Is vitamin D worth taking? What the biggest trials actually found

I make a point of getting at least 10 minutes of sun a day for vitamin D. Even so, when my regular doctor checked my blood, my vitamin D came back a little low, and he handed me a supplement. The gut specialist who treats my Crohn's disease had tested it too, and didn't tell me to take one.

So before starting, I looked up what vitamin D pills actually do.

The short version

• In huge trials, vitamin D pills didn't prevent heart attacks, cancer, broken bones or early death in people who weren't already low.
• One big trial found 22% fewer autoimmune diseases. In real numbers that was 2 or 3 fewer cases per 1,000 people, and it faded within two years of stopping the pills.
• If you're genuinely low, fixing it matters. Some researchers think that's where the real benefits are, but no trial has proven it yet.
• If you take it, a modest daily dose seems better than big monthly ones, and more isn't better. Stay under 4,000 IU a day unless your doctor says otherwise.

The trials everyone was waiting for#

For years, studies noticed that people with low vitamin D were more likely to get heart disease, cancer and much else. So researchers gave people vitamin D to see if those problems went down.

That's what the VITAL trial did. For about five years, nearly 26,000 American adults took either 2,000 IU of vitamin D a day or a dummy pill. Their average age was 67. Vitamin D made no real difference to cancer, heart attacks, strokes or deaths.¹ A separate look at the same people found no difference in broken bones either.²

In Australia, the D-Health trial gave over 21,000 people aged 60 to 84 a big monthly dose or a dummy pill, also for about five years. It didn't help them live longer. There were slightly fewer heart attacks and strokes, but too few to rule out chance.³⁴

Most people in these trials weren't deficient to begin with. But that describes most of us, too. According to the US National Institutes of Health, most Americans have enough vitamin D in their blood, even though most get less than the recommended amount from food.⁵

Both trials were also in people over 50, so they tell us less about people in their 40s.¹³

If you were hoping it would help your mood, trials haven't found that vitamin D supplements prevent or treat depression either.⁵

Two of the bluntest critics called VITAL "a decisive verdict" against vitamin D pills for the general population.⁶ Dr. JoAnn Manson, who led VITAL, puts it more gently: "It usually takes a profound deficiency of vitamin D to have a clear benefit from supplementation."⁷

The surprise, and what happened next#

VITAL had also been tracking autoimmune diseases, the ones where your immune system attacks your own body, like rheumatoid arthritis, psoriasis and some thyroid diseases. Immune cells respond to vitamin D, which is why scientists had wondered about this for years.⁸

In 2022 they reported 22% fewer confirmed autoimmune diseases in the vitamin D group.⁸ The study's first author said, "Until now, we have had no proven way of preventing them, and now, for the first time, we do."⁹

In real numbers, each group had about 12,900 people. 123 on vitamin D got an autoimmune disease, against 155 on the dummy pill. That's 2 or 3 fewer cases for every 1,000 people over five years. And the result only just passed the bar for counting as more than chance.

The next part got far fewer headlines. Researchers kept following most of the people for two more years after they stopped the pills, and the difference disappeared.¹⁰

Some cases also turned up late: 65 more people had actually gotten sick during the trial. Once they were counted, the 22% dropped to about 15%, too small to rule out chance.¹⁰

A 2024 review that combined 18 studies found that, overall, vitamin D didn't change the risk of getting an autoimmune disease.¹¹

So one large, well-run trial saw a plausible signal. But it's a single, borderline result, and if it's real, it seems to last only while you keep taking the pills.

The best argument for vitamin D#

If most people in the trials already had enough vitamin D, of course extra didn't help. Topping up a full tank doesn't take you further. The people who'd benefit are the ones running low, and the big trials mostly didn't test them.

One research group checked this using health records from a large British study called UK Biobank. In people like the trial volunteers, they also saw no effect. But in people who were deficient, they estimated roughly a fifth to a quarter fewer deaths.¹²¹³

That's a serious argument. And everyone agrees that if you're truly deficient, fixing it matters, especially for your bones.⁵¹⁴

But nobody in that study was actually given pills. People with low vitamin D are often sick, inactive or stuck indoors, so it's hard to tell whether the vitamin or the rest is to blame. And when the real trials checked, people with lower levels didn't get bigger benefits.²³

How you take it seems to matter#

Daily seems better than big occasional doses. When researchers combined 14 trials, people taking vitamin D every day had slightly fewer cancer deaths, about 12% fewer. People taking a big dose once a month or less saw no benefit.¹⁵

That's early evidence, but it lines up with the experts. The Endocrine Society, a group of hormone specialists, prefers small daily doses over big occasional ones for people over 50 who need vitamin D.¹⁴

And more isn't better. In one trial, healthy adults aged 55 to 70 took vitamin D every day for three years. Some took 400 IU, some 4,000 and some 10,000. The higher doses ended up with slightly lower bone density in the wrist.¹⁶

In another trial, older women took one huge dose a year. They had more falls and more broken bones than women on a dummy pill: 15% and 26% more.¹⁷

The upper limit for adults is 4,000 IU a day unless a doctor prescribes more. Too much vitamin D pushes up the calcium in your blood and, in extreme cases, can damage your kidneys. That almost always comes from supplements.⁵

Can sun and food cover it?#

In summer, usually. In winter, not so much. Researchers in Valencia, Spain, worked out how long you'd need in the midday sun to make a day's worth of vitamin D. In July it took about 7 minutes. In January it took more than two hours.¹⁸ Further north it's worse: the NHS says people in the UK can't make enough from sunlight between October and early March.¹⁹

Food is hard too. Oily fish like salmon and trout are among the best sources, and egg yolks have a little. But the NIH says getting enough from food alone is difficult unless it's fortified, like some milks and cereals.⁵

Where I land, as someone with Crohn's#

I'm not a doctor, which is why everything here comes from the research. But I am in one of the groups where vitamin D gets more attention. The NIH says conditions that make it harder to absorb fat, including Crohn's, make you more likely to run low.⁵

A review of studies in people with Crohn's and similar gut diseases found fewer flare-ups with vitamin D. People whose Crohn's was quiet were about half as likely to flare.²⁰ Those studies are small, a few hundred patients in total. So if you have one of these diseases, they're a reason to talk to your gut specialist, not a dose to copy.

I'll take the supplement my doctor gave me during the winter. I haven't seen anything against taking it, even if there aren't clear advantages either. In summer I'll try to get more sunlight instead.

Winter is my weak spot. Even though winters in Spain are pretty mild compared to a lot of places, I don't get outside much when it's cold, and I've been trying to change that. This winter I'm training for a marathon, so I'll be out running a lot more than usual.

What I'd actually do#

1. Know why you're taking it. If it's for your heart, cancer or "general health," and you're not in a risk group, the big trials say it probably isn't doing much. A standard dose is cheap and low-risk. It just isn't a shield.
2. If you take it, take a modest dose every day. The official daily amount for adults is 600 IU, or 800 IU over 70.¹⁴ The UK's NHS suggests everyone consider 400 IU a day from October to early March.¹⁹ (Some labels use mcg instead: 400 IU is 10 mcg.) Pick D3 over D2, since it raises blood levels more.⁵ Skip big weekly or monthly doses, and stay under 4,000 IU unless your doctor prescribes more.
3. Know whether you're in a group that needs a closer look: over 75, rarely outdoors, darker skin, skin mostly covered, a gut condition like Crohn's that makes it harder to absorb fat, weight-loss surgery, or certain medications like steroids.⁵¹⁴¹⁹ If you are, ask your doctor what makes sense for you. If you do get tested, 20 ng/mL (50 nmol/L) or more is enough for most people, and under 12 ng/mL (30 nmol/L) counts as deficient.⁵ For healthy adults, the Endocrine Society doesn't recommend routine vitamin D testing.¹⁴

If you only do one thing: check whether you're in one of those groups. If you are, talk to your doctor. If you're not, don't count on a vitamin D pill to protect you from much.

I've always preferred getting what I need from sunlight and food, and I don't want to depend on a pill at any time of year. But from what the experts say, winter sun may not be enough on its own, so in winter I'll let the supplement fill the gap. I'll keep getting outside anyway, including the cold mornings when it's hard to make myself go out. Wish me luck.