Quick answer

Vitamin D helps your body absorb calcium and is needed for normal bones, muscles and immune function. Adults need 15 to 20 mcg (600 to 800 IU) a day. In large trials, adults who already had enough gained no measurable extra benefit from 2,000 IU a day, so it matters most if you run low.^1,2,3

Key facts

For vitamin D, 1 mcg equals 40 IU, so a 50 mcg softgel is 2,000 IU.¹
Few foods contain much vitamin D naturally. Fortified milk, plant milks and cereals provide most of it in American diets.^1,2
A blood level of 20 ng/mL (50 nmol/L) or more is adequate for most people.²
The adult upper limit is 100 mcg (4,000 IU) a day from food, drinks and supplements combined.²
Vitamin D3 raises blood levels more, and keeps them up longer, than vitamin D2.¹

What vitamin D does

Vitamin D is a fat-soluble vitamin with an unusual source: your skin makes it when bare skin meets sunlight, and food and supplements add the rest. Whatever the source, it starts out inactive. The liver converts it to 25-hydroxyvitamin D, the form a blood test measures, and the kidneys convert that into the active hormone.¹

The active hormone tells the gut to absorb calcium and phosphate, the minerals bone is built from. Muscle and immune cells carry receptors for it too, which is why vitamin D comes up in talk about strength and immunity. The expert panel that set US intakes found the evidence firm only for bone; for other outcomes it was too thin or too contradictory to draw conclusions.¹

The skin makes vitamin D when bare skin is exposed to the sun. Clouds, smog, older age and darker skin reduce how much it makes, and sunlight through a window makes none.^1,2

What the evidence shows

Each claim below carries a grade and the amount studied. The strongest evidence is simply that a supplement raises blood levels. The other three describe what vitamin D normally does in the body. Trials that gave extra vitamin D to adults who already had enough found no added gain, so read them as reasons not to run low.^1,3

Raises and maintains blood levels of vitamin D*^1,3

Strong evidence

In the VITAL trial, 2,000 IU a day raised average blood 25-hydroxyvitamin D from 29.8 to 41.8 ng/mL after one year among the 1,644 participants who had repeat blood tests, while levels on placebo barely changed. NIH notes that vitamin D3 raises blood levels more, and keeps them up longer, than vitamin D2.

Amount studied: 2,000 IU (50 mcg) of vitamin D3 a day

Helps the body absorb calcium and maintain normal bones*^1,4,5

Moderate evidence

This is vitamin D's best-established role and the basis of official intake recommendations. It matters most when intake and sun exposure are low. In adults who mostly had adequate levels already, extra vitamin D gave no added benefit for bones: VITAL tested 2,000 IU a day in 25,871 people for about five years and found no difference from placebo.

Amount studied: The recommended 15 to 20 mcg (600 to 800 IU) a day from all sources; VITAL tested 2,000 IU a day on top of diet

Contributes to normal muscle function*^1,4,6,7

Moderate evidence

Low vitamin D can cause muscle weakness, and 'contributes to the maintenance of normal muscle function' is a nutrient-function statement permitted in the EU. Extra vitamin D has not improved strength in trials: a pooled analysis of 54 placebo-controlled trials (8,747 people) found no benefit for muscle strength or physical performance, and a three-year trial of 2,000 IU a day in 2,157 adults aged 70 and over found no gain in physical performance. Read this as a reason not to run low, not as a strength aid.

Amount studied: The recommended daily intake; DO-HEALTH tested 2,000 IU a day for three years

Contributes to normal immune function*^1,4,8

Moderate evidence

NIH lists immune function among vitamin D's roles, and 'contributes to the normal function of the immune system' is a permitted nutrient-function statement in the EU. Whether supplements help people get through the year with fewer respiratory illnesses is unsettled: the latest pooled analysis, covering 40 trials and 61,589 people, found a small effect that was not statistically significant.

Amount studied: The recommended daily intake from food, sun and supplements

How much you need

Adults aged 19 to 70 need 15 mcg (600 IU) of vitamin D a day, the same amount during pregnancy and breastfeeding.^1,2

Adults aged 71 and older need 20 mcg (800 IU) a day.^1,2

For vitamin D, 1 mcg equals 40 IU. So 10 mcg is 400 IU, 15 mcg is 600 IU, 50 mcg is 2,000 IU and 100 mcg is 4,000 IU.¹

These amounts count every source, and they were set on the assumption that you get little sun. Most Americans take in only 4 to 5 mcg a day from food and drink, so sunlight, fortified foods and supplements make up the gap. The dosage guide covers each age group and the upper limits.¹

Where vitamin D comes from

Fortified foods provide most of the vitamin D in American diets. Almost all US milk is fortified with about 3 mcg (120 IU) a cup, and many plant milks, breakfast cereals and some brands of orange juice, yogurt and margarine have vitamin D added. Cheese and ice cream are usually not fortified.^1,2

Fatty fish such as trout, salmon, tuna and mackerel, and fish liver oils, are among the best natural sources. Three ounces of cooked sockeye salmon has 14.2 mcg (570 IU), and a tablespoon of cod liver oil 34 mcg (1,360 IU).¹

Supplements come in two forms, D2 and D3. Both raise blood levels, and the forms guide compares them. Because vitamin D dissolves in fat, it is best taken with a meal or snack that has some fat in it.^1,2

Who tends to run low

Groups more likely to run low include breastfed babies, older adults, people who rarely expose their skin to the sun, people with darker skin and people with a higher body weight.^1,2

In US survey data from 2011 to 2014, most people had enough vitamin D in their blood. 18% had levels in the inadequate range (12 to under 20 ng/mL) and 5% had very low levels (below 12 ng/mL).¹

The Endocrine Society's 2024 guideline suggests against taking more than the recommended intake for healthy adults aged 19 to 74, and against routine blood testing. It suggests supplements for children and teens, adults 75 and older and during pregnancy, without recommending a specific dose.^1,9

In short, a supplement makes the most sense if you get little sun on your skin, have darker skin, are older, eat little fatty fish or fortified food, or have been told your level is low. If none of that applies, food and sun may already cover you.^2,9

What's in ours

Our Vitamin D3 softgel gives 50 mcg (2,000 IU) of D3, the daily amount tested in the VITAL and DO-HEALTH trials. Several other products in our range contain smaller amounts, shown here as printed on their labels.^3,7

What’s in ours

Vitamin D3: Vitamin D-3, 50 mcg (2,000 IU) per serving, from the label
Men's Multi: Vitamin D, 10 mcg per serving, from the label
Ashwagandha Complex: Vitamin D3, 20 mcg per serving, from the label
Immune Support: Vitamin D3, 40 mcg per serving, from the label

Side effects, medicines, and who should ask first

The upper limit for adults and children aged 9 and over is 100 mcg (4,000 IU) a day from all sources, including during pregnancy and breastfeeding. It is 25 mcg (1,000 IU) for babies up to 6 months, 38 mcg (1,500 IU) from 7 to 12 months, 63 mcg (2,500 IU) at ages 1 to 3 and 75 mcg (3,000 IU) at ages 4 to 8. Signs of toxicity are unlikely below 250 mcg (10,000 IU) a day, but NIH notes that even intakes below the upper limit might cause harm over time.^1,2

Too much vitamin D raises blood calcium. Signs include nausea, vomiting, muscle weakness, confusion, pain, loss of appetite, dehydration, passing a lot of urine, great thirst and kidney stones. They are linked to blood levels far above normal, usually over 150 ng/mL (375 nmol/L).^1,2

Ask your pharmacist or doctor first

Thiazide diuretics, water pills such as hydrochlorothiazide, chlorthalidone and indapamide, reduce the calcium lost in urine. Together with vitamin D supplements, which raise calcium absorption, they might push blood calcium too high, especially in older adults and people with reduced kidney function or an overactive parathyroid gland. This applies to thiazides, not to every kind of water pill.^1,2

Expected from how the two work, not shown in studies

Ask your pharmacist or doctor first

High intakes of vitamin D, especially from supplements, might make atorvastatin, lovastatin and simvastatin less potent, because they appear to compete for the same enzyme that breaks them down. Statins may also reduce the body's own production of vitamin D.^1,2

Reported in studies

Your doctor may want to check on you

Corticosteroid tablets such as prednisone can reduce calcium absorption and interfere with how the body uses vitamin D. In a US survey, very low vitamin D levels were twice as common among people taking oral steroids (11%) as among those who were not (5%).^1,2

Reported in studies

Ask your pharmacist how far apart to take them

Orlistat, a weight-loss medicine, reduces the absorption of fat-soluble vitamins, including vitamin D. Its label advises a daily multivitamin with vitamins A, D, E and K taken at a separate time from orlistat, and in its trials low vitamin D levels were more common on orlistat than on placebo.^2,10,11

On the medicine's label

Ask your pharmacist or doctor first

Bile acid sequestrants such as cholestyramine can block the absorption of fat-soluble vitamins, including vitamin D. The cholestyramine label lists low vitamin D among the effects of long-term use, and MedlinePlus lists vitamin D supplements among the products that interact with it.^12,13

On the medicine's label

Vitamin D adds up across products. Our Vitamin D3 softgel (50 mcg) and our immune formula (40 mcg) together give 90 mcg before food, close to the 100 mcg adult limit, and adding Ashwagandha Complex (20 mcg) takes you past it. Take one or the other, not both.²

Ask your doctor first if you are pregnant or nursing, or before giving vitamin D to a child
Ask first if you have high blood calcium, kidney disease, kidney stones or an overactive parathyroid gland
Ask first if you have a digestive condition that limits fat absorption or have had weight-loss surgery²
Count everything else that contains vitamin D: multivitamins, calcium with vitamin D, cod liver oil and prescription vitamin D

Questions people ask

What does vitamin D do for your body?

It lets your gut absorb calcium and phosphate, the minerals bones are made of, and it is needed for normal muscle and immune function. That is why running low matters. Taking more than you need has not shown an extra gain in trials of adults who already had enough.^1,3

Are the benefits of vitamin D different for men?

The recommended amount is the same for men and women: 15 mcg (600 IU) a day up to age 70 and 20 mcg (800 IU) after that. Men average a little more from food, about 5.1 mcg a day against 4.2 mcg for women, but both fall well short of the recommendation from food alone.^1,2

Is vitamin D good for your skin?

Your skin makes vitamin D, but the sources we use do not show that vitamin D supplements change the skin itself. Sunscreen limits how much vitamin D the skin makes, and health experts still advise an SPF of 15 or more whenever you are out in the sun for more than a few minutes.²

Do babies need vitamin D?

Breastfed babies do, because breast milk alone does not give them enough: the advice is a 10 mcg (400 IU) supplement each day from shortly after birth. Babies who are not breastfed need one too until they drink about a liter of fortified formula or milk a day. Check with your pediatrician.^1,2

See what we make

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See the Full Measure line Ashwagandha Complex Immune Support Men's Multi

Sources

*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.