Does vegetarianism prevent colon cancer? Maybe... but I've got a better lifestyle change for you.
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An article appearing in JAMA internal medicine looked at around 100,000 Seventh-day adventists, and found that ht risk of colorectal cancer was significantly reduced among those who ate a vegetarian diet. But beware of large studies, folks - this statistically significant result isn't terribly clinically meaningful. You'd have to change 5300 people into vegetarians to prevent one case of colon cancer per year. Interestingly, you'd only need to convert 3000 people to Seventh-day Adventism to get the same benefit.
See my video on medpagetoday.com: