Most Adults Don’t Need More Than 800 IU of Vitamin D, Experts Say

Vitamin D is one of the most popular supplements there is. And the doses on the shelf vary wildly.

So how much do you actually need?

In 2024, an expert panel went through the big trials to answer that. Their answer is probably less than you’d guess.

How much vitamin D should I take?

For most healthy adults, 600 IU (15 micrograms) a day. Over age 70, it’s 800 IU (20 micrograms).

That’s the US Recommended Dietary Allowance. It counts vitamin D from food, drinks and supplements together.

The UK sets a lower bar. The NHS says 10 micrograms (400 IU) a day is enough for most people.

It suggests everyone consider a supplement in autumn and winter, when the sun is too weak to make enough.

Both agree on the ceiling: no more than 4,000 IU (100 micrograms) a day for adults, unless a doctor advises it.

Your skin also makes vitamin D in sunlight. But clouds, older age and darker skin reduce how much it makes, says the US National Institutes of Health (NIH).

Food helps a little. Most US milk is fortified with about 120 IU per cup, and fatty fish like salmon and mackerel are good natural sources.

What the research shows

A 2024 guideline built on 151 studies

The main source is a 2024 guideline from the Endocrine Society, a large medical society for hormone doctors. It appeared in The Journal of Clinical Endocrinology & Metabolism.

An expert panel asked 14 questions about who should take vitamin D. A separate team searched the research and pooled results from 151 studies.

The panel looked at extra vitamin D. That means more than the daily amounts above, taken without a blood test first.

Healthy adults aged 50 to 74: little or no benefit

This is the clearest result. Here’s what extra vitamin D did in randomised trials of adults aged 50 to 74:

The typical dose in these trials was 1,500 to 2,000 IU a day. That’s well above the daily amount.

The panel rated this evidence as high certainty. Its verdict: vitamin D had “little or no beneficial impact” in this age group.

Under 50, there were fewer trials. They also found no clear benefit, for example on chest and throat infections.

Who might benefit

The panel did suggest extra vitamin D for four groups:

• Children and teens aged 1 to 18, to prevent rickets and possibly chest and throat infections.
• Adults aged 75 and over, because it may lower the risk of death.
• Pregnant women, because it may lower the risk of pre-eclampsia and other problems.
• Adults with high-risk prediabetes, because it may slow the move to diabetes.

The benefit for over-75s was small. In 25 trials with 49,879 people, vitamin D meant about 6 fewer deaths per 1,000 people.

The doses in those trials were modest. The weighted average was about 900 IU a day.

Skip the blood test

The guideline also suggests against routine vitamin D blood tests for healthy people. That includes people with obesity or darker skin.

The reason: no trial has shown which blood level prevents disease. The Endocrine Society no longer backs its old target of 30 ng/mL (75 nmol/L).

The catch

Here’s the part I’d keep in mind. Most people in these trials already had decent vitamin D levels when they started.

Many could also keep taking their own supplements, often 400 to 800 IU a day. So the trials mostly tested extra vitamin D on top of enough.

That’s why the panel says some people could reasonably take a supplement anyway. It means people who get little sun, such as housebound people, or too little from food.

All the panel’s advice is “conditional”. Most people would follow it, but some could reasonably choose differently.

Who funded it

The Endocrine Society paid for the guideline, and says no other organisation provided money. Panel members’ conflicts of interest were managed under the society’s policy.

When more becomes too much

Daily beats big occasional doses

Some people take one large dose a week or a month instead of a daily pill. The guideline looked at this too, in 15 studies with 53,527 people.

For adults 50 and over, it suggests a daily, lower dose instead. Big occasional doses showed a trend toward more fractures, especially doses above 100,000 IU.

Doses given less often than every 12 weeks were also linked to more falls.

Higher daily doses and bone loss

A 2019 trial in JAMA tested high daily doses in Calgary, Canada. It included 311 healthy adults aged 55 to 70, followed for 3 years.

People took 400, 4,000 or 10,000 IU a day. Bone density at the wrist fell in every group, but it fell more with higher doses.

Bone strength didn’t differ clearly between the groups. The authors said the results “do not support a benefit” of high doses for bone health.

Whether high doses actually harm bones needs more research, they added.

What too much can do

The NIH says very high blood levels can cause nausea, vomiting, muscle weakness, confusion and kidney stones. Extremely high levels can cause kidney failure, irregular heartbeat and even death.

This almost always comes from supplements. You can’t get too much vitamin D from the sun.

What earlier research found

A review of the big trials

A 2022 review in Nature Reviews Endocrinology looked at three large trials: VITAL, ViDA and D2d. Together they included more than 30,000 people.

In adults who already had enough vitamin D, supplements didn’t prevent cancer, heart events, falls or type 2 diabetes.

The authors concluded that extra vitamin D in such people “does not provide demonstrable health benefits”. But they agreed that severe deficiency should be corrected.

The review also covered over 60 genetic studies. Most found no effect of lifelong lower vitamin D on disease.

The biggest single trial: VITAL

VITAL was published in The New England Journal of Medicine in 2019. It gave 25,871 US adults either 2,000 IU of vitamin D a day or a dummy pill.

After about 5.3 years, vitamin D didn’t lower the risk of cancer. It didn’t lower the risk of heart attacks, strokes or heart deaths either.

There was a hint of fewer cancer deaths, but the result wasn’t statistically certain. Most people started with healthy levels, averaging 30.8 ng/mL.

VITAL was funded by the US National Institutes of Health. A supplement company, Pharmavite, donated the vitamin D pills.

One huge dose a year raised falls

A 2010 trial in JAMA gave 2,256 women either 500,000 IU of vitamin D once a year or a dummy pill. The women were aged 70 or older and at high risk of fracture.

The vitamin D group had 15% more falls and 26% more fractures. The risk of falling was highest in the first 3 months after each dose.

How it fits together

The research points one way. For people who already get enough, extra vitamin D adds little, and very big doses may backfire.

How much should you trust this?

Strong. The advice rests on large randomised trials with tens of thousands of people. For adults aged 50 to 74, the evidence was rated high certainty.

What makes it convincing

• The guideline pooled 151 studies, and its main conclusions rest on randomised trials.
• Very large trials such as VITAL, with 25,871 people, found the same thing.
• US and UK health bodies set the same 4,000 IU upper limit for adults.

What makes me cautious

• Most trial participants already had decent vitamin D levels, so the results may not apply to people who are truly low.
• For adults under 50, the trials were fewer, and the evidence was rated very low certainty.
• The best dose for the groups who may benefit is still unclear.
• Many trials let people keep taking their own vitamin D.

What this means for you

• Know your number. For most adults, it’s 600 IU (15 micrograms) a day, or 800 IU over 70.
• Count your multivitamin. Most multivitamins already contain 800 to 1,000 IU, the guideline notes.
• Don’t go over 4,000 IU a day unless a doctor tells you to.
• Prefer a daily dose to big weekly or monthly ones, especially if you’re over 50.
• Over 75, pregnant, or have prediabetes? Ask your doctor about a supplement. The guideline suggests one for these groups.
• Rarely outdoors, or have dark skin? The NHS suggests 10 micrograms a day all year round.
• Have a medical condition? Some conditions, such as gut problems that affect absorption, need different doses. Follow your doctor’s advice.

The NIH vitamin D fact sheet lists the daily amounts and upper limits by age.

In this press conference, members of the Endocrine Society panel explain the new vitamin D guideline:

What we still don’t know

1. Does extra vitamin D help people who are truly low? Few large trials have tested only people with low levels.
2. What’s the best dose for older adults? The trials used a wide range of doses.
3. Which blood level matters? No trial has shown which level prevents disease.
4. Are years of high doses safe? Most trials lasted only a few years.

My take: take enough, not more

What I like about this guideline is how practical it is. It answers the question most people actually have: should I take more?

I’m less convinced by the idea that more vitamin D means better health. The big trials keep finding the same thing: once you have enough, extra doesn’t help.

So my reading is simple. Aim for the daily amount from food, sun or a modest supplement.

Skip the mega-doses. And if you’re over 75 or pregnant, talk to your doctor.