Oral cancer is a common head and neck cancer malignancy that seriously affects patients' quality of life and increases the health care burden. Moreover, there is a lack of comprehensive reviews of previous research on factors associated with oral cancer. The aim of the current umbrella review was to provide a comprehensive and systematic summary of relevant studies, to grade the quality of evidence of relevant studies, and to provide guidance for the prevention of oral cancer.

The PubMed, Web of Science, Medline, Cochrane Library, Embase and CNKI databases were searched for relevant meta-analyses and systematic reviews. Two authors extracted the data separately and assessed the quality of the studies using the AMSTAR-2(A Measurement Tool to Assess Systematic Reviews-2) scale.

A total of 28 meta-analyses were included in this review:13 meta-analyses of dietary factors and 8 meta-analyses of behavioural habits and 7 meta-analyses of factors that were difficult to categorise as dietary and behavioural. Alcohol, tobacco (chewing tobacco/secondhand smoke exposure), betel quid , processed meat, periodontal disease, HPV (especially HPV16 and HR-HPV) infection, and chronic mechanical irritation were positively correlated with the risk of oral cancer. Vitamin C-rich citrus fruits were negatively associated with the risk of oral cancer . Fish, milk and dairy products except butter, coffee and tea intake may be negatively associated with the risk of oral cancer, but subgroup analyses of studies of fish and milk revealed significant effects only for European populations. Additionally, green tea showed the strongest protective effect among teas. High and moderate coffee intake was negatively associated with the risk of oral cancer. Oral hygiene maintenance was also negatively associated with the risk of oral cancer.

Reducing or eliminating alcohol consumption, refraining from direct or indirect exposure to tobacco products, and reducing betel quid chewing may lead to a reduced risk of oral cancer. Reducing the intake of processed meat, moderate intake of coffee, the consumption of green tea of appropriate temperature and strength, fish and citrus fruit intake, and oral hygiene, preventing periodontal diseases and HPV infections, and reducing mechanical stimulation of the oral mucosa caused by various reasons can exert protective effects against oral cancer.