Exercise training and tumour metastasis in mice: influence of time of exercise onset

The present study was designed to investigate the factor of timing of activity onset in determining the effects of exercise on experimental metastases. After 9 weeks of wheel running activity (W) or remaining sedentary (S), male C3H/He mice received an i.v. injection of CIRAS 1 tumor cells. Mice were then randomized into sedentary and wheel running groups (SS, SW, WS, WW) for a further 3 weeks. Although the average number of lung tumours did not differ by activity condition, the groups exercised prior to tumour injection had a greater incidence in the lowest tertile of tumour distribution relative to sedentary controls (SS = 8/20 vs. WS = 16/19, p < 0.005; SS vs. WW = 14/18, p < 0.05). In a replication trial, a similar outcome was present in the lowest tertile: increased incidence of animals exercised prior to injection (SS = 3/9 vs. WW = 7/9: p < 0.05). These data suggest that exercise training before, but not after, i.v. exposure to tumour reduces the number of lung metastases and that the timing of exercise onset influences the extent of experimental metastasis in this model.