Vitamin D benefits: what the research does and doesn't show

The large trials of vitamin D supplements do not show a change in cancer, heart attacks and strokes, fractures or depression in generally healthy adults. Positive results are narrower: fewer respiratory infections in people who started very low, and a borderline autoimmune result in one trial. Each finding follows, with who and what dose.

What the evidence doesn't support

Search results for "vitamin D3 benefits" promise a long list. These are the common ones the large trials do not back up:

• A change in cancer or heart-event rates in healthy adults. Not found at 2,000 IU a day in the largest trial.
• Fewer fractures, fewer falls or better bone density. Not found in the VITAL fracture analysis or in a pooled analysis of 81 trials.
• Protection against depression or better mood scores in older adults without depression. Not found.
• Benefits from doses far above the upper limit. A three-year trial found lower, not higher, bone density at 4,000 and 10,000 IU a day than at 400 IU.
• Energy, skin, hair, kidney or heart claims for the vitamin by itself. We found no trial we could cite for energy, skin, hair, kidney function or erectile function, so no statement is made about them. The heart finding is the null result above.

These are results about supplements in the people studied, mostly adults without a diagnosed deficiency. They are not findings about people with a diagnosed deficiency, who should be managed by a clinician.

What the trials found

Cancer and cardiovascular events: no effect

VITAL randomised 25,871 US men aged 50 and over and women aged 55 and over (20% Black), not selected for low vitamin D, to 2,000 IU of vitamin D3 a day or placebo. After a median 5.3 years, invasive cancer occurred at a rate no different from placebo (hazard ratio 0.96, 95% CI 0.88 to 1.06) and so did major cardiovascular events (0.97, 0.85 to 1.12). No excess of high blood calcium or other adverse events was identified. Certainty: high for this population and dose; it cannot speak to people who started deficient.

Fractures: no effect

The VITAL fracture analysis, in the same 25,871 adults on 2,000 IU of D3 for a median 5.3 years, found no significant effect on total fractures (hazard ratio 0.98, 95% CI 0.89 to 1.08), nonvertebral fractures (0.97) or hip fractures (1.01). It did not vary by baseline blood level, age, sex or body weight. A 2018 meta-analysis of 81 randomised trials (53,537 adults, vitamin D2 and D3 pooled, many doses) also found no effect on total fractures (relative risk 1.00, 0.93 to 1.07), hip fractures or falls, and no clinically relevant change in bone density at any site. Results were similar in trials above 800 IU a day. Certainty: high.

Depression and mood: no effect

In 18,353 VITAL participants (mean age 67.5, without clinically relevant depressive symptoms at the start), 2,000 IU a day of D3 for a median 5.3 years made no significant difference to depression or clinically relevant depressive symptoms (hazard ratio 0.97, 95% CI 0.87 to 1.09). The average change in mood score was 0.01 points. The authors concluded the findings do not support using vitamin D3 to prevent depression. Certainty: high in this population. The trial did not test people already being treated for depression.

Higher doses and bone density: lower at the high end

In a three-year trial of 311 healthy adults aged 55 to 70 with starting blood levels in the normal range, daily vitamin D3 at 400, 4,000 or 10,000 IU was compared. Radial bone density fell by 1.2% on 400 IU, 2.4% on 4,000 IU and 3.5% on 10,000 IU. The authors said the findings do not support a benefit of high-dose vitamin D for bone health. Certainty: moderate; one centre, and whether the difference is harmful was not established.

Respiratory infections: a benefit in people who started very low

A 2017 individual-participant meta-analysis of 25 randomised trials (11,321 people aged 0 to 95, a range of doses, D2 or D3) found vitamin D lowered the odds of an acute respiratory infection (adjusted odds ratio 0.88, 95% CI 0.81 to 0.96). The effect was concentrated in daily or weekly dosing rather than large single doses (0.81 versus 0.97), and in people whose starting blood level was below 25 nmol/L (0.30), against 0.75 for those above it. Certainty: rated high by the authors, but the large effect is in a small group who were very low at the start, and doses varied by trial.

Autoimmune disease: borderline in one trial

In the VITAL autoimmune analysis (the same 25,871 adults, 2,000 IU of D3 a day, median 5.3 years), confirmed autoimmune disease occurred in 123 people on vitamin D and 155 on placebo (hazard ratio 0.78, 95% CI 0.61 to 0.99, P=0.05). The interval nearly touches 1, it is one trial, and it needs replication before anyone treats it as established. Certainty: low to moderate.

Vitamin D3 benefits in plain terms

Vitamin D is needed by the body: it helps absorb calcium and phosphorus and is needed for normal bone (NIH Office of Dietary Supplements). That is a statement about the nutrient, and it is separate from whether taking more of it changes outcomes. For adults who are not low, the trials above found no change in the outcomes they measured. The one consistent signal is in people who start very low, which is a blood test and a clinician's call, not a guess.

Questions people ask

What is vitamin D3 good for?

In the body: absorbing calcium and phosphorus and normal bone. As a supplement: the trials above found no effect on cancer, cardiovascular events, fractures or depression in generally healthy adults at 2,000 IU a day, and a respiratory-infection benefit mostly in people starting very low.

What does vitamin D3 do for your body, for men, or for the body as a whole?

The nutrient's role is the same for everyone: calcium and phosphorus absorption and normal bone, with muscle, nerve and immune cells also using it (NIH ODS). We found no trial we could cite that changes this for men specifically.

Is vitamin D3 good for your heart, bones, kidneys, skin or energy?

Heart: no effect on major cardiovascular events at 2,000 IU. Bones: no effect on fractures or falls, and bone density barely moved in 81 pooled trials. Kidneys, skin and energy: we found no trial to cite, so we say nothing. For kidneys the point to know is the safety one: see who should not take vitamin D.

Is vitamin D3 good for the immune system?

Vitamin D is needed by immune cells as part of normal function (NIH ODS). The trial results are the respiratory-infection and autoimmune findings above: a real signal mostly in people who started very low, and one borderline trial.

Are there benefits of vitamin D3 with K2?

We found no trial we can cite that tested D3 plus K2 against D3 alone, so no claim is made for the combination. Vitamin K2 is a separate vitamin. The pairing question is covered in magnesium glycinate with vitamin D3 and K2.

Do different doses (1,000, 2,000, 5,000, 10,000 IU) have different benefits?

The only dose-comparison trial above found lower radial bone density at 4,000 and 10,000 IU than at 400 IU. The VITAL results are all at 2,000 IU. The pooled trials report similar results above 800 IU. 5,000 IU (125 mcg) is already over the adult upper limit of 100 mcg: see how much vitamin D should you take a day.

Do capsules or liquid drops have different benefits?

We found no trial we can cite comparing the health benefits of capsules and drops. The form studies measure blood levels, not outcomes.

When to talk to a doctor

Vitamin D is not a treatment for cancer, heart disease, fractures, osteoporosis, depression or any other condition, and nothing here should replace the care for one. See a clinician if you have been told your level is low, you have a bone, kidney or calcium-related condition, or you are considering any dose above the 100 mcg (4,000 IU) adult upper limit. A blood test of 25-hydroxyvitamin D is how status is checked.