Are you saying that those of us who have breast cancer probably developed it in our teenage years?

Leslie Bernstein, who's done much of this work, was herself a world class swimmer and “vigorous” refers to more than an hour a day of elevated cardio workout. The question of whether more moderate exercise is a benefit is still open, although as you know there are many good reasons why women should exercise regularly.

One of the theories why exercise does work to reduce the risk of breast cancer comes from the work of Brad Lowe and others. They have shown that women who exercise regularly and vigorously have lower amounts of circulating levels of free estradiols, and the lower amounts of free estradiols appear to be associated with a lower risk of breast cancer.

Alcohol has been shown to increase the amount of circulating estradiols, and so does hormone replacement therapy. So women who are on HRT and who drink regularly have an even greater risk of breast cancer, perhaps because both alcohol and HRT increase the amount of circulating estradiols.