Vitamin D testing - The USPSTF questions the use of routine Vitamin D testing
Despite the great interest in Vitamin D testing there have in fact been few studies which have demonstrated that low levels are the actual cause of any of the problems it has been linked to. This is important. Just because low levels are associated with certain illnesses does not mean they are the cause. We may have the relationship backwards. It is possible that low levels of vitamin D may not be causing the illness but instead the illness may be causing the low vitamin D levels. Or it maybe the low vitamin D is just a marker for poor health habits and its those habits that are causing illness not the low vitamin D.
Even if low vitamin D levels are the cause of some of these illnesses that does not necessarily mean that we can correct or prevent the illness by correcting the vitamin D level. Unfortunately there are few controlled trials demonstrating that supplementing low levels are either beneficial or safe.
One of the most important concerns with vitamin D levels is how we define "low vitamin D". Some studies and my own experience has shown that more than 80% of healthy patients are found to have vitamin D levels that would be defined as low by the current definition. For a level of anything to be considered abnormal it should either be "unusual" compared to the average individual or we should have evidence that when the vitamin falls below or above a certain level it can cause some form of illness or disease (high levels of vitamins can cause illness just as low levels can).
If the average person is 5 foot 6 we would not define 5 foot 7 as being short, but this is what's happening with vitamin D levels. The average healthy person in my own office seems has a vitamin D level of about 19 but anything below 20 is defined as low.
The second criteria for defining a measurement as being abnormal is that such levels have to be shown to cause disease. It is possible that even a level that is average could still be too low for good health. As discussed above though, few studies have really shown a causal relationship between low vitamin D levels and most of diseases for which links have been suggested.
There are also concerns about whether current vitamin D testing actually tell us what the true vitamin D levels are in our bodies. Vitamin D is a fat soluble vitamin. It is mostly stored in our liver and fatty tissues with a small amount circulating in the blood. When we measure the level in the blood we are making certain assumptions that may not be accurate. It is possible to have low blood levels of vitamin D and yet have plenty of vitamin D stored in our tissues and visa versa.
The USPSTF has determined that there are no proven benefits associated with routine vitamin D measurements but there are also concerns about the possible costs associated with routine testing. Not only are there costs associated with the test itself and with possible unnecessary treatments but there is also the possibility that treating levels currently defined as being low could actually be harmful. Many people think of vitamins as essentially harmless but vitamins are medications and it is well established that vitamin supplementation can under some circumstances cause disease and even increase the risk of cancer. Although currently there is no evidence linking vitamin D to an increased cancer risk, such studies can take decades to complete.
What the USPSTF recommendation is saying is that we may some day have evidence to support the use of vitamin D measurements and treatment but much more research needs to be done to answer this question. Without such evidence caution is advised because we don’t know if the benefits outweigh the real risks of doing these tests and possibly using treatments that could potentially be harmful.