Dueling opinions on vitamin D

“Curiouser and curiouser,” cried Alice in her journey through Wonderland. And so might we primary care clinicians, soldiering forward to stamp out disease and promote good health in a sometimes confusing world. Our mission is made more challenging when there is not a firm foundation of evidence about which tools in our arsenal truly work and which do not.

Take vitamin D for example. Who among us has never ordered a vitamin D blood level, or prescribed vitamin D supplementation for a patient? Hearing no reply, I shall continue. And just what did we accomplish by checking a blood level or recommending this vitamin? Ah, there lies the rub. Whether this tool is useful, harmful, or neither, depends on which experts you opt to believe.

Two reputable sources have recently come out with somewhat contradictory recommendations on the question of vitamin D. The BMJ just published a large umbrella review on vitamin D, looking at whether measuring its levels or prescribing it as a treatment have any utility. They identified 107 systematic reviews and 74 meta-analyses looking at the significance of vitamin D blood levels and found the results obtained from this test were not meaningful. Some non-significant trends were noted linking low vitamin D levels to increased rheumatoid arthritis activity and higher rates of colorectal cancer, pediatric hypertension, pediatric rickets, bacterial vaginosis during pregnancy, and falls in the elderly. But no clear benefit for these conditions was noted from vitamin D supplementation. The only potential benefits from vitamin D supplementation were less dental caries in children, lower parathyroid hormone levels in CKD patients, and increased birth weight and maternal vitamin D levels. The authors conclude, “On the basis of this umbrella review, highly convincing evidence of a clear role of vitamin D with highly significant results in both randomised and observational evidence does not exist for any outcome.” The authors also point out they are not denying all utility for vitamin D measurement or treatment. Their principal point is that high-quality evidence to support its efficacy has not been found. Even for osteoporosis.

That is one viewpoint. Let us move on to see what the Cochrane Reviews have to say about vitamin D. The Cochrane Reviews are certainly reputable, reliable, and authoritative. In January of this year, they published a review looking at the potential benefits and harms of vitamin D for preventing mortality. They examined 56 trials involving close to 100,000 patients. Their conclusion? Vitamin D3 supplementation may reduce mortality. Treating 150 people over five years will save one life. Not bad, really. The Cochrane Review had a more narrow focus than the BMJ article, but nonetheless found a potential benefit from vitamin D supplementation.

So, what are we to do about vitamin D? On this issue, the jury is still out. Vitamin D supplementation is unlikely to be harmful, though it may increase the risk of kidney stones. But whether it is helpful remains to be seen. Cochrane says probably yes. BMJ says probably no. Curious indeed. And more validation for why, when describing our chosen profession, we refer to the art and science of medicine.

Richard Fleming, MD

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