Vitamin D barely touched colds in big trials, but its autoimmune results were more intriguing

Key takeaways

• In the VITAL trial of 25,871 older U.S. adults, 123 of 12,927 people taking 2,000 IU (50 mcg) of vitamin D3 a day developed a confirmed autoimmune disease over about five years, compared with 155 of 12,944 on placebo⁹.
• In two years of follow-up after people stopped the pills, vitamin D’s autoimmune protection was no longer statistically clear, while fish oil’s appeared to last¹⁰¹¹.
• In a 2021 analysis, 61.3% of people taking vitamin D had at least one respiratory infection, compared with 62.3% on placebo⁵.
• In a 2017 analysis, treating 4 people who started very low with daily or weekly vitamin D prevented one respiratory infection, compared with 33 people overall, though a 2021 update couldn’t confirm that pattern⁴⁵.
• The Endocrine Society’s 2024 guideline suggests vitamin D for ages 1 to 18, pregnancy, high-risk prediabetes and people over 75, and advises against routine blood tests in healthy people¹².

VITAL was built to answer big questions. It gave thousands of U.S. adults vitamin D3, fish oil, both or placebos⁹¹⁰. Its best-known results, on cancer and heart disease, came out close to even; our article on vitamin pills in trials covers them.

The researchers also tracked a quieter outcome: autoimmune diseases, in which the immune system attacks the body’s own tissue. Examples include psoriasis, rheumatoid arthritis and autoimmune thyroid disease¹⁰¹¹.

In 2022, they reported that 123 of the 12,927 people taking vitamin D had developed a confirmed autoimmune disease, compared with 155 of the 12,944 on placebo⁹. That’s a 22% lower risk⁹. By our math, it works out to about 9.5 versus 12 cases per 1,000 people, or roughly 2 to 3 fewer cases per 1,000 over about five years.

Colds were a different story. Big trials of vitamin D against colds, flu and COVID-19 mostly came up empty, especially for people who weren’t deficient to begin with⁵⁶⁸.

This article covers what vitamin D does in immune cells, what the infection and autoimmune trials found, who benefits, and how much is safe.

Almost every immune cell has a vitamin D receptor

Vitamin D from sun or food does nothing until your body converts it, first in the liver and then mainly in the kidneys, the NIH Office of Dietary Supplements (ODS) explains³.

Nearly all immune cells, including B and T cells, carry receptors that respond to vitamin D³. Some, such as macrophages and dendritic cells, can activate vitamin D on their own, right where they work³.

Research suggests vitamin D slows viruses from copying themselves, calms inflammation and boosts regulatory T cells, the immune system’s brakes³. Our article on the immune system’s two layers explains how those brakes work.

Biology like this makes vitamin D sound like an immune switch. But a receptor on a cell doesn’t guarantee fewer infections in people. For that, you need randomized trials, in which chance decides who gets the vitamin and who gets a placebo.

The infection trials found a small benefit, mostly in people who were low

In 2017, a team funded by Britain’s National Institute for Health Research pooled raw data from 25 trials and 11,321 people, from newborns to age 95⁴. Our article on immune “boosting” gives the overall drop. Two details matter more.

First, the schedule mattered. Daily or weekly doses helped, while large doses given weeks or months apart, called bolus doses, did not⁴.

Second, starting level mattered. Among people below 25 nanomoles per liter (10 ng/mL), a deficient level, treating 4 people with daily or weekly vitamin D prevented one infection⁴. Overall, the number was 33⁴.

By 2021, the same team had found 46 trials³⁵. In 43 trials reporting the main outcome, 61.3% of people taking vitamin D had at least one respiratory infection, compared with 62.3% on placebo⁵. By our math, that’s about 1 fewer sick person per 100.

Daily doses of 400 to 1,000 IU (10 to 25 mcg) and children ages 1 to 15 showed the clearest benefit⁵. This time, no group sorted by starting level showed a clear benefit on its own⁵. That update had no outside funding⁵.

Rating: Promising but early for daily vitamin D in people who start very low, and a small effect at best for everyone else.

Three large trials found no protection

Australia. The D-Health trial gave 21,315 adults ages 60 to 84 either 60,000 IU of vitamin D or a placebo once a month for up to five years⁷⁸. By our math, that averages about 2,000 IU a day, delivered in one big monthly dose.

The odds of a respiratory infection were the same in both groups⁸. People on vitamin D had symptoms for about half a day less, on average⁷. Most weren’t deficient: blood levels in the placebo group averaged 77.5 nmol/L⁸. Australia’s National Health and Medical Research Council paid for the trial⁷.

Britain. CORONAVIT tested a “test and treat” approach during the pandemic. People offered a home blood test who came back below 75 nmol/L got six months of daily vitamin D, with no placebo⁶.

Among those offered 3,200 IU (80 mcg) a day, 5.0% had a confirmed respiratory infection. With 800 IU (20 mcg), it was 5.7%, and with no offer, 4.6%⁶. COVID-19 rates were 3.0%, 3.6% and 2.6%⁶. None of the differences were statistically significant⁶.

CORONAVIT’s funders included charities, government research bodies and supplement makers such as Pharma Nord, which supplied capsules. The paper says funders had no role in the analysis⁶.

Mongolia. Here, 8,851 schoolchildren took 14,000 IU (350 mcg) once a week for three years. Most started below 20 ng/mL, yet they had no fewer respiratory infections or cases of tuberculosis³.

Why the mixed picture? Most adults in the big trials weren’t deficient, and large spaced-out doses tended to do poorly⁴⁸. Even so, the Mongolian result shows that being low doesn’t guarantee a benefit³. Our immune article also describes a federal complaint against a seller accused of claiming that vitamin D and zinc could treat COVID-19.

VITAL’s autoimmune finding, and what happened when people stopped

VITAL enrolled 12,786 men 50 and older and 13,085 women 55 and older, and followed them for a median of 5.3 years⁹. The vitamin D groups took 2,000 IU (50 mcg) of vitamin D3 daily⁹.

Fish oil groups had 130 cases versus 148 on placebo, a gap that wasn’t statistically clear⁹. Vitamin D’s 123 versus 155 just cleared the usual bar⁹. NIH grants paid for the trial, and Pharmavite, a supplement company, prepared the vitamin D capsules and placebos⁹.

The benefit took time. Rheumatologist Karen Costenbader, a VITAL investigator, said the groups only began to pull apart after two to three years¹⁰. When the researchers set aside cases from the first two years, the gap widened to 54 on vitamin D versus 87 on placebo⁹. The effect also looked stronger in people with low or normal body weight¹⁰.

Then the pills stopped. In 2024, the team reported on 21,592 participants followed for two more years. Vitamin D’s protection was no longer statistically significant, while fish oil’s benefit appeared to last at least two years¹¹. Costenbader said vitamin D’s effect faded quickly once people quit¹⁰.

The study has limits. An outside expert noted it wasn’t large enough to judge single diseases¹¹, and Costenbader said autoimmune thyroid disease was hard to confirm¹⁰. VITAL also tested prevention in older adults. It says nothing about treating an autoimmune disease you already have.

Rating: Promising but early. Our megadose article covers high-dose vitamin D protocols for multiple sclerosis, and a later article in this series covers supplement cautions for people with autoimmune conditions.

Who the 2024 guideline says should take vitamin D

In June 2024, the Endocrine Society, a medical group for hormone specialists, published a guideline on vitamin D for preventing disease¹²¹³. Its advice is worded as suggestions, a sign that the evidence leaves room for judgment¹³. A March 2026 summary in American Family Physician lists the doses behind each suggestion¹².

The guideline advises against routine vitamin D blood tests for healthy children and adults. The reason: testing hasn’t been shown to improve health, and research hasn’t settled on a target level¹². Testing can make sense with low blood calcium, gut conditions that block absorption, kidney losses or conditions that break vitamin D down faster¹².

The U.S. Preventive Services Task Force also found too little evidence to judge screening adults without symptoms, and ODS notes that no national medical group recommends screening everyone¹.

For healthy adults ages 19 to 74, the guideline suggests no vitamin D beyond the standard daily amount¹². Our article on how intakes are set explains why that amount is 600 IU in the U.S., and our life-stage map covers babies.

The vitamin D decision card

This card pairs the Endocrine Society’s 2024 suggestions with the intake ranges and average trial doses listed in a 2026 American Family Physician summary¹². Microgram figures are our math (40 IU equals 1 mcg¹). Upper limits come from ODS¹.

Who you are	What the 2024 guideline suggests	Suggested intake (average dose in trials)	When to test	Safety note
Child or teen, 1 to 18	Take vitamin D to prevent rickets; may cut respiratory infections	300 to 2,000 IU (7.5 to 50 mcg); average 1,200 IU (30 mcg)	Not routinely	Upper limit 2,500 IU (63 mcg) at ages 1 to 3, 3,000 IU (75 mcg) at 4 to 8, 4,000 IU (100 mcg) from 9
Pregnant	Take it; may lower preeclampsia, preterm birth and newborn death	600 to 5,000 IU daily or weekly; average 2,500 IU (63 mcg)	Not routinely	Upper limit 4,000 IU (100 mcg) unless a clinician directs more3
Healthy adult, 19 to 74	Nothing beyond the standard 600 IU (15 mcg)	Not applicable	Not routinely	Stay under 4,000 IU (100 mcg) from all sources12
High-risk prediabetes	Take it with lifestyle changes to slow the move to diabetes	842 to 7,543 IU; average 3,500 IU (88 mcg)	Not routinely	The top of this range is above the upper limit; use such doses only under a clinician’s care3
Over 75	Take it; may lower the risk of dying; prefer daily over large spaced doses	400 to 3,333 IU; average 900 IU (23 mcg)	Not routinely	Standard amount after 70 is 800 IU (20 mcg)1
Low calcium, malabsorption or kidney problems	Testing may be appropriate	Set by your clinician	Yes, with your clinician	Treatment doses can exceed the upper limit for a short time2
Taking orlistat, a statin, a steroid or a thiazide	Talk with your prescriber about vitamin D	Not applicable	Ask your prescriber	See interactions below2

Food and sun come first, with limits

Many natural-health readers would rather get vitamin D from sunshine and food than from a capsule. That’s a sound instinct, with two limits worth knowing.

Sunlight helps your skin make vitamin D, and experts don’t think too much sun causes vitamin D toxicity, ODS says¹. Tanning beds are different: frequent use can push blood levels well above 375 to 500 nmol/L¹.

Few foods are rich in vitamin D. Fatty fish and UV-exposed mushrooms lead, and our food-versus-supplements article lists them. Everyday foods add smaller amounts, ODS reports¹:

• Fortified soy, almond or oat milk, 1 cup: 100 to 144 IU (2.5 to 3.6 mcg)
• Fortified breakfast cereal, 1 serving: 80 IU (2 mcg)
• Sardines canned in oil, 2: 46 IU (1.2 mcg)
• Egg, 1 large: 44 IU (1.1 mcg)
• Light tuna canned in water, 3 ounces: 40 IU (1 mcg)

Broccoli, almonds, apples and whole wheat bread have essentially none¹. By our math, reaching 600 IU from eggs alone would take about 14 of them.

How much is too much

These are the daily upper limits, as listed by ODS¹. They count food, drinks and supplements together².

Age	Upper limit
0 to 6 months	1,000 IU (25 mcg)
7 to 12 months	1,500 IU (38 mcg)
1 to 3 years	2,500 IU (63 mcg)
4 to 8 years	3,000 IU (75 mcg)
9 years and older, including pregnancy	4,000 IU (100 mcg)

A clinician may prescribe more for a short time to correct a deficiency². The limits don’t apply to people taking vitamin D under a physician’s care³.

Too much vitamin D raises the calcium in your blood. ODS links toxicity to blood levels above 375 nmol/L (150 ng/mL) and says it almost always comes from supplements, through manufacturing errors, self-dosing or prescribing mistakes¹. Our vitamins and minerals guide shows how excess levels have climbed in national data, and our megadose article describes real cases.

At ordinary doses, trouble is rare. In the 2017 analysis, high blood calcium (0.5%) and kidney stones (0.2%) were uncommon and split evenly between vitamin D and control groups⁴.

Interactions. ODS flags four kinds of medicine². Two can leave you with less vitamin D: prednisone and similar steroids lower blood levels, and the weight-loss drug orlistat cuts how much you absorb. One works the other way, since thiazide water pills combined with vitamin D supplements can drive blood calcium too high. And high-dose vitamin D may blunt some statins. If you take any of these, ask your prescriber how vitamin D fits; don’t change the medicine on your own.

A later article in this series covers asthma, including research on vitamin D and severe attacks.

Questions readers ask

The bottom line

Vitamin D works inside immune cells, but big trials show it’s not a cold shield for people who already have enough. The clearest infection benefit showed up in people who started very low and took it daily or weekly. The autoimmune signal from VITAL is more intriguing, yet modest, and it faded after people stopped.

The 2024 guideline keeps things simple: vitamin D for children and teens, pregnancy, high-risk prediabetes and people over 75, the standard amount for other adults, and no routine blood tests for healthy people. Stay under the upper limit unless a clinician is guiding a higher dose.

Related reading

• Your immune system works more like a team than a volume knob, and some habits help it
• Vitamin pills mostly failed big prevention trials, though a few still earn their place
• Babies Need Vitamin D. Most Healthy Kids Need None. A Supplement Map by Age
• Some Vitamins Tolerate Megadoses. For Others, the Safety Margin Is Thin
• Vitamin C helped ultramarathoners dodge colds, while most people get a smaller payoff

Sources

1. NIH Office of Dietary Supplements. “Vitamin D: Fact Sheet for Health Professionals.” National Institutes of Health, accessed October 2026. ods.od.nih.gov
2. NIH Office of Dietary Supplements. “Vitamin D: Fact Sheet for Consumers.” National Institutes of Health, updated November 8, 2022. ods.od.nih.gov
3. NIH Office of Dietary Supplements. “Dietary Supplements for Immune Function and Infectious Diseases: Fact Sheet for Health Professionals.” National Institutes of Health, accessed October 2026. ods.od.nih.gov
4. Martineau AR, Jolliffe DA, Hooper RL, et al. “Vitamin D Supplementation to Prevent Acute Respiratory Tract Infections: Systematic Review and Meta-Analysis of Individual Participant Data.” BMJ, 2017;356:i6583. pubmed.ncbi.nlm.nih.gov
5. Jolliffe DA, Camargo CA, Sluyter JD, et al. “Vitamin D Supplementation to Prevent Acute Respiratory Infections: A Systematic Review and Meta-Analysis of Aggregate Data From Randomised Controlled Trials.” Lancet Diabetes and Endocrinology, 2021;9(5):276-292 (University of Birmingham research record). research.birmingham.ac.uk
6. Jolliffe DA, Holt H, Greenig M, et al. “Effect of a Test-and-Treat Approach to Vitamin D Supplementation on Risk of All Cause Acute Respiratory Tract Infection and Covid-19: Phase 3 Randomised Controlled Trial (CORONAVIT).” BMJ, 2022;378:e071230. pubmed.ncbi.nlm.nih.gov
7. QIMR Berghofer Medical Research Institute. “Clinical Trial Finds Vitamin D Does Not Ward Off Colds and Flu.” News release, January 14, 2021. qimrb.edu.au
8. Pham H, Waterhouse M, Baxter C, et al. “The Effect of Vitamin D Supplementation on Acute Respiratory Tract Infection in Older Australian Adults: An Analysis of Data From the D-Health Trial.” Lancet Diabetes and Endocrinology, 2021;9(2):69-81 (Bond University research record). research.bond.edu.au
9. Hahn J, Cook NR, Alexander EK, et al. “Vitamin D and Marine Omega 3 Fatty Acid Supplementation and Incident Autoimmune Disease: VITAL Randomized Controlled Trial.” BMJ, 2022;376:e066452. pubmed.ncbi.nlm.nih.gov
10. The Rheumatologist (American College of Rheumatology). Report on VITAL autoimmune findings and a presentation by Karen Costenbader, MD, MPH. the-rheumatologist.org
11. MDedge. “New Findings on Vitamin D, Omega-3 Supplements for Preventing Autoimmune Diseases” (on Costenbader KH, et al., Arthritis and Rheumatology, 2024). February 7, 2024. mdedge.com
12. Dakkak M, He AM. “Vitamin D for Prevention of Disease: Guidelines From the Endocrine Society.” American Family Physician, March 15, 2026;113(3):291-293. aafp.org
13. D2d Study, Tufts Medical Center. “Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline.” June 4, 2024. d2dstudy.org