For infants, nothing packs the same nutritional punch as breast milk.

This is why public health officials put so much emphasis on encouraging mothers to breastfeed their children.

In spite of breast milk’s reputation as a superfood for babies, it still falls short in one area.

“Breast milk is an incredible nutrient for children and has many positives, but the thing that it doesn’t have very much of is vitamin D,” Dr. Jonathon Maguire, a pediatrician and researcher at St. Michael's Hospital in Toronto, told Healthline.

Maguire is the author of a new study, published today in the American Journal of Public Health. It recommends that children who are still breastfeeding after one year of age may need to continue taking vitamin D supplements to avoid health problems such as rickets.

Read More: Surprising Benefits of Vitamin D »

Current Guidelines on Vitamin D

Vitamin D promotes the formation of strong bones and teeth by helping the body absorb and use calcium and phosphorus.

Severe vitamin D deficiency can lead to rickets, a weakening and softening of the bones.

Vitamin D is found in foods like fatty fish — salmon and mackerel — and egg yolks. It’s also added to many foods such as milk, formula, and soy milk.

The body can produce its own vitamin D when the skin is exposed to sunlight. People living closer to the poles, though, may not receive enough sun exposure to make vitamin D in sufficient amounts.

Diet and low sun exposure are both behind the lack of vitamin D in breast milk.

“Moms’ vitamin D levels just aren’t that high,” Maguire said, “and the vitamin D isn’t passed from the breast milk to the children.”

Which is why professional organizations such as the American Academy of Pediatrics and the Canadian Paediatric Society recommend that breastfed children are supplemented with 400 international units (IU) of vitamin D every day for the first year of life.

Now, with the success of public health campaigns to promote breastfeeding, more children are breastfeed past their first birthday.

As a result, researchers are taking a closer look at what’s going on with the vitamin D levels of these children.

“The issue is that after a year of age, it was unclear what happens to children’s vitamin D levels,” Maguire said. “They’re eating other foods, so they’re getting vitamin D presumably from other foods, but they’re also breastfeeding.”

Read More: Effects of Vitamin D Deficiency »

Vitamin D Levels Can Drop

In the new study, researchers measured the vitamin D levels in the blood of more than 2,500 children between the ages of 1 and 5.

The children were participating in TARGet Kids!, a collaboration between St. Michael's Hospital and The Hospital for Sick Children in Toronto.

“For each additional month of breastfeeding beyond a year of age, the vitamin D level goes down and it keeps going down,” Maguire said. “But for children who are continuing to receive vitamin D supplementation, the vitamin D level in their blood does not go down.”

By age 2, children who were still breastfeeding had a 16 percent increased risk of being deficient in vitamin D. By age 3, that increased to 29 percent.

More studies are needed to confirm these results and see if they apply to other groups of children, especially those living in sunnier areas.

This research will be needed before organizations like the American Academy of Pediatrics change their current guidelines.

Even so, some welcome this study.

“We are seeing a lot of moms who are choosing to breastfeed their babies past a year. So I think that is really important research,” Tamara Melton, registered dietician and Academy of Nutrition and Dietetics spokesperson, told Healthline.

Read More: Extra Facts on Vitamin D »

Simple Vitamin D Supplements

Other studies have looked at whether giving nursing mothers larger doses of vitamin D can increase the amount of vitamin D in their breast milk. This may not be the easiest approach.

“We’ve known for a long period of time that giving a very inexpensive vitamin D supplement to children who are breastfeeding just plain works,” Maguire said.

Most supermarkets and pharmacies carry liquid vitamin D drops suitable for children.

“I used them when I was breastfeeding my daughters and I just put a little drop on their tongue,” Melton said. “A couple of drops and they were good to go.”

The most challenging part for many mothers is remembering to give their child the drops each day.

“I usually suggest doing it with the first feeding in the morning,” Melton said. “Put it next to your coffee, or something like that, so you see it and grab it.”