Dear Dr. Eva,
My old doctor used to test me every year or so for vitamin D, but my new doctor says it’s not necessary. What do you think?
Healthy So Far
Like fashion, nutrition, and politics, medical testing goes through fads. About ten years ago, many doctors began checking their patients’ vitamin D blood levels. It had long been known that vitamin D, along with calcium, is important for bone health and that vitamin D supplements can be useful for adults who have osteoporosis (loss of bone density) and low levels of vitamin D. Not many foods are naturally high in vitamin D, but vitamin D is one of only two vitamins that can be made in the body. Vitamin D is made with sunlight, and many people living in Northern locations with less sunshine have low vitamin D levels. For growing children, lack of vitamin D leads to a serious bone disease called rickets, with weakness and curving of the bones of the arms and legs, sometimes causing permanent deformity. vitamin D has been added to cows’ milk and other milk products, and to bread, since the 1930’s. Since 2007, it has also been added to some types of orange juice. As a result, rickets is now extremely rare in the US and other developed countries.
The idea of widespread testing for vitamin D became popular around 2010 after small studies showed that people with many serious problems including heart attacks, cancer, diabetes, dementia (senility), stroke, schizophrenia, depression, and frequent falls all were more likely to have low vitamin D levels. Although it is known that serious bone disease in adults only happens when vitamin D levels are under 10 ng/mL, it became common to recommend supplements for everyone with a level under 30 ng/ml. As a result, one in four adults age 60 and older now takes vitamin D supplements, compared with one in 400 ten years ago. Vitamin D tests are now the fifth-most-common lab test covered by Medicare, at a cost of $365 million in 2016. Vitamin D testing has increased ten-fold since 2007.
Larger and better designed studies published in the last few years have shown that vitamin D supplements do not prolong life, decrease the chance of cancer or heart disease, or even decrease the chance of broken bones. Recent studies of vitamin D to treat depression, fatigue, arthritis, and chronic pain have not shown any benefits, even in people with low vitamin D levels. In 2014 the United States Preventive Services Task Force (USPSTF) advised that there is not enough evidence to recommend for or against testing healthy people for vitamin D. USPFTS is now doing research to find out whether or not it’s useful to test for and treat low vitamin D levels. Despite this and other published warnings that it may not be useful, widespread testing continues.
Any time a single test result seems to be linked to many different conditions, we should take a second look. Treatments once thought to be useful for multiple unrelated conditions have usually turned out to be useless or even dangerous. This happened with vitamin E. Not so long ago, after initial research results looked promising, many primary care doctors and specialists recommended taking vitamin E to prevent heart attacks and dementia. When larger, better studies were completed, we learned that vitamin E does not prevent heart attacks or dementia. Besides that, death rates are higher when people take large doses of vitamin E. Similar events occurred when early studies suggested that estrogen might prevent heart attacks and dementia, then larger studies showed this was not the case.
It’s a basic principle of medical research that correlation is not causation. In other words, just because two things happen together, like low vitamin D levels and cancer, it does not necessarily mean that one of those things is causing the other. To find out if one of two factors found together is causing the other, it’s necessary to do comparison studies. This means looking at two similar groups of people where one group is given a treatment and the other is given a different treatment, or no treatment. When that type of study was done, it turned out that even though people with colon cancer have lower vitamin D levels than people who don’t have colon cancer, healthy people who take vitamin D supplements are not any less likely to get colon cancer than people who don’t take vitamin D. Also, when people who have colon cancer and low vitamin D levels are given vitamin D supplements, they don’t survive any longer than cancer patients who do not take vitamin D. So if low vitamin D does not cause cancer, why do people who have colon cancer have low vitamin D levels? There could be several reasons. Maybe having cancer interferes with the body’s ability to produce vitamin D. Or maybe it’s just that people who are ill don’t get outside as much, so they don’t make as much vitamin D.
Should you get vitamin D testing? The current evidence is that, if you don’t have osteoporosis, there is no reason for you to get tested. There’s also no known harm in being tested, although that is still under study. But why do something with no known benefit, even if it’s harmless?