How do we know exercise helps prevent Breast Cancer?

Lots of research has been done into whether exercising affects breast cancer.
A huge US study tracking the wellbeing of 65,000 nurses since the 1980s has found that those women who developed breast cancer had a whopping 50% greater chance of surviving if they did moderate exercise such as walking for three to five hours a week. Even walking for just one hour a week increases your chances of a longer life, the researchers found.
It appears that women who have the hormonal forms of cancer (which means hormones such as oestrogen can make the tumours grow) are more likely to benefit from exercising.
Hormonal cancer is the most common form of breast cancer. It's thought that the reason exercise helps is that it lowers hormone levels, and lower hormone levels reduce the chances of the cancer coming back.
Meanwhile, studies carried out in Britain and Spain have found that women who exercise may recover more quickly after treatment for breast cancer and generally feel better about themselves. They're also not as tired and less prone to depression.
"All the research is very encouraging," says Auckland breast physician Sonja Freese.
"We know that exercise can give you physical as well as emotional and mental strength and improve your general wellbeing. Another issue for some women is putting on weight after treatment, for example, the drug tamoxifen can lead to weight gain, and exercise helps with that."