Leisure time television watching, computer use and risks of breast, colorectal and prostate cancer: A Mendelian randomisation analysis

Abstract

Background: Sedentary behaviours have been associated with increased risks of some common cancers in epidemiological studies; however, it is unclear if these associations are causal.

Methods: We used univariable and multivariable two-sample Mendelian randomisation (MR) to examine potential causal relationships between sedentary behaviours and risks of breast, colorectal and prostate cancer. Genetic variants associated with self-reported leisure television watching and computer use were identified from a recent genome-wide association study (GWAS). Data related to cancer risk were obtained from cancer GWAS consortia. A series of sensitivity analyses were applied to examine the robustness of the results to the presence of confounding.

Results: A 1-standard deviation (SD: 1.5 h/day) increment in hours of television watching increased risk of breast cancer (OR per 1-SD: 1.15, 95% confidence interval [CI]: 1.05-1.26) and colorectal cancer (OR per 1-SD: 1.32, 95% CI: 1.16-1.49) while there was little evidence of an association for prostate cancer risk (OR per 1-SD: 0.94, 95% CI: 0.84-1.06). After adjusting for years of education, the effect estimates for television watching were attenuated (breast cancer, OR per 1-SD: 1.08, 95% CI: 0.92-1.27; colorectal cancer, OR per 1-SD: 1.08, 95% CI: 0.90-1.31). Post hoc analyses showed that years of education might have a possible confounding and mediating role in the association between television watching with breast and colorectal cancer. Consistent results were observed for each cancer site according to sex (colorectal cancer), anatomical subsites and cancer subtypes. There was little evidence of associations between genetically predicted computer use and cancer risk.

Conclusions: Our univariable analysis identified some positive associations between hours of television watching and risks of breast and colorectal cancer. However, further adjustment for additional lifestyle factors especially years of education attenuated these results. Future studies using objective measures of exposure can provide new insights into the possible role of sedentary behaviour in cancer development.

Keywords: Mendelian randomisation; breast cancer; colorectal cancer; prostate cancer; sedentary activities.

FIGURE 1

Associations of leisure time television watching and computer use with breast and colorectal cancer after adjusting for the four secondary traits. The black dot corresponds to the 1‐SD odds ratio and the corresponding error bar to the 95% confidence interval. Each error bar refers to the associations of leisure time television watching or computer use with breast or colorectal cancer after adjusting for the specific trait on the left side of the graph. (A) leisure time television watching‐breast cancer associations; (B) leisure time computer use‐breast cancer associations; (C) leisure time television watching‐colorectal cancer associations; (D) leisure time computer use‐colorectal cancer associations. BMI, body mass index; IVW, inverse‐variance weighting.

FIGURE 2

Bidirectional associations of leisure time television watching and computer use with the four secondary traits: ΒΜΙ, years of education, smoking and alcohol. The solid lines correspond to the effects of time television watching and computer use on the four secondary traits while the dashed lines correspond to the effects of the four secondary traits on time television watching and computer use. The black colour corresponds to statistically significant associations and the grey colour to non‐significant. All the results, odds ratios and 95% confidence intervals correspond to a 1‐SD change in the levels of the variables. BMI, body mass index.