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Background: Pre-diagnostic physical activity may lower the risk for developing colon cancer and is potentially associatedwith improved prognosis when diagnosed with colorectal cancer. Body composition and immune cell infiltration in the tumour microenvironment are also associated with prognosis in colorectal cancer. The physical activity guidelines are the same in colorectal cancer patients as in the healthy population and the national health care programme for colorectal cancer in Sweden recommends individualised support for physical activity to all patients.

Aims: The overall aim of this thesis was to evaluate pre-diagnostic physical activity, assessed using self-reported questionnaire, from physical tests, by using an accelerometer, using interviews and its relationship with colorectal cancer with regard to immune cell infiltration of the tumour, body composition, and the patient’s own experiences.

Results: Study I: Pre-diagnostic physical exercise more than three times a week was associated with increased numbers of cytotoxic T cells in the tumour front (OR 2.91, 95% CI 1.25-6.75) and centre (OR 2.92, 95% CI1.31-6.50)

Study II: Low pre-diagnostic physical exercise was not associated with sarcopenia, i.e. low muscle mass (OR1.37, 95% CI 0.86-2.19) nor myosteatosis, i.e. low muscle quality (OR 0.95, 95% CI 0.59-1.51) at the time of colorectal cancer diagnosis

Study III: After adjusting for multiple testing no significant results were seen in either the descriptive statistics or in the logistic regression model adjusted for age and sex.

Study IV: Various experiences were expressed towards physical activity described in three main categories “I’ll fight the cancer and come out stronger”; “the diagnosis makes no difference”; and “the cancer is an obstacle for physical activity”.

Conclusions: Self-reported physical exercise may be associated with increased numbers of cytotoxic T cells in the microenvironment of colorectal cancer. However, this was neither confirmed nor rejected when assessing physical activity and fitness more objectively in a smaller cohort. Little physical exercise in middle age was not associated with sarcopenia or myosteatosis at the time of colorectal cancer diagnosis but when present together, cancer-specific mortality risk was increased. Physically active individuals have a wide spectrum of attitudes and experiences toward physical activity when diagnosed with colon cancer.