Vitamin D is unlike any other nutrient. For starters, your body can make its own when your skin is exposed to sunlight (though not in the winter for most of the U.S. and not if you’re wearing sunscreen). What’s more, many people get too little vitamin D, because it occurs naturally in few foods except fatty fish.

Getting enough vitamin D is essential for preventing osteoporosis (or brittle bones), but in recent years, scientists have tested whether it can prevent or treat other diseases.

What trials of vitamin D for asthma have found

Asthma attacks cause symptoms like wheezing, coughing, chest tightness, and shortness of breath. Severe cases can be fatal. Could vitamin D help, as some studies suggest?

Scientists reviewed nine trials testing vitamin D against a placebo on nearly 1,100 children and adults. Most had mild or moderate asthma and continued to take their usual asthma medication. The studies lasted four to 12 months.

Vitamin D supplements reduced the risk of severe asthma attacks requiring a hospital admission or visit from 6 percent to 3 percent. However, they had no impact on lung function or day-to-day symptoms.

“This is an exciting result, but some caution is warranted,” said lead author Adrian Martineau, of Queen Mary University of London.

First, more trials are needed to find out if vitamin D can help people with severe asthma. “Second, it is not yet clear whether vitamin D supplements can reduce risk of severe asthma attacks in all patients, or whether this effect is just seen in those who have low vitamin D levels to start with,” noted Martineau.

Does vitamin D lower the risk of colorectal cancer?

Scientists leading 17 studies worldwide pooled their data on roughly 5,700 participants with—and 7,100 without—colorectal cancer. Compared to people with vitamin D blood levels between 20 and 24.9 ng/mL, those with levels between 30 and 34.9 ng/mL had a 19 percent lower risk of colorectal cancer over an average of six years, and those with levels between 35 and 39.9 had a 27 percent lower risk. The recommended range for healthy bones is 20 to 30 ng/mL.

This kind of study can’t prove that higher blood vitamin D levels lower the risk of colorectal cancer (because something else about people with higher levels may explain their lower risk). Stay tuned. Other studies are underway.

Heard that vitamin D can curb Covid-19 symptoms?

Researchers studied 237 hospitalized Covid patients. Those who were randomly assigned to get one huge dose of vitamin D (200,000 IU) did not leave the hospital any sooner than those who got a placebo. Nor were the vitamin D takers less likely to die or need intensive care or a ventilator.

Roughly 30 more trials on vitamin D and Covid are underway. Until you hear otherwise, don’t expect vitamin D to fight Covid.

Vitamin D doesn’t help older people with symptoms of depression

Dutch scientists randomly assigned 151 people aged 60 to 80 to take either vitamin D (1,200 IU) or a placebo each day. While all had “depressive symptoms,” anyone with a diagnosis of major depressive disorder was excluded.

The participants also had at least one physical limitation (like difficulty walking, climbing stairs, rising from a chair, or dressing) and low blood levels of vitamin D (6 to 20 nanograms per milliliter in the winter or 6 to 28 ng/mL in the summer).

After one year, the vitamin D takers fared no better with their symptoms or limitations than the placebo takers.

Can vitamin D supplements lower your risk of type 2 diabetes?

The Vitamin D and Type 2 Diabetes (D2d) trial randomly assigned roughly 2,400 adults with prediabetes to take either a placebo or 4,000 IU of vitamin D a day.

When the study began, 4 percent of the participants had “deficient” blood levels of vitamin D (less than 12 ng/mL), 17 percent had “inadequate” levels (12 to 19 ng/mL), and 78 percent had “adequate” levels (at least 20 ng/mL). (Nationwide percentages are similar.)

After 2½ years, the vitamin D takers had no lower risk of diabetes than the placebo takers. Another recent trial also came up empty.

Vitamin D may help prevent benign paroxysmal positional vertigo

BPPV occurs when tiny crystals in the inner ear get dislodged. Researchers randomly assigned 1,050 people who had been treated for BPPV—with a simple maneuver that moves the crystals back into place—to either an intervention or an observation group.

Of the 500 people in the intervention group, 348 had insufficient vitamin D levels (under 20 ng/mL) when they entered the study. Only they were given vitamin D (400 IU) and calcium carbonate (500 mg) twice a day.

After a year, the full intervention group had 24 percent fewer episodes of vertigo than the observation group. The researchers calculated that four people with insufficient vitamin D would need to take vitamin D and calcium for a year to prevent one case of BPPV.

Can vitamin D protect your vision?

Age-related macular degeneration (AMD) is a leading cause of vision loss in U.S. adults.

Researchers randomly assigned roughly 25,000 adults to take vitamin D (2,000 IU), fish oil (1,000 milligrams), both, or a placebo every day for five years. The supplements had no impact on whether AMD occurred or got worse.

What to do

Stay tuned until more studies are done. In the meantime, take a multivitamin or a supplement with the RDA for vitamin D (600 IU a day up to age 70 and 800 IU over 70).

Don’t assume that more is better. In one study, people who got 4,000 IU a day for 3 years lost more bone than those who got 400 IU a day.