[DigitalToday AI Reporter] An analysis has found that a pregnant mother’s nutritional status can determine not only a fetus’ bone health but also lifelong immune function and the formation of the gut microbiome.

Because a fetus cannot be exposed to sunlight and cannot produce vitamin D on its own, the mother’s blood vitamin D level is reflected directly in the baby. Yet 1 in 4 pregnant and breastfeeding women is deficient in vitamin D, underscoring the need for careful management, it said.

Seokjin Kim (김석진), head of the Good Bacteria Research Institute, explained in a recently released video how a mother’s vitamin D level affects immune system development in fetuses and infants, and offered practical guidance.

Kim said low maternal vitamin D in early pregnancy, or the first trimester, can hinder placental formation and raise inflammation risk, increasing the likelihood of preterm birth. More notably, vitamin D is involved in signalling in hematopoietic stem cells, which create immune cells. If vitamin D is lacking during pregnancy, hematopoietic stem cells do not receive vitamin D signals and the blueprint for producing immune cells itself is altered, which can leave traces in immune cell composition even after the baby becomes an adult. A large-scale study found that babies born to mothers who consumed enough vitamin D during pregnancy tended to have fewer asthma or recurrent wheezing symptoms in childhood.

Vitamin D also plays a role in the first gut bacteria seeding process passed from mother to baby. A cohort study published in 2024 found that babies born to mothers who were vitamin D deficient during pregnancy had significantly lower diversity of beneficial gut bacteria at 6 months of age. Vitamin D strengthens the intestinal mucosal barrier and provides a foundation for beneficial bacteria to take hold.

Special caution is also needed during breastfeeding after childbirth. Breast milk is an almost perfect nutrient source, but it is exceptionally low in vitamin D and does not meet an infant’s recommended daily intake. For this reason, paediatric societies worldwide commonly recommend that breastfed babies receive separate vitamin D supplements from immediately after birth. One option is for the mother to consume enough vitamin D to raise its concentration in breast milk, but whether to supplement directly and the dosage should be discussed with a paediatric specialist.

Kim said, "Health is not inherited through genes but through the environment parents create, and one of the key keys is vitamin D," as he proposed practical guidelines.

He said women should check blood vitamin D levels in advance from the family planning stage and, if deficient, use supplements after consulting an obstetrics and gynaecology specialist. During breastfeeding after birth, mothers should keep their levels steady to support their own recovery and improve breast milk quality, and build a habit of taking walks in the sun for about 15 to 20 minutes a day with the baby. He also stressed that specific supplement doses and methods must be decided through consultation with medical staff, taking into account the characteristics of the pregnant woman and the baby.