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Also, exercise reduces colon cancer risk by 40-50% and breast cancer risk by 30-40%. Then there are other risk factors such as poor diet, insufficient vitamin D, etc. Overall it seems highly unlikely that cancer is mostly "bad luck".

http://www.cancer.net/navigating-cancer-care/prevention-and-...

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50% of what though? If you have 1 in a million chance, increasing that to 1.5 in a million isn't a big change. If "bad luck" afflicted 1 in 100,000 then then bad luck would still have a much bigger effect, despite lifestyle changes clearly being significant.

(These numbers totally made up)

Do these percentages take into account causation vs correlation? Apparently (surprisingly), it's still very difficult to prove causation without a laboratory experiment.

Perhaps people who are likely to exercise have a certain genetic disposition to begin with.

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