Abstract
The primary aim was to investigate the effects of nut and seed consumption on markers of glucose metabolism in adults with prediabetes. Studies with a randomized controlled trial (RCT) design, comparing the effects of a diet containing nuts or seeds against a diet without nuts or seeds in adults with prediabetes, were considered eligible. Primary outcome measures included fasting plasma glucose (FPG), 2-hour plasma glucose (PG) during oral glucose tolerance test (OGTT) and glycated haemoglobin (HbA1c) concentrations. Studies were identified by searching PubMed and Scopus electronic databases and by checking full texts and reference lists of relevant records. Risk of bias was assessed using the Cochrane Collaboration’s tool. We included 5 RCTs involving 371 adults with prediabetes or at risk of diabetes; 3 RCTs investigated the effects of whole nut consumption and 2 the effects of ground flaxseed consumption. Consumption of 57 g/day pistachios or mean intake of 60 g/day almonds for 4 months improved FPG and fasting plasma insulin (FPI) concentrations, insulin resistance, cellular glucose uptake in lymphocytes and β-cell function. Consumption of 56 g/day walnuts for 6 months was not found to affect FPG or HbA1c concentrations. Consumption of 13 g/day flaxseed for 3 months improved FPG and FPI concentrations and insulin resistance. In a second study, however, flaxseed consumption was not found to affect markers of glucose metabolism. The risk of bias was generally low, thus the reported results could be reliable. Further investigation of nut and seed consumption effects in the field of prediabetes is warranted.
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Key Points
Prediabetes is considered an intermediate state of glucose metabolism between glucose homeostasis and type 2 diabetes (T2D). Subjects with prediabetes have increased blood glucose levels, but they do not meet the criteria for being diagnosed with T2D. Prediabetes is defined by the presence of isolated impaired fasting glucose (IFG), isolated impaired glucose tolerance (IGT) or both; IFG refers to increased fasting plasma glucose (FPG) levels after 8-12 hours of overnight fast, whereas IGT refers to increased post-prandial plasma glucose (PG) levels after consumption of 75 g glucose during a 2-hour oral glucose tolerance test (OGTT).
This paper presents a systematically review of randomized controlled trials which investigate the effects of nut and seed consumption on markers of glucose metabolism in adults with prediabetes. A secondary aim was to report other cardio-metabolic effects of nut and seed consumption in the same population, according to the aims of each study.
FPG concentrations were significantly decreased in the flaxseed low-dose treatment (13 g/day) compared to the flaxseed-free treatment. Fasting plasma insulin concentrations and HOMA-IR were significantly decreased in the flaxseed low-dose treatment (13 g/day) compared to both the flaxseed high-dose (26 g/day) and flaxseed-free treatments. No significant differences were found for plasma fructosamine across treatments. After 3 months in the flaxseed low-dose treatment, FPG and FPI concentrations, as well as HOMA-IR were decreased. Treatment with a flaxseed dose twice as high as the low-dose, did not improve markers of glucose metabolism. Treatment with either a low-dose or a high-dose flaxseed dietary supplement for 3 months did not significantly affect markers of inflammation (i.e. serum concentrations of IL-6 and CRP) or adiposity (i.e. plasma concentrations of adiponectin).
Significant differences were found across the three treatment arms with respect to systolic blood pressure (SBP); this was decreased in the flaxseed high-dose (40 g/day) and the flaxseed low-dose (20 g/day) treatments, whereas SBP was increased in the flaxseed-free treatment. No significant differences were found for diastolic blood pressure across treatments. In the present systematic review of RCTs evaluating the effects of nuts or seeds in adults with prediabetes, flaxseeds improved markers of glucose metabolism. Anthropometric measures, blood pressure and blood lipid profile were not significantly affected by nut consumption, whereas systolic blood pressure was improved by flaxseed consumption. Simple, efficient and sustainable dietary strategies for the prevention of T2D, more and well-powered RCTs are needed to ascertain whether nuts and seeds could be a healthy snack for adults with prediabetes.