Getting enough vitamin D is essential so kids’ bones grow strong and their immune systems ward off illness.

Vitamin D gets into the body through absorption of sunlight and ingestion of food. From April through the end of October, spending just 15 to 30 minutes outside in the middle of the day with hands and face exposed will stimulate the skin to make all the vitamin D your child needs. In fact, on a sunny summer day, a child wearing only a bathing suit can generate 10,000 to 20,000 international units (IU) of vitamin D after 15 to 30 minutes. In a neat biological trick, a person’s body can’t “overdose” on vitamin D created by the sun.

Foods such as salmon, sardines, tuna, cod-liver oil, egg yolks and shiitake mushrooms contain a lot of vitamin D. Many kids don’t seem to love these vitamin D super foods, so luckily store-bought milk is often fortified with vitamin D, as are many cereals and even orange juice. Not all dairy products are fortified with vitamin D, so read the labels to be sure.

When vitamin D supplements are helpful

During the fall and winter, when the sun’s rays aren’t at an angle that will produce vitamin D in the skin, it’s good to give your child a vitamin. Most over-the-counter children’s vitamins contain 600 IU of vitamin D, which is the recommended daily allowance set by the U.S. Food and Drug Administration for kids 1 year old and older.

Children who are obese, who have dark skin, who rarely go outside, or who wear clothing that covers most of their skin may need supplements to ensure they have adequate levels of vitamin D all year round. Some medications interfere with vitamin D, so make sure your child’s pediatrician knows of all medicines they take.

There are risks if kids take in too much vitamin D from supplements and food, including an increased chance of developing kidney stones. Parents need to calculate the amount their child gets from fortified milk, other food, and vitamin supplement to make sure the total amount does not exceed:

1,000 to 1,500 IU a day for infants

2,500 to 3,000 IU a day for children 1 to 8 years old

4,000 IU a day for children 9 years and older

The sunscreen paradox

Parents know that it’s important to protect their child’s skin from dangerous sunburns, skin damage, and future skin cancers by using sunscreen. But sunscreen can reduce vitamin D production by 95% (SPF 8) to 99% (SPF 15).

What’s a parent to do?

“In reality, this is not much of a problem from the perspective of vitamin D,” says Michael A. Levine, MD, Director of the Center for Bone Health at Children’s Hospital of Philadelphia (CHOP). “We usually recommend that parents allow children to have 10 to 15 minutes of sun time before applying sunscreen. This allows them to have sufficient exposure to sunlight to meet their vitamin D requirements. Even after application of sunscreens, kids continue to make some vitamin D, as most kids — and adults, too — actually don’t apply adequate amounts of sunscreen before going out into the sun. And, many forget to reapply sunscreen as directed after several hours or after going into water. So, sunscreen use really does not interfere with getting enough vitamin D.”

Babies need a vitamin D boost

Because a typical mother’s breast milk does not give her baby enough vitamin D, breastfed babies need vitamin D supplementation. Either the baby can be given drops of 400 IU a day directly or the mother can take 5,000 IU a day (half the safety threshold), which will fortify her milk with enough vitamin D for the baby.

Although all standard infant formulas are fortified with vitamin D, vitamin D supplementation is also recommended for formula-fed babies. “A baby would need to drink a quart of formula per day to get the recommended amount of vitamin D,” Dr. Levine says, “and young infants may not take in that much.” Consult with your baby’s pediatrician or healthcare provider to make sure they are getting all the vitamin D they need.

Vitamin D testing

Most kids never need vitamin D testing. Blood testing measures the major circulating form of vitamin D called 25-hydroxy vitamin D, often abbreviated to 25(OH)D. Although some labs report levels below 30 ng/mL (nanograms per milliliter) as low, for children, most primary care providers find that levels greater than 20 ng/mL of 25(OH)D are adequate.

Long-term benefits of adequate vitamin D

Vitamin D is important for more than just building and maintaining strong, healthy bones and teeth. The vitamin supports nervous system health, strengthens defenses against infections, and may also improve lung and heart health.

There have been hints vitamin D may actually help prevent some diseases. But, for the most part, the studies that identify associations between disease risk and low levels of vitamin D have not been confirmed by well-designed studies. For instance, the idea that vitamin D supplements may prevent diabetes was recently dealt a blow in a rigorous study of adults. In June, a National Institutes of Health-funded study of 2,423 adults reported that daily vitamin D supplements failed to prevent type 2 diabetes.

Even though the study didn’t include children, it’s doubtful that high-risk kids — those with a sibling with type 1 diabetes or a parent with type 2 — would have their risk of diabetes reduced by taking vitamin D supplements, said Dr Levine.

The biggest bonus of vitamin D continues to be strengthening bones and preventing rickets. As long as your child is getting enough vitamin D, if research eventually shows it does prevent other diseases, your child will be covered.