Vitamin D: What Your Skin Does, & What The Two Biggest Pill Trials Found

Your skin is a manufacturing site. Ultraviolet B light hits a molecule called 7-dehydrocholesterol sitting in the skin, splits it into previtamin D3, and your own body heat then rearranges that into vitamin D3.

That arrangement is older than every supplement aisle on earth, and it is the reason the pill conversation is more interesting than either side usually admits.

What the design actually depends on

The mechanism is not controversial. What varies enormously is how much of it happens, and the list of variables is longer than most people realise: skin pigment, sunscreen use, age, time of day, season, and latitude all change the yield (Holick, Advances in Experimental Medicine and Biology, 2008).

Which is why “just go outside” is honest advice in Kansas in July and useless advice in Kansas in January. Above a certain latitude in winter, the UVB simply does not arrive at the angle required, and no amount of standing in the cold fixes that.

There needs to be a renewed appreciation of the beneficial effect of moderate sunlight for providing all humans with their vitamin D requirement for health.

Holick, Advances in Experimental Medicine and Biology, 2008

Then somebody tested the pills, properly, twice

This is where it gets useful, because both of these are large, randomised and placebo-controlled, which is the standard almost nothing in the supplement world is held to.

D2d randomised 2,423 adults with prediabetes to 4000 IU of vitamin D3 a day or placebo, and watched for new diabetes (Pittas et al., New England Journal of Medicine, 2019).

After a median 2.5 years, diabetes occurred in 293 people on vitamin D and 323 on placebo. The hazard ratio was 0.88, with a confidence interval running from 0.75 to 1.04 and a p-value of 0.12. That interval crosses 1, which means the trial did not demonstrate a benefit.

VITAL went bigger: 25,871 US adults randomised to 2000 IU a day or placebo, followed a median of 5.3 years, with invasive cancer and major cardiovascular events as the two primary endpoints (Manson et al., New England Journal of Medicine, 2019).

Endpoint	Hazard ratio	Result
Invasive cancer of any type	0.96 (0.88 to 1.06)	Not met
Major cardiovascular events	0.97 (0.85 to 1.12)	Not met
Death from any cause	0.99 (0.87 to 1.12)	No difference

Neither primary endpoint was met. No excess hypercalcemia or other adverse events turned up either, so this is a story about absent benefit rather than about harm.

The detail that changes how you read both

Here is the part that almost never gets quoted, and it is the hinge of the whole argument.

The pills worked. In D2d, blood 25-hydroxyvitamin D in the supplement group went from 27.7 to 54.3 ng/mL by month 24, while the placebo group sat at 28.8. The level roughly doubled, exactly as intended.

So this is not a story about a supplement that failed to be absorbed. It is a story about a supplement that did precisely what it said on the label, in thousands of people, for years, and the diseases did not care.

That is a genuinely important finding, and it points somewhere specific. For decades, observational studies found that people with low vitamin D got more of almost everything bad. The natural reading was that low vitamin D caused it. These trials test that reading directly, and it did not hold.

The likelier explanation is that a low vitamin D level is often a marker rather than a cause: of being indoors, of being unwell, of being inactive, of carrying more body fat, which sequesters it. Raising the number in the blood does not undo the life that produced the number.

What these trials do not say

Be precise here, because both camps overstate this one.

Neither trial selected people for deficiency. D2d enrolled adults at high risk of diabetes regardless of their baseline vitamin D level, and the average was already around 28 ng/mL, which is not deficient. VITAL enrolled broadly too. So neither answers the question “what happens when you correct a genuine deficiency”, which is a different and better-established question.

Neither tested sun against pills head to head. Nobody has. The comparison in this article is between a supplement and a placebo, not between a supplement and an afternoon outdoors.

And vitamin D’s established job is still its established job. Deficiency causes rickets in children and worsens bone loss and fracture risk in adults. That was never in dispute and these trials do not touch it. What they tested was the much larger set of claims that grew up afterwards.

Where the design shows up

The thing worth sitting with is the direction of the arrow.

Sunlight on skin does not just make vitamin D. It sets your body clock, which runs sleep and metabolism and hormone timing. It is usually attached to being outside, which is attached to moving, which is attached to seeing other people. A blood level of 25-hydroxyvitamin D is downstream of all of that.

A capsule delivers exactly one item off that list. It can raise a number in a blood test without touching anything the number was standing in for.

Which is probably the honest summary of what happened in both trials, and it is a general lesson rather than a vitamin D lesson. When a measurement travels with health, the measurement is sometimes the cause and is often just the evidence.

“God made the two great lights, the greater light to govern the day.” Genesis 1:16, NASB

The verse carries the conviction. Twenty-eight thousand randomised participants carry the limits of the capsule.

What to do this week

Get outside, for the whole list of reasons rather than for the one. Short, regular, skin-type-appropriate time in daylight, and more of it in the middle of the day during the months when UVB actually reaches your latitude. Never to the point of burning, which buys you nothing and costs you real skin damage.

Put mushrooms on the windowsill before you cook them. This genuinely works and almost nobody does it. Button mushrooms measured about 5 micrograms of vitamin D per 100 g dry weight untreated, and about 410 after UVB exposure. We wrote it up properly in what the mushroom trials actually measured. It is D2 rather than D3, and the figures are dry weight, so read it as a useful addition rather than a replacement for sun.

If you are going to supplement, test rather than guess. Ask for a 25-hydroxyvitamin D level and let the number drive the decision. That is cheap, it is specific to you, and it is the one approach neither of the trials above argues against. High-dose regimens in particular are a conversation with a clinician, not with a shelf.

And hold the two things together. The capsule is a tool for a measured deficiency. It is not a substitute for the life that would have made the deficiency unlikely.

Read this next: what morning daylight does to your clock, with trial evidence behind it: morning daylight worked as well as melatonin. The other end of the same day: light in the bedroom at night. Why a whole food keeps beating its own extract: what food as medicine advice never mentions. And how to read a study claim at all: should you really trust the science.

Sources

1. Pittas AG, Dawson-Hughes B, Sheehan P, et al. “Vitamin D Supplementation and Prevention of Type 2 Diabetes.” The New England Journal of Medicine, 2019;381(6):520-530. https://pubmed.ncbi.nlm.nih.gov/31173679/
2. Manson JE, Cook NR, Lee IM, et al. “Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease.” The New England Journal of Medicine, 2019;380(1):33-44. https://pubmed.ncbi.nlm.nih.gov/30415629/
3. Holick MF. “Sunlight, UV-radiation, vitamin D and skin cancer: how much sunlight do we need?” Advances in Experimental Medicine and Biology, 2008;624:1-15. https://pubmed.ncbi.nlm.nih.gov/18348443/
4. Simon RR, Phillips KM, Horst RL, Munro IC. “Vitamin D mushrooms: comparison of the composition of button mushrooms treated postharvest with UVB light or sunlight.” Journal of Agricultural and Food Chemistry, 2011;59(16):8724-8732. https://pubmed.ncbi.nlm.nih.gov/21736377/

Article metadata retrieved from PubMed. Photograph from Pexels. Scripture quoted from the New American Standard Bible.

Educational content from True Health Solutions. We do not diagnose, treat, cure or prevent disease, and nothing here is a substitute for care from someone who knows your history. The body heals. We write about what supports that design.