After finishing my Master of Public Health in 2016, I started working in the Department of Epidemiology and Preventive Medicine at Regensburg University, Germany. Our goal is to investigate the influence of modifiable lifestyle factors, such as physical activity, on chronic diseases. Physical activity has been shown to reduce the risk of cancers of a number of sites, including cancers of the bladder, breast, colon, endometrium, esophagus, kidney, lung, and stomach. However, for other cancers there is insufficient knowledge.1
The incidence of malignant melanoma, the most aggressive form of skin cancer, has risen globally over the past decades. Unlike other cancers, malignant melanoma often occurs in middle-aged people.2 Identifying factors and behaviours that have the potential to prevent malignant melanoma is thus an important public health goal.
Three years ago, former colleagues of mine published a meta-analysis on the relations of physical activity and cardiorespiratory fitness to malignant melanoma.3 Surprisingly, cohort studies showed a positive association between physical activity and the risk of developing malignant melanoma. Why should physical activity, a behaviour that has been shown to have a protective effect on cancer development, cause malignant melanoma? The authors of that meta-analysis concluded that it could be due to the confounding effect of sun exposure during outdoor physical activity and recommended further prospective studies with comprehensive adjustment for UV radiation-related skin damage.3
Within the framework of my doctoral work on „Measurement of physical activity in epidemiologic studies and associations with cancer incidence“, one of my goals was to close this important research gap. The UK Biobank, a large prospective cohort study, appeared to be the perfect data base, as it provided detailed information on physical activity and extensive data on a number of key potential confounding factors, including UV exposure and UV sensitivity.
In our analyses, physical activity did not have an influence on risk of malignant melanoma, neither with nor without adjustment for UV exposure and UV sensitivity. Possibly, in Great Britain, variation in UV exposure is too small or weather conditions favour indoor sports. Despite finding a null association between physical activity and malignant melanoma, it turned out that body height, an indicator of genetic, environmental, nutritional and hormonal risk factors for malignant melanoma4, acted as a strong confounder in our basic models, resulting in a positive association between grip strength, a surrogate measure of muscular strength, and malignant melanoma incidence. After full adjustment, this positive association vanished and grip strength was no longer associated with risk of malignant melanoma.
Thus, in summary, it once again became clear to me how important it is to identify important confounders in advance and to carefully take them into account in the analysis.
- 2018 Physical Activity Guidelines Advisory Committee. 2018 Physical Activity Guidelines Advisory Committee Scientific Report. Washington, DC: U.S. Department of Health and Human Services (2018).
- Rastrelli, M., Tropea, S., Rossi, C. R. & Alaibac, M. Melanoma: epidemiology, risk factors, pathogenesis, diagnosis and classification. In Vivo 28, 1005-1011 (2014).
- Behrens, G., Niedermaier, T., Berneburg, M., Schmid, D. & Leitzmann, M. F. Physical activity, cardiorespiratory fitness and risk of cutaneous malignant melanoma: Systematic review and meta-analysis. PloS one 13, e0206087 (2018).
- World Cancer Research Fund/ American Insitute for Cancer Research. Continuous Update Project Expert Report 2018. Diet, nutrition, physical activity and skin cancer. https://www.wcrf.org/wp-content/uploads/2021/02/skin-cancer.pdf (2019).