Here are three widespread beliefs about healthy living that may seem to be based on common sense but that research has revealed to be either partially or entirely wrong.
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Human beings have survived for millennia because most of us make good decisions about our health most of the time. These decisions are usually based on common sense, defined by the American Heritage dictionary as: "Sound judgment not based on specialized knowledge."

However, common sense can also lead one astray. As an anonymous philosopher noted, "Common sense is the sense that tells you the world is flat." Sometimes a person can't make a sound judgment without specialized knowledge.

With that in mind, here are three widespread beliefs about healthy living that may seem to be based on common sense but that research has revealed to be either partially or entirely wrong.

1. Take extra calcium for strong bones. By far, the most common perception I hear about bone health is that taking supplemental calcium -- or consuming plenty of calcium-rich dairy products -- is essential. I think this idea is widespread because it seems so reasonable: Bones get their strength from calcium, so taking calcium pills or drinking milk daily seem like the best routes to building and maintaining them.

The problem: Bones are actually made of a complex of minerals (calcium is just one of them) and specialized connective tissue known as collagen. Most American diets, even bad ones, provide more than enough calcium for bone health, especially for men. The populations with the highest calcium intakes actually have the highest rates of hip fractures in later life. I now recommend supplemental calcium for only some women, and only at modest dosages. When it comes to bone health, what's usually lacking in American diets and lifestyles is adequate vitamin D, which regulates the absorption and use of calcium, and weight-bearing exercise, which is essential to stimulating the complex process of bone-building. I endorse the advice of my friend Walter Willett, M.D., Dr.P.H., who chairs the department of nutrition at the Harvard School of Public Health: "If you want to prevent fractures, don't drink the milk, but take your cow for a walk!"

2. Saturated fat clogs arteries. Google the exact phrase "artery-clogging saturated fat" and you'll get 188,000 hits. The demonization of saturated fat has been going on for nearly four decades now, ever since the Senate Select Committee on Nutrition and Human Needs featured the phrases "(i)ncrease carbohydrate consumption" and "(r)educe saturated fat consumption" in its landmark 1977 report. Although public perceptions have changed somewhat, I still find that anyone who eats a lot of butter, lard, coconut oil or beef is likely to be warned that heart attack is imminent. Again, the idea sticks because it seems to align with common sense: We've all seen drainpipes clog with grease, so it's easy to imagine something similar happening with saturated fat (roughly defined as fat that remains solid at room temperature) in our arteries.

But human metabolism is far more complex than a pipe. Fats are transported in the bloodstream inside lipoproteins, tiny, sophisticated biochemical assemblies that bear no resemblance to undifferentiated masses of grease; in fact, certain lipoproteins can actually remove cholesterol from arterial walls. The "clogging" scenario was based on shaky research that, in my view, has now been refuted. A large-scale meta-analysis published in the March 2010 issue of the American Journal of Clinical Nutrition found "no significant evidence" that dietary saturated fat is associated with increased heart disease risk. (For a report on the skewed perspective preventing leading dietary advisory committees from adopting this view, see this 2011 article from Nutrition). I'm still not comfortable recommending that people eat saturated fat with abandon, but it's clear to me that sugar, flour and oxidized seed oils create inflammatory effects in the body that almost certainly bear most of the responsibility for elevating heart disease risk.

3. Stay out of the sun to avoid cancer. Again, this seems sensible -- ultraviolet rays can burn the skin, so it makes a kind of intuitive sense to conclude that, over time, this abuse could manifest as cancerous lesions. The flip side, however is that extreme fear of solar exposure keeps many people -- even in sunny Arizona where I live -- in a chronic state of vitamin D deficiency, which may raise the risk of several types of cancer, as well as of Type 1 diabetes, multiple sclerosis, schizophrenia and high blood pressure. In short, it is at least possible that avoiding sunlight via staying indoors or never venturing out without sunscreen may cause more serious disease, including cancer, than it prevents. My recommendations: Take 2,000 IU of vitamin D3 daily, and follow the quite reasonable solar exposure recommendations of vitamin D researcher Michael F. Holick, Ph.D., M.D.

One of the characteristics of "specialized knowledge" is that new evidence can compel it to change. That means none of the conclusions above are unassailable -- they simply represent, in my view, the best inferences we can draw right now. The bottom line is that the human body is complex and subtle, and oversimplifying -- as common sense sometimes impels us to do -- can be hazardous to your health.

Andrew Weil, M.D., invites you to join the conversation: Become a fan on Facebook, follow him on Twitter, and check out his Daily Health Tips Blog.

Dr. Weil is the founder and director of the Arizona Center for Integrative Medicine and the editorial director of

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