DEAR DR. ROACH: Please give your take on the importance of vitamin D. There’s been a lot of research in the past 15 to 20 years. I have read that it greatly reduces cancer risk for most types of cancer and that it activates anti-infection genes and anti-autoimmune disorder genes and reduces the rate of Alzheimer’s disease.

I also have heard that the dose needs to be high enough to get to the middle of the “normal” range, and that for adults this means a dose of 5,000 to 7,000 IU.

Considering the above, it really annoys me that the major organizations fail to publicize prevention information. Perhaps they have too much invested in expensive cure research and thus a financial incentive not to emphasize prevention. — J.M.

ANSWER: I have seen many proposed preventive treatments for cancer fail when they get tested in large trials. I am thus cautious not to overstate the benefits of promising small trials and epidemiological data, such as the kind you mention above. It is clear that vitamin D has significant benefits in specific circumstances, especially in bone health, where inadequate vitamin D is associated with a greater risk of osteoporosis and fracture. And although the studies do show that people with higher levels of vitamin D are less likely to develop some cancers, especially in the colon, a study published in October 2015 found that vitamin D did not prevent the development of polyps.

Large trials are ongoing. They will give us much more information within a few years. Until then, I remain cautious about vitamin D and confident that the major organizations wish to see more proof before making recommendations. I reject the suggestion that research organizations are deliberately withholding information on a cancer cure or prevention for profit; I know too many dedicated and honorable researchers to believe it.

The standard dose of vitamin D is 1,000 to 2,000 IU. Although 5,000 or even 7,000 is unlikely to cause harm, I wouldn’t recommend a dose that high without cause.

DEAR DR. ROACH: I am a 64-year-old male. I had a stroke and lost vision in my right eye in 2005. I am right-handed. I had very high blood pressure at the time, and it’s now under control with drugs. Since then, I can no longer wear polarized sunglasses. I have light-sensitive eyes, but most over-the-counter sunglasses are polarized.

My ophthalmologist and other eye specialists tell me they’ve never heard of my problem occurring. With polarized glasses on, I see in 3-D. Blue lines in the roadway and sewer lids or any metal covering appear to be 3-D to me. I have to be careful stepping over the handicap space lines because they look like curbs.

Have you ever heard of this, or am I unique? If I can locate nonpolarized lenses, then I have no problem. Thank you. — J.O.

ANSWER: While everyone is unique, this is a condition I have heard of before. In fact, I know people who deliberately watch television with one eye in order to enhance the 3-D effects. Polarized light does tend to accentuate lines and edges, so it doesn’t surprise me that with your one working eye you have an accentuated 3-D sensation — your brain has learned to use cues that people with two working eyes might miss. In your case, the brain has interpreted things so strongly that you are having some optical illusions.

Sadly, I don’t have any practical advice beyond finding nonpolarized lenses.

Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu or request an order form of available health newsletters at 628 Virginia Dr., Orlando, FL 32803. Health newsletters may be ordered from www.rbmamall.com.

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