Allergies and risk of colorectal cancer: a systematic review and meta-analysis of observational studies.

A history of allergy or allergic condition has been reported to be associated with reduced risk of some types of malignancies. However, the understanding of this association for colorectal cancer (CRC) is controversial. We conducted a meta-analysis of CRC risk in individuals who had history of allergy compared to those without the history of allergic condition. Pumbed and Embase databases were searched for relevant studies. The adjusted relative risk (RR) and 95% confidence interval (CI) were pooled using the random-effects model. Nine studies, including 775, 178 individuals, were eligible for inclusion. The pooled estimate showed no significant association between history of allergy and CRC risk (adjusted RR 1.01, 95 % CI 0.88-1.17). Subgroup analyses confirmed the neutral association stratified by tumor location (colon: n = 6 studies; adjusted RR 1.01, 95 % CI 0.81-1.25; rectum: n = 6 studies; adjusted RR 0.94, 95% CI 0.77-1.15; colorectum: n = 3 studies; adjusted RR 0.92, 95 % CI 0.70 to 1.21), sex (male: n = 4 studies; adjusted RR 0.93, 95 % CI 0.81-1.07; female: n = 6 studies; adjusted RR 0.94, 95 % CI 0.80-1.09) or by allery type (asthma: n = 5 studies; adjusted RR 1.16, 95 % CI 0.96-1.42; hay fever: n = 4 studies; adjusted RR 0.93, 95 % CI 0.86-1.03). Meta-analysis of existing evidence provides a neutral association between allergies and CRC risk. Future well-designed prospective cohort studies should be conducted to better understand this association.