To systematically investigate the effects of dietary
flavonoids and
flavonoid subclasses on the risk of smoking-related
cancer in observational studies.
METHODS: Summary estimates and corresponding standard errors were calculated using the multivariate-adjusted odds ratio (OR) or relative risk (RR) and 95% CI of selected studies and weighted by the inverse variance.
RESULTS: A total of 35 studies, including 19 case-controls (9,525 cases and 15,835 controls) and 15 cohort studies (988,082 subjects and 8,161 cases), were retrieved for the meta-analysis. Total dietary
flavonoids and most of the
flavonoid subclasses were inversely associated with smoking-related
cancer risk (OR: 0.82, 95% CI: 0.72-0.93). In subgroup analyses by
cancer site, significant associations were observed in aerodigestive tract and
lung cancers. Total dietary
flavonoid intake was significantly associated with aerodigestive tract
cancer risk (OR: 0.67, 95% CI: 0.54-0.83) marginally associated with
lung cancer risk (OR: 0.84, 95% CI: 0.71-1.00). Subgroup analyses by smoking status showed significantly different results. The intake of total
flavonoids,
flavonols,
flavones, and
flavanones, as well as the
flavonols quercetin and
kaempferol was significantly associated with decreased risk of smoking-related
cancer in smokers, whereas no association was observed in non-smokers, except for
flavanones. In meta-analysis for the effect of subclasses of dietary
flavonoids by
cancer type, aerodigestive tract
cancer was inversely associated with most
flavonoid subclasses.
CONCLUSION: The protective effects of
flavonoids on smoking-related
cancer risk varied across studies, but the overall results indicated that intake of dietary
flavonoids, especially
flavonols, was inversely associated with smoking-related
cancer risk. The protective effects of
flavonoids on smoking-related
cancer risk were more prominent in smokers.