The period from birth until a child reaches 1,000 days old is a “golden time” when the foundation is laid for immunity that can shape a lifetime. During this time, vitamin D serves as a key regulator that helps immune cells function properly, beyond simply strengthening bones. But modern lifestyles mean most pregnant women and infants suffer vitamin D deficiency, increasing the importance of supplementation.

Kim Seok-jin (김석진), head of the Good Bacteria Research Institute, recently introduced in a video how to manage vitamin D during the first 1,000 days, which influences a child’s immune strength, and provided guidance on safe supplementation.

Kim said the body’s cells have receptors (VDR) that receive vitamin D signals. These receptors exist throughout the body, including in bones as well as the brain, heart, pancreas, liver, muscles and immune cells. What is known so far is that there are as many as 570 to 600 types of gene switches turned on and off by vitamin D, and these switches regulate normal immune cell activity.

Kim said that during pregnancy, a fetus cannot synthesise vitamin D on its own because it is in a dark space in the womb, so its level depends entirely on the mother’s condition. But because of sunscreen use and more indoor 생활, 8 out of 10 pregnant women are deficient. Research findings support that children born to mothers who lacked vitamin D during pregnancy are more likely to be exposed to immune diseases as they grow.

The situation does not change much after birth. Newborns have limited outdoor activity, and a mother’s breast milk also contains very little vitamin D. The U.S. National Institutes of Health (NIH) and paediatric societies worldwide recommend not only vitamin D intake for mothers, but also separate supplementation for babies from immediately after birth, such as through infant vitamin D oil.

On supplementation doses for newborns and infants, Kim said: “Vitamin D is expressed in international units, IU (International Unit), and generally a level of 200 to 400 IU a day is recommended for a newly born baby.” He added: “It is safe if you take it while following the recommended dosage of verified supplements sold on the market.”

Kim stressed that vitamin D, unlike water-soluble vitamins excreted in urine, is a fat-soluble substance stored in the body’s fat tissue. If taken excessively over a long period, it can accumulate and cause side effects, he said. He added that it is desirable to check levels through periodic blood tests and adjust the appropriate amount after consulting a specialist.