As winter approaches, concerns about vitamin D deficiency are growing due to reduced outdoor activities and longer indoor hours caused by the cold weather. Vitamin D plays a crucial role in strengthening our bones and teeth, and it is an essential nutrient for the growth of children and adolescents during their growth periods.
In particular, vitamin D is synthesized in the skin when exposed to sunlight, so the risk of deficiency naturally increases in winter when sunlight is scarce.
While some vitamin D can be obtained through food, the amount that can be met through common foods is only about 30% of the total requirement.
Some foods, such as salmon, mackerel, and egg yolks, contain relatively high levels of vitamin D, but it is difficult to get enough intake from these alone. Ultimately, most vitamin D is naturally synthesized in the body when the skin is exposed to ultraviolet B (UV-B) rays, so caution is needed during seasons like winter when sunlight exposure decreases.
Sunlight Through Windows is Useless
Many people believe that they can synthesize enough vitamin D just by getting sunlight indoors through windows, but this is a misconception.
Ultraviolet B, or UV-B, does not pass through glass, so the sunlight that comes in through windows has no effect on vitamin D synthesis. In fact, for our bodies to produce vitamin D, the skin must be directly exposed to sunlight outdoors.
According to statistics from the Health Insurance Review and Assessment Service, the number of children and adolescents aged 0-19 who visited hospitals due to vitamin D deficiency increased by 165% from 4,254 in 2014 to 11,310 in 2024. This significant increase is largely influenced by the rise in indoor activities due to smartphone use and video watching.
Professor Park Ki-young of the Pediatrics Department at Gangneung Asan Hospital emphasized, "Adequate vitamin D intake is very important during childhood after 6 months of age and during the rapid growth period of adolescence," adding, "A deficiency can lead to reduced bone growth or decreased bone density."
He continued, "When it’s cold and you get sunlight indoors through windows, it may seem like you’re getting enough sunlight, but in reality, it has no effect. Vitamin D can only be synthesized when you are directly exposed to UV rays outdoors."
Can Vitamin D Be Supplemented Through Food?
The recommended daily intake of vitamin D varies depending on age, pregnancy, or breastfeeding status, but it is generally recommended to be between 400-800 IU.
In the winter in Seoul, exposing about half of the body, such as the face and limbs, to sunlight for about 90 minutes during well-lit daytime hours can sufficiently meet the vitamin D requirement. During this time, it is advisable not to apply sunscreen, as it can hinder vitamin D synthesis.
However, taking excessive supplements just because you feel you are lacking in vitamin D can actually harm your health. Taking vitamin D in amounts significantly exceeding the recommended intake can lead to abnormally high calcium levels in the body, resulting in various side effects such as vomiting, constipation, high blood pressure, and kidney stones. In fact, vitamin D toxicity usually occurs from excessive supplementation.
Especially for infants under 6 months old, their skin is very delicate, so the World Health Organization (WHO) does not recommend direct sunlight exposure. During this time, infants should supplement vitamin D through breast milk or formula.
Professor Park stated, "While a deficiency in vitamin D can have some impact on height growth, it has not been medically proven that taking more will increase height," and advised, "Since it can be sufficiently supplemented through sunlight or food, indiscriminate intake of supplements should be avoided."
