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On March 16th 2015, the Journal of the American Medical Association published a meta-analysis of human trials on vitamin D and blood pressure. Their finding: vitamin D does not lower blood pressure 1. This analysis challenges numerous other findings on vitamin D and human health, so before we lose faith in this essential vitamin, we should discuss some important caveats with this study and what is known about vitamin D.
First, let’s point out the important fact that a multitude of factors influence blood pressure, and therefore determining whether a single nutrient can cure any ailment is a very difficult task. Furthermore, there is never a one-size-fits-all cure for a disease; it is important to rule out the potential that a deficiency may cause the body to not be able to function properly.
Secondly, the methods and results of the analysis need to be carefully weighed before any conclusions can be drawn regarding this research. For this study, data was combined from 27 randomized controlled trials (RCTs) that included only 3092 participants. Studies were included regardless of the form of vitamin D used, ignoring the well-known fact that vitamin D3 (and not D2) supplements are the most effective form of vitamin D. Based on the results from the aforementioned study, the authors conclude that their findings do not justify the use of vitamin D supplements to lower blood pressure.
Before discussing the pitfalls of this study, let’s recall one important fact: vitamin D is an essential nutrient that is very difficult to get through the diet. Our bodies require vitamin D to function optimally, and due to the typical American diet and lifestyle, most Americans are not getting enough of this essential vitamin. Vitamin D is necessary for the relaxation of blood vessels, and vitamin D supports heart health as well as kidney health 2, all of which are essential for healthy blood pressure.
Now back to the JAMA study: it is important to note that this study was a meta-analysis of previous trials, meaning that previous results from many differently designed and conducted studies were pooled and analyzed according to a statistical model, but no new evidence was gathered. Meta-analyses are controversial in the scientific community because several conditions are critical to a sound meta-analysis, and small violations of those conditions can lead to misleading results. This particular meta-analysis pooled data from studies performed between 1966 and 2014, studies that have different interventions, doses, and patient populations, to reach a general conclusion on the effectiveness of vitamin D in improving blood pressure outcomes. Several factors severely limit the value of such studies when assessing the role that vitamin D supplementation plays in regulating blood pressure.
Despite the authors’ statement that vitamin D does not appear to positively impact blood pressure, the outcomes of numerous other studies, including other meta-analyses do find a positive effect of vitamin D in supporting healthy blood pressure 5 6 7. The conflicting results that are presented in the JAMA study point to the need to look at the scientific literature as a whole, rather then let a single research paper establish dogma.
Overall, the findings of the recent JAMA study are not solid enough to refute previous findings and the indisputable fact that people with high blood pressure are significantly more likely to have low levels of vitamin D. Like all meta-analyses, this one is subject to its own bias based on study inclusion and exclusion criteria and methods of statistical analysis. It is for these reasons that other meta-analyses investigating the efficacy of vitamin D on blood pressure outcomes reach different conclusions 8.