What you should know about the popular supplement.

Vitamin D has been touted as a must-have vitamin in recent years as studies have shown that many Americans are deficient.

Getting vitamin D is important since it can help with calcium absorption and has roles in immune function and cell growth, among others. While the vitamin is found in some foods and can be obtained via ultraviolet light, nearly 50 percent of the population worldwide has insufficient levels, according to a 2012 study.

But understanding what’s the right amount of vitamin D for young children, pregnant women, and others, can be difficult.

This month, three studies shed more light on when pregnant women and babies should be getting vitamin supplements, and what’s the ideal amount.

The first study was published in the Journal of the American Medical Association (JAMA), and spearheaded by a University Hospitals Rainbow Babies & Children’s Hospital researcher. The team found infants born between 28 and 36 weeks of gestation who took a daily vitamin D supplement had a lower risk of wheezing.

When African-American infants arrive prematurely, they have an increased risk for recurrent wheezing. It’s also a risk factor for developing asthma later on.

Dr. Anna Maria Hibbs, the lead author and neonatologist in the division of neonatology and perinatal medicine at University Hospitals Rainbow Babies & Children’s Hospital, said that supplementing with 400 international units (IU) of vitamin D per day until six months of age can reduce the chance that the child will experience recurrent wheezing.

“Parents need to know African-American preterm infants are at high risk of wheezing in infancy,” Hibbs said. “I hope this study can highlight the burden of wheezing illness experienced by premature babies and the importance of targeting interventions that can lessen this burden.”

In the experiment, Hibbs studied 300 African-American infants born preterm between 2013 and 2016. The babies received a multivitamin until they were consuming 200 IU per day of vitamin D from formula or fortifiers added to their breastmilk.

Once they were receiving at least 200 IU per day from their diet, they received either 400 IU of vitamin D per day or a placebo until six months of age. Infants who were breastfed exclusively got a multivitamin with 400 IU per day. In total, 277 babies completed the trial.

Of them, 31.1 percent of infants who received vitamin D supplementation experienced recurrent wheezing, while 41.8 percent of infants in the diet-limited supplementation group did the same.

A second study in JAMA Pediatrics found that taking a higher dose of vitamin D does not increase bone strength or decrease the incidence of infections in infants.

In the study, 975 infants in Finland were split into two groups. One group took 400 IU of vitamin D daily, and the other took 1,200 IU of vitamin D daily.

Because there was not a significant change in bone strength or infection incidence, the authors say that taking 400 IU a day is adequate for maintaining vitamin D levels in children under the age of two.

But how much vitamin D pregnant women should take? That was the subject of the third study, which was also published in JAMA Pediatrics.

When the researchers conducted a meta-analysis of 24 clinical trials spanning 5,405 participants, they found that women who took up to 2,000 IU of vitamin D each day had a 55 percent lower chance of having an underweight infant.

These women also had a 65 percent lower chance of the baby dying before or shortly after birth.

In related news, a study published in The Lancet Diabetes & Endocrinology, researchers at the National Institutes of Health found that among women planning to conceive after a pregnancy loss, those who had sufficient levels of vitamin D were more likely to become pregnant and have a live birth, compared to women with insufficient levels of the vitamin.

Pegah Jalali, RDN, a registered dietitian from New York City, said there has been a lot of misinformation about vitamin D throughout the past decade.

“Most women are likely deficient prior to pregnancy, so they will require more than the recommended amounts or the amounts provided in prenatal vitamins,” she told Healthline.

The recommended daily allowance for adult women is 600 IUs daily.

Most women, and physicians for that matter, still think of vitamin D solely as related only to the bones, said Bruce Hollis, PhD, a professor at the Medical University of South Carolina.

His 2012 study touted the benefits of women getting 4,000 IU per day, something the American Pregnancy Association promotes.

“Truth is, vitamin D in relation to pregnancy impacts many biological systems. It will prevent complications of birth such as preeclampsia, preterm birth, and gestational diabetes. It will also prevent asthma development in the newborn in later life,” Hollis told Healthline. “Beyond that, it is also likely to aid in brain development and prevent autism as well as protection from multiple sclerosis later in life.”

In his practice, all women are told to take 4,000 IU per day while pregnant, and 6,000 IU per day during lactation. As a result, breastfeeding infants don’t have to take an additional supplement, he noted.

The American Academy of Pediatrics recommends breastfed and partially breastfed babies receive 400 IU per day of vitamin D until they are no longer taking breastmilk.

Elizabeth Shaw, a registered dietitian from San Diego, explained that even breastmilk can’t provide everything.

“While breastmilk is the gold standard for feeding your baby, it’s lacking in adequate vitamin D to properly nourish your baby who needs 400 IU per day,” Shaw told Healthline.

Experts agree that prenatal vitamins do not give women enough vitamin D — so knowing your levels, and then supplementing if needed, is key.

But knowing your levels also means ensuring people aren’t taking too much vitamin D.

“Current prenatal vitamins, which contain 400 IU vitamin D, come nowhere near meeting the true requirement for vitamin D in either the pregnant or lactating women,” Hollis said. “If all women are taking just a prenatal vitamin, their needs for vitamin D are nowhere close to being met.

Lyssie Lakatos and Tammy Lakatos Shames, registered dietitians from New York, said having a medical expert monitor your blood levels is key to ensure your body has enough vitamin D.

“We believe playing it on the safer side and checking blood levels is the way to go rather than blindly supplementing. You never know how someone is metabolizing vitamin D, whether they are getting plenty from their diet,” they told Healthline.

They pointed out that too much of vitamin D can also be harmful.

“Taking high doses of vitamin supplements in the past has sometimes proven to be problematic. Too much vitamin D can cause damage to the heart, blood vessels, and kidneys,” they added.

Stephanie McKercher, a registered dietitian from Colorado, echoed this caution against women taking too much vitamin D.

“Some women may not realize that vitamin D can actually be toxic in high amounts, so it’s important to get tested before starting a high-dose vitamin D supplement,” she said.