Vitamin D for Kids: Dosing, Deficiency, and When to Test

Vitamin D deficiency in children is more common than most parents realise. NHANES data analysed by the CDC's National Center for Health Statistics indicate that roughly 1 in 5 US children and adolescents are at risk of vitamin D inadequacy (serum 25-OH-D below 50 nmol/L), and the rate is markedly higher in Black and Hispanic children because deeper skin pigmentation slows skin synthesis of vitamin D. Cases of nutritional rickets — once thought eliminated — have reappeared in paediatric clinics worldwide.

Why Children Are Vulnerable

Vitamin D is made in the skin when UVB sunlight reaches it, but most children in northern latitudes spend too little time outdoors and use sunscreen when they do. Unfortified breast milk contains only about 15–50 IU of vitamin D per litre — well below the 400 IU/day the American Academy of Pediatrics (AAP) recommends for infants. Exclusively breastfed infants therefore need a vitamin D supplement from the first days of life unless the mother is taking a high enough dose to enrich her milk.

Guidelines and Dosing by Age

The American Academy of Pediatrics, the Institute of Medicine (IOM/NAM), and the Endocrine Society recommend:

• Infants 0–12 months: 400 IU/day.
• Children 1–18 years: 600 IU/day (RDA); higher intakes (1,000–2,000 IU/day) may be needed in children at risk of deficiency.
• Tolerable Upper Limit (IOM): 1,000 IU/day for infants under 6 months, 1,500 IU/day for 6–12 months, 2,500 IU/day for ages 1–3, 3,000 IU/day for ages 4–8, and 4,000 IU/day for ages 9–18.

D3 (cholecalciferol) is the preferred form because it raises serum 25-OH-D more effectively than D2 (ergocalciferol). D3 from lichen is available for vegan families.

When to Test

Routine testing is not recommended for all healthy children. Endocrine Society guidance suggests measuring 25-OH-D in children with limited sun exposure, deeper skin pigmentation, obesity, malabsorption disorders (coeliac disease, inflammatory bowel disease, cystic fibrosis), exclusive breastfeeding without supplementation, or use of anticonvulsants or glucocorticoids. Most clinicians treat 25-OH-D levels of 50–75 nmol/L (20–30 ng/mL) as adequate for bone health, and aim for at least 50 nmol/L. Doses above 4,000 IU/day in children should be supervised by a clinician because of the risk of hypercalcaemia from over-supplementation, including from over-the-counter "gummy" products.