Even after controlling for a variety of dietary and nondietary factors, those eating plant-based diets appear to have lower overall cancer rates.
The same reason it was so difficult to study cancer among coffee-drinkers, is the same reason it’s so difficult to study cancer among meateaters. Even if they found lower cancer rates among those eating vegetarian, maybe it’s just because they exercise more, or smoke less, or inhale less diesel fumes because all own a prius.
So the way you get around that is you study a group of healthy meateaters who, for example, smoke just as infrequently as the group of vegetarians you’re trying to study—to equal things out, control for nondietary factors. So you don’t just classify people into meateaters, fish-only eaters, and vegetarians, you adjust for smoking—past smoking, current smoking, the amount of smoking, cigarette smoking, cigar smoking, pipe smoking, alcohol consumption, body mass index, physical activity level, and for women, how many children they’ve had—which can be protective against breast cancer, whether they were on the pill or not. Anything they could think of to factor everything else aside and just focus in on what they were interested in, whether or not one eats meat.
Now controlling for obesity, is not really fair to the vegetarians. We know that vegetarians are significantly more likely to be thin, which we know is protective against cancer, so by comparing vegetarians only to thin meateaters, it undercuts one of the benefits of eating vegetarian. It effectively erases one of the reasons why eating vegetarian may reduce cancer rates. But they weren’t interested in indirect ways in which meat might cause cancer, like meat leading to obesity leading to cancer. They wanted to study meat and cancer more directly.
And to do that you have to handicap the vegetarians even further. Maybe the reason vegetarians are so healthy is not because they eat less meat, but because they eat more plants. So vegetarians were compared to meateaters who on average ate about the same amount of fruits and vegetables every day. It may not have been easy, but they were able to dig up thousands meateaters who ate 4-5 servings of fruits and veggies a day—about as much as the vegetarians were eating.
Again, this put the vegetarians at a comparative disadvantage by removing one of the key benefits of more plant-based diets which is… more plants. By comparing vegetarians to omnivores who don't eat a lot of meat and have a high fruit and vegetable intake, this could reduce the chance of observing lower cancer rates in the vegetarians, but they wanted to isolate out the meat component. So they did, they compared vegetarians only to healthy meateaters with healthier diets, and still found the incidence of all cancers combined was lower among vegetarians.
To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring watch the above video. This is just an approximation of the audio contributed by veganmontreal.
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I've covered findings from the EPIC studies before in Meat & Multiple Myeloma, Bowel Movement Frequency, Thousands of Vegans Studied, EPIC Study and Low Meat of No Meat?. Tomorrow I'll cover the findings from this paper in more depth and I have a few more videos in the pipeline on it, so stay tuned! And the same diet that may prevent cancer may help treat it too, see Cancer Reversal Through Diet?. For more on plant-based nutrition, see the 101 videos I have on plant-based diets, and hundreds of other videos on more than a thousand subjects. PS: the reference in this video to teasing out the cancer and coffee connection is dealt with in Coffee and Cancer.
For more context, check out my associated blog posts, Poultry and Penis Cancer.