Are you wasting money on vitamin D?
The narrative arc of vitamin D is the same with every single disease:
An association is found between people with low vitamin D levels and worse prognosis
Small or observational studies suggest vitamin D supplementation leads to better outcomes
More well done and randomized studies with bigger sample sizes suggest that supplementing with vitamin D has no impact on length or quality of life
But despite the unimpressive evidence, vitamin D testing and supplementation remains big business: we are talking about billions of dollars each year spent on testing vitamin D levels and buying supplements.
Is everyone wasting their money here?
Let’s take a look.
Vitamin D deficiency can be very bad. But for most people, it isn’t
Severe vitamin D deficiency can be devastating, particularly for children.
But the introduction of vitamin D fortified foods like milk basically eliminated rickets, the worst manifestation of vitamin D deficiency, in the United States.
And since most adults are deficient in vitamin D and we’re not all walking around with crippling fractures, it’s not a huge leap of faith to suggest that we probably doesn’t require all that much vitamin D to promote minimal bone health.
But there may be a difference between the amount needed to prevent a catastrophic deficiency and the amount needed to promote optimal health.
Low vitamin D levels are linked to basically every health problem you can think of - obesity, coronary artery disease, cancer, Alzheimer’s disease, Parkinson’s disease, congestive heart failure.
If you can think of a disease, I bet you that you can find a link between low vitamin D levels and that disorder.
The clinical trials for vitamin D are not very impressive
The big randomized trials for vitamin D have almost universally been disappointing.
The VITAL trial is probably the most impactful of these trials. This trial followed over 25,000 people over 6 years to monitor the impact of vitamin D supplementation on cancer and heart disease.
The study found no difference in heart disease or cancer risk between the group that got vitamin D and the group that got placebo:
But the VITAL investigators didn’t just stop there.
VITAL is so impactful that you’ve seen news reports on the substudies looking at things like fractures and depression that have made it into the press as big disappointments for those of us on the vitamin D bandwagon.
So the reason that almost every doctor you ask about this is skeptical of vitamin D supplementation to prevent disease is because of huge, long-term studies like VITAL that basically show there’s no benefit from supplementing everyone with vitamin D.
But we shouldn’t pretend that the large trials looking at vitamin D are all that definitive
These trials aren’t the nail in the coffin for vitamin D. They certainly have weaknesses, which have been picked apart by people much smarter than I am.
It’s worth noting that there has never been a single vitamin D trial that took patients with vitamin D deficiency and randomized some to vitamin D supplements and some to placebo, making sure to evaluate whether supplementation actually cured the deficiency.
In other words, no one has actually tested whether fixing vitamin D deficiency improves health outcomes. We’ve only tested whether giving people vitamin D supplements does.
That may seem like a trivial distinction, but I don’t think that it is. If you’re going to treat someone because of a biomarker, you should measure that biomarker to see if your treatment has been successful.
Even though the trials aren’t perfect, it’s ok to remain skeptical
It’s easy to dismiss the studies that we have and just point to the plausible mechanisms and say that vitamin D should work.
But the story of clinical trials of vitamin D has been so universally disappointing, that I wonder whether the lack of study design to test this question actually matters.
At some point, if the benefit of vitamin D was really that great, wouldn’t we see some signal in the data?
If vitamin D deficiency really is so catastrophic for our health, why don’t the randomized trials show even a tiny hint of a benefit?
And don’t start with me on the observational studies
Observational data - data where you survey a population and look at the risk of disease or death in people who do “X” versus people who don’t - doesn’t actually inform you of very much.
The amount of bias and confounding in these trials generally renders the results meaningless; that’s why I’m so dismissive of basically all nutritional research.
So while you can pull up a trial like this one, which shows a large increase in risk of death from COVID in patients who didn’t supplement with vitamin D compared to those who did, I would argue that it tells you essentially nothing about what vitamin D does.
The reason for that is because people who take supplements are different than people who don’t - they exercise more, they eat better, and they are less likely to be overweight or smoke.
If you had to bet, which do you think made them less likely to die from COVID - was it the vitamin D supplement or was it the years of eating better and exercising more?
A high level of vitamin D obtained via lifestyle may not be the same as a high level of vitamin D obtained from supplements
My big suspicion here is that not all vitamin D is created equal. That’s partly because of how complicated vitamin D metabolism is and partly because of the differences between those who are naturally deficient and those who aren’t.
Maybe we shouldn’t confuse a high vitamin D level obtained from supplementation as being the same thing as a high vitamin D level obtained from lifestyle.
After all, think about the lifestyle patterns that a high vitamin D is a marker for:
Being outside and getting natural sunlight, which requires that you feel well enough to do that. It probably closely associates with the type of person who is both healthy enough to do some physical activity as well as folks who choose to be active.
A diet that’s rich in nutritious foods, particularly animal products. You don’t find much vitamin D in crappy processed foods. And you won’t find any in the Impossible Burger.
So is it really the high vitamin D levels that cause the reduced risk of chronic disease, or it is the fact that people who eat better and spend more time outside are generally healthier?
My conclusion is that vitamin D supplementation is pretty harmless, but it’s unlikely to have much benefit if your levels are in the normal range.
It’s probably also unlikely to have much benefit if you are regularly exercising and eating nutritious foods.
For most people, supplementing if your levels are low makes sense. But vitamin D isn’t like exercise, where everyone who uses the intervention has a benefit.
And while it’s good to remember that the absence of evidence isn’t evidence of absence, I’m not ready to make much of a leap of faith in the direction of vitamin D for everyone.