Why a nutrient once embraced by medicine is now unfairly sidelined
Just 15 years ago, vitamin D was widely embraced by doctors as essential to health. Deficiencies were seen to be common, and observational studies linked low blood levels of D to everything from weak bones to cardiovascular health and poor immunity. Now, the medical profession has cooled—insisting vitamin D offers little benefit beyond bone health.
What has changed? The problem isn’t the nutrient—it’s how we study it. In this article, I explain why applying drug-style trials to essential nutrients like vitamin D leads to confusion, not clarity—and why it’s time to rethink how we evaluate vitamin D and other nutrients and their role in health.
Why Vitamin D Can’t Be Tested Like a Drug
In modern medicine, the randomized controlled trial (RCT) is considered the “gold standard” of scientific evidence. And for pharmaceutical drugs, that’s exactly right. A well-designed RCT can tell you whether a new drug lowers blood pressure, reduces pain, or clears an infection—usually within weeks. But what happens when we apply that same model to vitamin D, or any other essential nutrient?
The answer: we get muddled results, misleading headlines, and public confusion. That’s not because vitamin D doesn’t work. It’s because it doesn’t work like a drug. And trying to test it as though it does frequently creates more confusion than clarity.
Drugs and Nutrients Are Fundamentally Different
Let’s start with the basics. A drug is an outside agent introduced to change some physiological process in the body – lower cholesterol, block a receptor, suppress inflammation. It’s optional. It’s not required for survival. Vitamin D is entirely different. It’s a nutrient—an essential compound your body must have to function normally. It plays a role in bone formation, hormone regulation, immune function, inflammation control, and even gene expression.
So when we ask, “Does vitamin D work?” we’re not talking about an optional add-on. We’re asking whether giving more of something essential improves function, particularly in people who may not be getting enough.
Why a Classic RCT Doesn’t Work for Vitamin D
Here are four reasons why a standard RCT – the kind used to test drugs – breaks down when applied to vitamin D:
1. You Can’t Ethically Randomize People to “No Vitamin D”
In a true RCT, one group gets the treatment, and the other gets a placebo. But we can’t ethically deprive anyone of vitamin D over time. We know deficiency leads to rickets in children, osteomalacia in adults, and increases the risk of respiratory infections, autoimmune disease, and more. So what do researchers do? They compare different doses—a low-dose “control” group and a higher-dose intervention group. But both groups may still be getting some vitamin D, especially if they’re exposed to sunlight or already taking over-the-counter supplements. This “contamination” makes it hard to detect meaningful differences between groups.
2. Baseline Vitamin D Status Is Often Ignored
Most vitamin D trials do not sort participants based on whether they are deficient, insufficient, or adequate at the start. This is a major flaw. If someone already has a reasonable blood level of vitamin D (say, 100 nmol/L), giving them more may make no difference. But someone who starts at a very low level (say, 30 nmol/L) may see significant benefits from supplementation. When studies average outcomes across the entire group, the strong effects in the deficient subgroup is likely to be diluted by the lack of response in the already-sufficient group. The result? The conclusion often reads “no significant benefit,” even though some participants clearly did benefit. This is like testing the impact of iron supplements without checking first which patients in the study are anemic.
3. Everyone Can Still Make Vitamin D from Sunlight
Unlike a drug, vitamin D isn’t just some pill you swallow. It’s produced in your skin when you’re exposed to sunlight. So unless you are going to keep your trial participants indoors for the duration of the study, even people in the placebo group may make their own vitamin D throughout the trial. How much they make will depends on where they live, what season it is, their skin pigmentation, how much time they spend outdoors, whether they wear sunscreen or cover up, and many other factors. All of this introduces enormous variability—and uncertainty.
It’s nearly impossible to control for this kind of background exposure in a trial. So again, the differences between “treatment” and “control” groups often shrink—not because the nutrient has no effect, but because the trial couldn’t be tightly controlled in the way a drug study can be.
4. Nutrient Effects Are Long-Term and Synergistic
Drugs are typically expected to work within days or weeks. But nutrients don’t work that way. Vitamin D supports the long-term functioning of complex systems like the immune system, bone remodeling, inflammation control, and gene regulation. These systems don’t fail overnight—and they don’t improve overnight either. So expecting a short-term trial to show dramatic outcomes (like fewer infections, lower cancer risk, or longer lifespan) misunderstands the preventive, slow-building role of nutrients. What’s more, vitamin D depends on adequate magnesium to be activated, so giving vitamin D in someone who is magnesium deficient may be ineffective and reveal little unless this and other co-nutrients are also in place.
So, What’s the Alternative?
Does this mean we can’t study vitamin D at all? Of course not. But we need to use different kinds of evidence, and interpret it with nutritional context in mind.
- Observational studies have consistently shown that people with low vitamin D levels are at higher risk of infections, bone fractures, cardiovascular disease, and certain cancers.
- Mechanistic studies show how vitamin D regulates immune cells and gene expression.
- Intervention trials, when done in deficient populations, often show clear benefits—reduced respiratory infections, better bone health, and even improved outcomes in autoimmune diseases.
Together, these form a converging body of evidence. It may not look like the “perfect” drug trial—but it is still meaningful, and in many cases, actionable.
The Bottom Line
We don’t need to throw out RCTs. But we do need to stop demanding drug-style proof from something that isn’t a drug. Vitamin D is an essential nutrient, not a treatment. The right question isn’t “Does it work?” but “Who needs more, and what happens when they don’t get enough?” When we reframe the question, the evidence we need, and how to get it becomes a lot clearer.
And one final point: the fact that official Recommended Dietary Allowances (RDAs) for vitamin D exist at all is an acknowledgment that the body requires a minimum daily amount in order to maintain health. So we already have broad scientific agreement that if vitamin D intake falls short of this threshold, there are likely to be consequences – some of which may be serious, like rickets in children or osteomalacia in adults. Nutrient deficiencies are not neutral. Left unaddressed, they compromise the body’s ability to maintain normal functioning.
Correcting a deficiency isn’t “treatment.” It’s restoring the conditions required for on-going health. So when clinicians find individuals with clear evidence of a vitamin D deficiency, it is not just good practice to correct it, but an ethical obligation.
