diets emphasizing tree nuts at a median dose of 56 g/d significantly lowered HbA1c (MD = −0.07% [95% CI:−0.10, −0.03%]; P = 0.0003) and fasting glucose (MD = −0.15 mmol/L [95% CI: −0.27, −0.02 mmol/L]; P = 0.03) compared with control diets. No significant treatment effects were observed for fasting insulin and HOMA-IR, however the direction of effect favoured tree nuts.LimitationsMajority of trials were of short duration and poor quality.ConclusionsPooled analyses show that tree nuts improve glycemic control in individuals with type 2 diabetes, supporting their inclusion in a healthy diet. Owing to the uncertainties in our analyses there is a need for longer, higher quality trials with a focus on using nuts to displace high-glycemic index carbohydrates.Trial RegistrationClinicalTrials.gov NCT01630980
Highlights
Tree nuts are a healthy source of vegetable protein [1], unsaturated fatty acids [1,2], fibre [3], antioxidants [4], vitamins, minerals, and phytochemicals [5]
Diets emphasizing tree nuts at a median dose of 56 g/d significantly lowered Hemoglobin A1c (HbA1c) (MD = 20.07% [95% CI:20.10, 20.03%]; P = 0.0003) and fasting glucose (MD = 20.15 mmol/L [95% CI: 20.27, 2 0.02 mmol/L]; P = 0.03) compared with control diets
No significant treatment effects were observed for fasting insulin and homeostasis model assessment of insulin resistance (HOMA-IR), the direction of effect favoured tree nuts
Summary
Tree nuts are a healthy source of vegetable protein [1], unsaturated fatty acids [1,2], fibre [3], antioxidants [4], vitamins (i.e. folic acid, vitamin B6, niacin, tocopherols), minerals (i.e. magnesium, potassium, calcium), and phytochemicals (i.e. phytosterols) [5]. Over the last two decades, a large body of evidence regarding tree nut consumption and related health outcomes has emerged from both epidemiological and controlled trials [1]. There is strong evidence that tree nuts lower LDL-cholesterol [8], which has resulted in an FDA qualified health claim [9] and their inclusion in heart association guidelines for cardiovascular risk reduction [10,11]. 2 recent systematic review and meta-analysis of prospective cohort studies showed no overall significant association between nut consumption and type 2 diabetes risk [7,16], which was consistent with a subgroup analysis of the PREDIMED trial that showed a nonsignificant reduction in diabetes risk for individuals consuming a Mediterranean diet supplemented with nuts in comparison to a low fat diet [17]. Tree nut consumption has been associated with reduced diabetes risk, results from randomized trials on glycemic control have been inconsistent
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