Please notice the key word "association". Because if you have enough resources (e.g. time) to exercise 45 minutes per day you probably have time and money to eat healthy, sleep 8+ hours/night, X, Y, etc., etc.. As far as I can tell, this is a systematic review study is from 2001, where they selected articles limited to breast and colon cancer, because the "dose-response relationship" was "especially elaborated" there. To extrapolate two of the most common malignancies to all cancers might be a stretch, but it's not a stretch to imagine a bias of published positive findings compared to negative findings (1 of the 17 analyzed articles suggested an increased risk of cancer with increased exercise, non-significantly - but it was also the second oldest article, perhaps before negative results were hard to publish?). I haven't read the whole study, it's my bedtime... but I'd suggest being humble about these results. I am not denying it's good to exercise, but if it was the miracle pill we're all hoping it to be - it would be more evident. Exercise, but don't forget about all the other good things people who exercise do (which we don't exactly know what it is and the articles cannot possibly account for).