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Dear Doctor: I had my thyroid removed because of papillary cancer, and my surgeon has advised me to take high levels of vitamin D. He says there may be a correlation between low vitamin D levels and the development of cancer and Alzheimer's disease. If I remember correctly, the normal blood range for vitamin D is between 30 and 60 nanograms per millileter. I take 8,000 units a day, which keeps my blood level near 60 ng/ml. What's the evidence for higher levels of vitamin D?

Dear Reader: You and I share the same past cancer diagnosis -- and the same search for answers. When I was 26, I was also diagnosed with papillary thyroid cancer. At the time, I racked my brain for why I developed this cancer. I was healthy at the time, had a good diet and exercised regularly. I was also outdoors a lot, so my vitamin D levels were good.

As with so many cancer diagnoses, I ultimately couldn't find any factor that would have increased my risk. True, there is a definite link between papillary thyroid cancer and radiation exposure, which can come from radiation fallout due to nuclear accidents, as well as excessive X-ray radiation to the neck and oral area. But I hadn't been exposed to nuclear fallout, and I'd had only the normally recommended dental X-rays. (Today's more modern X-rays expose patients to lower doses than previous generations.)

I haven't stopped looking for connections, however, between a diagnosis of disease, risk factors and possible preventive measures.

That brings us to your current question. The normal blood level for vitamin D is between 20 and 50 ng/ml. This level -- generally considered necessary for good bone health -- is recommended by the Institute of Medicine, which conducted a review of studies on the topic. Some organizations, such as the National Osteoporosis Foundation, recommend 30 to 50 ng/ml as the normal range. But both of these recommendations are about good bone health, not the prevention of cancer and Alzheimer's disease.

The World Health Organization looked at multiple retrospective studies evaluating a potential connection between vitamin D levels and colon cancer. They did find a link between low vitamin D levels and colon cancer, specifically that people with blood levels of 20 ng/ml or less had significantly higher colon cancer rates.

However, retrospective studies look at disease after the disease has happened, and it can be difficult to make a solid correlation that way. A prospective study assessing vitamin D concentrations prior to the diagnosis of colon cancer did not find any evidence that vitamin D levels had any relation to colon cancer.

Studies focused on prostate cancer have shown both benefit and increased risk among people with higher levels of vitamin D, so it would seem that supplementation does not appear to have an impact one way or another. With regard to breast cancer after menopause, an analysis of nine prospective studies showed a decrease in breast cancer rates among women with higher levels of vitamin D, but this decrease was not seen in women whose levels were above 35 ng/ml. Still another analysis of 18 studies assessing overall breast cancer risk in women did not find any benefit to vitamin D supplementation.

In summation, there may be slight benefit to minimal supplementation in women after menopause to get their levels to 35 ng/ml to decrease the incidence of breast cancer. But in men, the benefit of reduced cancer risk has not been shown.

In relation to cognitive decline, a study published in the journal Neurology in 2012 showed a slight increase in Alzheimer's disease in people whose vitamin D levels were lower than 20 ng/ml. The authors concluded that more studies are needed to evaluate the potential connection.

Let's say that there is a benefit to vitamin D supplementation in lowering the risk of cancer and Alzheimer's disease. To reach a level of 20 ng/ml, you probably would not need to take more than 400 units of vitamin D per day. For a postmenopausal woman to reach the benefit needed for a possible reduction in the risk of breast cancer, she probably would not need to supplement with any more than 800 units per day.

In light of the evidence, 8,000 units of vitamin D daily seems too high. The blood serum level of 60 ng/ml also may be too high. Although the topic needs to be studied more, higher amounts of vitamin D could potentially lead to greater bone turnover and possibly weaker bones.

Ask your doctor if you can reduce the amount of vitamin D that you are taking, as there currently is no evidence to support it.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

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