It is generally understood that vitamin D plays a role in exerting health benefits related to bone strength and condition. However, the significance of that role is not well understood. A number of misconceptions about vitamin D may have contributed to the ambiguity and should be addressed.
First, vitamin D is not really a vitamin. Vitamins are essential nutrients that the human body is incapable of producing. As such, vitamins must be obtained through diet. Vitamin D, on the other hand, is produced in mammalian skin upon exposure to UV rays from the sun.
Secondly, vitamin D is not directly responsible for exerting health benefits related to bone strength. Instead, vitamin D is converted to a pre-hormone, 25-hydroxyvitamin D, in the liver. This pre-hormone circulates in the blood and is measured to determine the vitamin D status of an individual. However, it (like vitamin D) is inactive under normal biological conditions. Conversion of the pre-hormone to the active hormone (1,25-dihydroxyvitamin D) takes place in the kidney. The biological effects of the active hormone (1,25-dihydroxyvitamin D) have been demonstrated at a number of cell and tissue sites, and its activity is strongly linked to calcium maintenance.
Thirdly, many consumers and healthcare providers are either unaware or simply fail to acknowledge that vitamin D status (when determined via measurements of serum levels of the pre-hormone, 25-hydroxyvitamin D) does not necessarily correlate to the same health outcomes in all individuals. This underappreciated detail about vitamin D status was explored in depth in The vitamin D paradox in Black Americans: a systems-based approach to investigating clinical practice, research, and public health – expert panel meeting report.
How does the relationship between vitamin D status and bone health differ in various segments of the population?
Studies show that low measures of vitamin D status correlate to increased risk of osteopenia and fractures in White and Mexican-Americans, but measures of vitamin D status do not correlate to the same health outcomes in Black Americans. Despite markedly low measures of vitamin D status in Black Americans, the incidence of falls, fractures, or osteopenia are significantly lower compared to White American counterparts with similar vitamin D status. The BMC Proceedings report describes discussions amongst expert panelists that revealed that Black Americans gain no skeletal benefits from high doses of vitamin D supplementation; and high levels of the current marker for vitamin D status are almost certain to result in adverse effects in this population.
Should Black Americans worry about low measures of vitamin D status?
There are many factors that affect vitamin D status and skeletal health in Black Americans. Data presented in the BMC Proceedings report shows that adiposity, skin pigmentation, genetics, and environment all contribute to differences in vitamin D status with respect to skeletal health in Black Americans vs. White Americans; but no one factor could fully explain the significance of low measures of vitamin D status with respect to skeletal health in Black Americans. A better understanding of these factors may be key to understanding mechanisms for improving bone health in all populations.
Where do we go from here?
Consumers and healthcare providers should appreciate the potential for adverse effects following over-supplementation with high doses of vitamin D in some individuals. While threshold values (deficient lower levels vs. safe upper levels) for individuals require further refinement, outcomes presented in the BMC Proceedings report suggest that the Recommended Daily Allowance values (400-600 IU/day) should be appropriate for most healthy populations.
Health scientists should appreciate the need to better understand the role of vitamin D and its significance in various segments of the population that are commonly underrepresented in clinical studies. A larger forum or workshop on the topic may help to stimulate more research on vitamin D with respect to bone health as well as implications beyond bone health.