Vitamin D: The Leap from Correlation to Effectiveness

In a more innocent time, Schlitz, the beer that made Milwaukee famous, was also the beer with sunshine vitamin D. “A priceless source of vigor,” claimed the advertising. Similarly sunny claims are bubbling up for vitamin D in this time of COVID-19. These claims are based on observed correlations. But thinking that vitamin D might have effectiveness for blunting the effects of COVID-19 is a big leap.

Two new studies offer some insight.

A Meta-Analysis of Correlations

In the International Journal of Infectious Diseases, Nanyang Liu et al report that low vitamin D levels correlate with a higher risk of COVID-19 infection. In a meta-analysis of ten case-control studies with 376,596 subjects, they found that infection with COVID-19 was 43 percent more likely in people with low vitamin D levels. This raises the possibility that low vitamin-D levels might raise the risk of infection.

However, the authors carefully point to the limitations of their study. Low vitamin D levels might be a cause of COVID-19 infection. Or it might be a consequence. Other limitations, including publication bias, are important.

They rightly conclude that this subject needs more research. But it’s not much of a leap to suggest that people with vitamin D deficiency need attention.

A Randomized Study of Vitamin D Supplementation

If you need a reason to doubt the benefits of supplementing with vitamin D, a new RCT might give you that. Hai Pham et al studied the effect of supplementing with 60,000 units of vitamin D monthly on the risk, duration, and severity of respiratory infections. They found no effect on the risk of infection. They found only small and insignificant effects on the duration and severity of symptoms.

Of course, the story might be different for COVID-19, but so far, results have been mixed.

So What?

It should be clear enough at this point, that vitamin D has no miracles to offer for COVID-19. If people are deficient it makes abundant sense to take a supplement, but not too much. The Harvard Medical School advises that more 4,000 units per day is the upper limit of what’s safe, unless your doctor has a specific reason for prescribing more.

Health plans don’t like to pay for unnecessary lab tests. But if you have real questions about a possible vitamin D deficiency, measuring your levels makes a lot of sense. It’s a straightforward blood test that’s not terribly expensive. If there’s a good reason, insurance will cover it.

Click here for the Liu study and here for the Pham study. For contrasting views on the value of vitamin D supplementation, click here, here, and here.

A Sunshine Holiday, watercolor by William Blake / WikiArt

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January 16, 2021