As for the survival rate, very few studies have been made, but extrapolations of data from other studies support the hypothesis that breast cancer patients may survive longer if they are physically active.
"At least weight gain and obesity are linked to relapse. Obesity seems to be a problem for everything. As for hormones, obesity is linked to levels of oestrogen, insulin and insuline-like growth factor (IGF), as well as immune function and all sorts of problems", says Courneya.
"In a study on breast cancer survivors, we found improvements in a number of immune system components as a result of exercise, for example, an increase in natural killer (NK-cells) cytotoxic activity. We also found changes in insulin-like growth factor, he says.
Courneya believes several factors may be involved in extending the survival rate for breast cancer patients who are physically active. Immunological and several humoral factors may play a role. Sex steroid and metabolic hormones may also be involved, as well as hormonal changes caused by reduced weight.
Courneya advises cancer patients not to gain weight at all unless they are underweight to begin with: "There seems to be a linear relationship between health and weight gain. But one should not weigh too little either, a BMI of 20 24 is ideal. The greatest risk is for obese patients with a BMI above 30." He also recommends physical activity for overweight patients who cannot achieve weight loss. "Physical activity is an independent factor, reducing risk independently of weight," he says.