Two reports published this week in the journal BMJ weren't exactly an April Fool's Day message about vitamin D, but they came close.
For the past few years, vitamin D has been gaining a reputation—not entirely earned—as a wonder vitamin that offers protection against some cancers, bone-weakening osteoporosis, heart attack, Alzheimer's disease, and other chronic conditions. Not so fast, caution the two reports.
In one of the BMJ studies, led by Evropi Theodoratou, a research fellow at the University of Edinburgh, researchers evaluated the results of 268 previous studies of vitamin D. Their conclusion: "highly convincing evidence of a clear role of vitamin D does not exist for any outcome, but associations with a selection of outcomes are probable." In other words, there's no solid proof that taking vitamin D helps.
The other BMJ study, led by Rajiv Chowdhury, a cardiovascular epidemiologist at the University of Cambridge, showed that low blood levels of vitamin D are linked to increased risks of dying prematurely from cardiovascular disease, cancer, and other causes. But whether supplementation with vitamin D can help people live longer and healthier requires more study.
One big unknown is whether low vitamin D causes chronic conditions, or whether chronic conditions cause low vitamin D levels. Another area of mystery is what type of vitamin D supplement would be best for health, and how much to take.
Testing for vitamin D
Some doctors routinely check their patients' vitamin D levels. The authors of one of the studies question this practice. They say the evidence so far doesn't show significant differences in health between young and middle-aged people with lower blood levels of vitamin D. The authors acknowledge that it might be different for older people, in whom improving the blood level of vitamin D may be beneficial.
If not everyone should be tested for vitamin D, who should have a blood test for it? Like so much else about vitamin D, there's no clear answer. I advise the test for people who have bone-thinning osteopenia or osteoporosis or who are at high risk of developing it, such as
- Caucasian and Asian women just before and any time after menopause, especially if they smoke or are thin
- women and older men with a family history of osteoporosis
- individuals who have had a hip, wrist, spine, or other fracture after age 50
- individuals who take a medicine that thins bones, such as a corticosteroid
Should you take vitamin D?
I plan to keep advising my patients to get the amount of vitamin D recommended by the Institute of Medicine:
- 600 IU of vitamin D a day for everyone ages 1 to 70
- 800 IU of vitamin D a day for those 71 and older
Food is usually the best way to get vitamins. But not vitamin D. Only a few foods—salmon, tuna, sardines, milk, fortified cereals, and some types of mushroom—can give you more than 100 IU per serving.
What about the way humans have gotten vitamin D for millions of years—from the sun? It's a hot-button issue—and a balancing act. Getting 10 to 15 minutes of sunlight on your face, arms, back, or legs without sunscreen a few times a week is enough to generate your body's vitamin D needs for a week. But too much exposure to the sun causes skin cancer.
If you rarely get out in the sun, or just aren't certain you are getting 600 to 800 IU of vitamin D a day, taking a supplement containing 400 to 1,000 IU is safe, inexpensive insurance.