Association Between Blood Circulating Vitamin D and Colorectal Cancer Risk in Asian Countries: A Systematic Review and Dose-Response Meta-analysis
Authors: Zhang Lin , Zou Huachun , Zhao Yang , Hu Chunlei , Atanda Adejare (Jay) , Qin Xuzhen , Jia Peng , Jiang Yu , Qi Zhihong 
Published: October 2019

ABSTRACTObjectivesTo assess the association between blood circulating Vitamin D levels and colorectal cancer risk in the Asian population.DesignThis is a systematic review and dose-response meta-analysis of observational studies that investigated the relationship between blood circulating Vitamin D levels and colorectal cancer risk in the Asian population.Data SourcesRelevant studies were identified through a literature search in MEDLINE, EMBASE, and Web of Science from January 1980 to 31 January 2019. Eligibility criteria: original studies published in peer-reviewed journals investigating the association between blood circulating Vitamin D levels and the risk of colorectal cancer and/or adenoma in Asian countries.Data extraction and synthesisTwo authors independently extracted data and assessed the quality of included studies. Study-specific ORs were pooled using a random-effects model. A dose-response meta-analysis was performed with generalized least squares regression. We applied the Newcastle-Ottawa Scale quality assessment to evaluate the quality of the selected studies.ResultsThe eight included studies encompassed a total of 2,916 cases and 6,678 controls. The pooled ORs of colorectal cancer for the highest versus lowest categories of blood circulating Vitamin D levels was 0.75 [95% CI, 0.58-0.97] up to 36.5 ng/mL in the Asian population. There was heterogeneity among the studies (I2=53.9%, Pheterogeneity=0.034). The dose-response meta-analysis indicated a significant linear relationship (Pnon-linearity=0.11). An increment of 16 ng/mL in blood circulating Vitamin D level corresponded to an OR of 0.79 [95% CI, 0.64-0.97].ConclusionsThe results of this meta□analysis indicate that blood circulating Vitamin D level is associated with decreased risk of colorectal cancer in Asian countries. The dose-response meta-analysis shows that the strength of this association among the Asian population is similar to that among the Western population. Our study suggests that the Asian population should improve nutritional status and maintain a higher level of blood circulating Vitamin D.Strengths and limitations of this studyOur study seeks to extend previous work by including a number of new studies and by distinguishing the Asian population explicitly.The number of included studies is not sufficient to provide a robust estimate, so the results should be interpreted in the context of the limitations of the available data.Heterogeneous definitions of blood circulating Vitamin D categories were used across studies. The variability in definitions could limit comparability between studies.Our study included seven case-control studies; the study design implies that the measurement of blood circulating Vitamin D is measured in individuals already diagnosed with colorectal cancer. Results from case-control studies need to be interpreted cautiously because of the potential for reverse causation.Time of blood sampling in relation to outcome ascertainment also varied among studies. Such cross-sectional measurements may not accurately reflect an individual’s Vitamin D status across time.