Folate and global health umbrella review series, part 2: syntheses on cancers.
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BACKGROUND: Folate has been examined extensively in relation to carcinogenesis due to its role in one-carbon metabolism impacting the synthesis of DNA and RNA, methylation processes, and genomic integrity. Current evidence on the relationship between folate status and the risk of cancer is equivocal: low or deficient folate status may contribute to an increased risk of cancers, while high-dose folic acid supplementation may have adverse effects on carcinogenesis. METHODS: We searched MEDLINE, Embase, CINAHL, the Cochrane Library, and the Database of Abstracts of Reviews of Effects up to February 2024 for systematic reviews and meta-analyses investigating the associations of folate (measured as dietary intake, supplementation, or blood concentrations) with any specific cancer outcome. Screening, data extraction, and risk of bias assessment were performed in duplicate. We assessed the credibility of the evidence using predefined criteria. RESULTS: We found 67 syntheses, of which 57 provided meta-analyses. Over half of the syntheses had a high risk of bias. We identified 168 unique associations (unique exposure - unique outcome - unique setting) across 10 cancer types, 3 system cancers, and total cancer. Of these, we assessed 15 directional associations (colorectal, oesophageal, and total cancers) to be at a highly suggestive level of credibility, and 17 directional and 10 null associations to be at a suggestive level of credibility. CONCLUSIONS: The available evidence for each category of unique association was generally limited. Highly suggestive associations were found for oesophageal, colorectal, childhood brain and spinal tumours and total cancers. More robust primary studies are warranted to follow-up the signal of a positive relationship reported for prostate cancer warranting further research. Evidence was weak for all but colorectal and oesophageal cancers, or the central nervous system cancers in children. REGISTRATION: PROSPERO: CRD42021265041.