Nut consumption and risk of cancer and type 2 diabetes: a systematic review and meta-analysis

Context: The identification of foods that can decrease the risk of cancer and type 2 diabetes may be helpful in reducing the burden of these diseases. Although nut consumption has been suggested to have a disease-preventive role, current evidence remains inconsistent. Objective: The aim of this systematic review and meta-analysis was to clarify the association between nut consumption and risk of cancer or type 2 diabetes. Data Sources: Six databases were searched for relevant studies from the time of database inception to August 2014. Reference lists of relevant review articles were hand searched, and authors were contacted when data were insufficient. Study Selection: Eligible studies included epidemiological studies (case-control and cohort) or clinical trials that reported an association between nut consumption and the outcome of type 2 diabetes or specific cancers. Data Extraction: Two investigators independently extracted descriptive, quality, and risk data from included studies. Data Synthesis: Random-effects meta-analysis was used to pool relative risks from the included studies. The I-2 statistic was used to assess heterogeneity. A total of 36 eligible observational studies, which included 30 708 patients, were identified. The studies had fair methodological quality, and length of follow-up ranged between 4.6 years and 30 years. Comparison of the highest category of nut consumption with the lowest category revealed significant associations between nut consumption and decreased risk of colorectal cancer (3 studies each with separate estimates for males and females, RR 0.76, 95% confidence interval [95% CI] 0.61-0.96), endometrial cancer (2 studies, RR 0.58, 95% CI 0.43-0.79), and pancreatic cancer (1 study, RR 0.68, 95% CI 0.48-0.96). No significant association was found with other cancers or type 2 diabetes. Overall, nut consumption was significantly associated with a reduced risk of cancer incidence (RR 0.85, 95% CI 0.76-0.95). Conclusions: Nut consumption may play a role in reducing cancer risk. Additional studies are needed to more accurately assess the relationship between nut consumption and the prevention of individual types of cancer, given the scarcity of available data.