Note: This article is heavy on biology, so if it is too long for you, just read the 1st and last paragraphs.
By the current medical exams, the vast majority of African Americans are Vitamin D deficient. Vitamin D is an essential dietary vitamin that and aids in calcium absorption and is synthesized in the body when the skin is exposed to sunlight. Therefore, diagnosing 80% of African Americans with Vitamin D deficiency, despite the fact that blacks have the best bone health in the nation, just seems wrong. This inconsistency led to a study by Dr. Ravi Thadani from Massachusetts General Hospital which examined various forms of vitamin D in 2,085 Baltimore residents.
The current test measures the form 25-hydroxyvitamin D which circulates the bloodstream and is tightly bound to a carrier protein. In order for the vitamin to be taken up, the kidneys must convert it to 1,25-dihydroxyvitamin D. Levels of protein-bound 25-hydroxyvitamin D are a good measure of the amount of bioavailable vitamin D for whites but not for blacks. Why? Because of the geographic origins of African Americans.
For Africans, Vitamin D is very easily formed because of the plentiful amount of sunlight. In fact, the reason Africans have darker skin is to prevent excessive production of Vitamin D which can be toxic at high levels. In order to compensate, Africans have a genetic polymorphism in the Vitamin-D carrier protein gene which causes less of this protein to be produced. This protein increases the half-life of Vitamin D in the blood so that it can be stored and used at a later time by the body. However, Africans do not need to store Vitamin D because of the amount of sunlight they are exposed to. The further north people migrated, the less sunlight they saw and therefore, the greater amount of carrier protein they needed. African Americans thus have a lower amount of protein-bound 25-hydroxyvitamin D which is what current tests measure. This would fool physicians into thinking their black patients are Vitamin D deficient. Thadani’s team measured the amount of bioavailable Vitamin D (the form that can be taken up by cells) and found that the amount in blacks is equivalent to that in Caucasians.
High levels of parathyroid hormone (think back to bio class or MCAT flashcards!) are also considered to be an indicator of Vitamin D deficiency. Vitamin D helps your body absorb calcium and PTH increases the blood’s calcium concentrations. When there is a severe Vitamin D deficiency, there is less calcium in the blood and parathyroid glands overcompensate by secreting a lot of PTH. In addition to a decreased level of protein-bound 25-hydroxyvitamin D, African Americans also have elevated PTH which would also prompt physicians to consider patients deficient. However, Thadani’s study showed that the levels of PTH were only slightly elevated in their black participants, suggesting that the relationship between Vitamin D and PTH may be different in blacks than in whites.
The conclusion of this study is that African Americans cannot be considered Vitamin D deficient simply by the current tests and many have been misdiagnosed. Instead, physicians should test for the amounts of bioavailable Vitamin D and the amount of carrier protein in their patients before diagnosis. This brings up important questions about the validity of medical diagnostic tests for all races and how the levels of specific molecules can differ among populations. America is no longer just a nation of Caucasians, and scientists and doctors should be conscious of medical differences among races and work to create tests and guidelines to effectively diagnose all Americans.
1. What other diagnostic exams could be racially biased?
2. What are the specific dangers of misdiagnosis based on these exams?
3. Is there harm in prescribing Vitamin D to patients who are not actually Vitamin D deficient?
4. Do the findings from this study apply to South Asians and others from naturally very hot/sunny places?
For More Information:
1. Full Paper from NEJM: http://www.nejm.org/doi/full/10.1056/NEJMoa1306357#t=article
2. NPR: http://www.npr.org/blogs/health/2013/11/20/246393329/how-a-vitamin-d-test-misdiagnosed-african-americans
3. PTH: http://www.mayoclinic.com/health/hyperparathyroidism/DS00396/DSECTION=causes
4. Vitamin D: http://en.wikipedia.org/wiki/Vitamin_D