How?

http://www.theatlantic.com/magazine/print/2010/11/lies-damned-lies-and-medical-science/8269/
When people start to learn about the Paleolithic model, and the discrepancies between what they've been told and what they experience when they eat meat, vegetables, nuts and seeds - they often say "How could this be?"
How could we not have been told this?  How could we have been told so many things which we find to be untrue when we test them for ourselves?
First answer: that's just the way it is.  This article explores one of the reasons why.
Courtesy www.crossfit.com
"In poring over medical journals, he was struck by how many findings of all types were refuted by later findings. Of course, medical-science “never minds” are hardly secret. And they sometimes make headlines, as when in recent years large studies or growing consensuses of researchers concluded that mammograms, colonoscopies, and PSA tests are far less useful cancer-detection tools than we had been told; or when widely prescribed antidepressants such as Prozac, Zoloft, and Paxil were revealed to be no more effective than a placebo for most cases of depression; or when we learned that staying out of the sun entirely can actually increase cancer risks; or when we were told that the advice to drink lots of water during intense exercise was potentially fatal; or when, last April, we were informed that taking fish oil, exercising, and doing puzzles doesn’t really help fend off Alzheimer’s disease, as long claimed. Peer-reviewed studies have come to opposite conclusions on whether using cell phones can cause brain cancer, whether sleeping more than eight hours a night is healthful or dangerous, whether taking aspirin every day is more likely to save your life or cut it short, and whether routine angioplasty works better than pills to unclog heart arteries."