Many breast-fed infants may not be getting enough vitamin D, a new study suggests.

Researchers surveyed 184 women who were breast-feeding, and found that most did not give vitamin D supplements daily to their infants. Because breast milk has low levels of vitamin D, the American Academy of Pediatrics recommends that breast-fed infants, and those who are fed formula in addition to breast milk, receive 400 international units (IU) of vitamin D per day, beginning in the first few days of life.

Inadequate levels of vitamin D in children can lead to the bone disease rickets. And while people can also get vitamin D from sunlight, the AAP advises that infants younger than six months avoid exposure to direct sunlight due to skin cancer risk. The supplement is a liquid, and can be given with a dropper straight into the baby's mouth.

The survey included 140 mothers who exclusively breast-fed their infants and 44 mothers who gave their infants both breast milk and formula. All of the participants received care at the Mayo Clinic in Rochester, Minnesota, where the study was conducted.

A little more than half of the mothers (55 percent) said they had given their infants vitamin D supplements in the past week, and just 42 percent said they had given their infants the recommended 400 IU of vitamin D per day.

When asked about why they didn't give vitamin D supplements to their infants, some mothers said they didn't know about the recommendation, and others thought that breast milk had everything their babies needed.

One woman said she "never even knew vitamin D supplementation was needed," according to the paper. Another said, "I feel like my breast milk was designed by God to give my baby what she needs. Babies have been fine and healthy without vitamin D supplementation for generations." [9 Good Sources of Disease-Fighter Vitamin D]

Some mothers who fed their infants formula said they thought the formula had all the vitamin D that their baby would need. However, a baby needs to be drinking at least 32 ounces (about 1 liter) of vitamin-D-fortified formula daily in order to receive an adequate amount of vitamin D, according to the Mayo Clinic.

Many of the mothers (76 percent) surveyed said they were taking a multivitamin that contained vitamin D. Although mothers can take vitamin D as a way to increase the amount of this vitamin in their breast milk, studies have found that they need to take around 4,000 to 6,400 IU per day in order to sufficiently enrich their breast milk, the researchers said. That's higher than the amount of vitamin D typically found in multivitamins.

Still, the majority of breast-feeding mothers in the study (88 percent) said they would prefer to take supplements themselves, rather than give them to their infants.

The researchers concluded that having mothers take additional vitamin D supplements could improve infants' vitamin D levels.

"Mothers take a prenatal vitamin after delivery, so additional vitamin D could be incorporated in the maternal supplementation routine," they said.

The study is published in the January/February 2017 issue of the journal Annals of Family Medicine.

Original article on Live Science.