Association of dietary nitrate and nitrite from plant sources with digestive system cancer risk: a systematic review and meta-analysis.

[BACKGROUND] Recent studies have indicated that dietary nitrate intake from plant sources offers numerous health benefits. However, the relationships between the intake of plant-derived nitrates and nitrites and the risk of digestive system cancers (DSCs) remain unclear. This study aimed to quantify the associations between dietary nitrate and nitrite from plant sources and the risk of DSCs via a systematic review and meta-analysis.

[METHODS] We conducted extensive literature searches of the PubMed, Embase, Scopus, Web of Science, CINAHL, Cochrane Library, CNKI, VIP, Wanfang, and CBM databases up to April 2024. STATA 14.0 was applied for meta-regression and meta-analysis, and fixed or random effects models were used to calculate the pooled relative risks and 95% confidence intervals. Sensitivity and subgroup analyses were conducted to explore sources of heterogeneity.

[RESULTS] Our analysis included 13 articles with a total of 897,585 participants, which included 10 different types of digestive tumors. The pooled results indicated no significant association between high (median: >134.9 mg/day) [RR = 0.92; 95% CI (0.82-1.03); P = 0.14] or moderate (median: 83.0-133.1 mg/day) [RR = 0.95; 95% CI (0.89-1.02); P = 0.186] nitrate intake and DSCs. Similarly, neither high (median: >0.91 mg/day) [RR = 0.91; 95% CI (0.84-1.00); P = 0.05] nor moderate (median: 0.75 mg/day) [RR = 0.96; 95% CI (0.89-1.04); P = 0.355] nitrite intake was correlated with DSCs. However, a negative association was observed between nitrate or nitrite intake and DSCs when the data were stratified by subgroup variables such as study type, sex, region, antioxidant intake, and fibre intake. A meta-regression dose‒response analysis revealed that the risk of gastric cancer was negatively associated with the median intake of plant-derived nitrates [slope= -0.0047 per mg/day; 95% CI (-0.0086--0.0008); P = 0.022].

[CONCLUSIONS] Plant-derived nitrate and nitrite intakes were not significantly associated with DSCs. This relationship may be affected by subgroup variables, and a dose‒response analysis indicated that higher nitrate intake was linked to a reduced risk of gastric cancer.