You need vitamin D for many reasons. Vitamin D is essential for the body’s health and functions related to muscles, bones, and nerves. But many are deficient in vitamin D, especially in the cold winter. What can we do to increase the vitamin D level in our body?

Sunlight exposure
Baking in the sun is the most effective way to synthesize vitamin D. Basking in the sun is the best way to get vitamin D through synthetic pathways. The human skin can naturally synthesize vitamin D. When ultraviolet rays reach the dermis, the “7-dihydrocholesterol” in skin cells is converted into vitamin D3. Previtamin D3 and vitamin D3 also absorb UV radiation and are transformed into various photoproducts, some of which have unique biological properties (1).
But, the sunlight in winter is less than in other seasons. We need to increase the time for sunlight exposure to compensate vitamin D synthesis. I highly recommend taking a 20-30 minute walk at least twice a week and going outside to enjoy the sun during lunchtime, when the sun is strong.
However, just sitting by a window on a clear day is not as effective as walking outside. It is known sunlight exposure through the window is of little use in vitamin D synthesis. Ultraviolet rays reaching the surface are 2 types: UV-A with a long wavelength of 315 to 400 nm and UV-B with a wavelength of 280 to 320 nm. Among them, UV-B is related to vitamin D synthesis, but most of them are blocked by glass windows due to its short wavelength. For the same reason, vitamin D may not be synthesized even when sunscreen is applied and exposed to the sun. Because of the weak transmittance of UVB, let’s go outside on a sunny day.

Eating vitamin D
Perhaps it is the most simplest way to increase vitamin D level. During the winter season, the sunlight is not enough to fulfill vitamin D, so supplements through food are somewhat necessary. There are plenty such as milk, cheese, eggs, and mushrooms. And fatty fish such as salmon and mackerel are also rich in vitamin D. One glass of milk contains about 120 IU of vitamin D, one medium-sized egg is about 40 IU, and 3 ounces of salmon have more than 500 IU of vitamin D.

Vitamin D’s uses
Vitamin D is an essential nutrient for maintaining a healthy body function such as muscles, bones, and nerves. It helps calcium absorption, increasing bone density and strengthening bones. Nerves are needed to send signals between the brain and the body and help muscle movement. It stimulates insulin synthesis and secretion by controlling calcium and phosphorus levels and directly affects pancreatic beta cells. It also has the effect of boosting immunity. Therefore, a lack of vitamin D can lead to osteoporosis and sarcopenia, which reduces iron absorption and increases the risk of anemia. However, be careful to take too many of vitamin D because excessive doses of more than 10000 IU per day may cause toxicity.

No need to worry about vitamin D deficiency if you often go outside and are exposed to the sun every day and eat fish or vitamin D-fortified foods. However, people with low outside activity in winter, who only live in a room and have no sunlight exposure, or over the age of 70 need to take vitamin D through adequate supplements. They are likely to lack vitamin D in the body because their body can’t make enough for poor synthetic ability. Aging actually affects the formation of 1,25-dihydroxy vitamin D(calcitriol), the active form of vitamin D (2). In general, it is recommended 800 IU daily in the form of vitamin D3.

References
1) Matthias Wacker &Michael F. Holick. Sunlight and Vitamin D; A global perspective for health. Dermato-Endocrinology, Volume 5, 2013 – Issue 1, Pages 51-108.
2) J. Christopher Gallagher, MD, MRCP. Vitamin D and Aging. Endocrinol Metab Clin North Am. 2013 Jun; 42(2): 319–332.
3) 최지우 기자, 일조량 줄어드는 겨울, 비타민D 효과적으로 보충하려면?, 헬스조선 2023/12/29 06:00.

How to Maximize Vitamin D?
Vitamin D is an essential nutrient for maintaining a healthy body function such as muscles, bones, and nerves. And dose many biological functions.
Basking in the sun is the most effective way to synthesize vitamin D.
But, the sunlight in winter is less strong. Highly recommend taking 30 minutes walk at least twice a week. For the weak transmittance of UVB, sitting beside the windows is not helpful.
Sometimes the sunlight is not enough to fulfill vitamin D, some intake from food are somewhat necessary. There are plenty such as milk, cheese, eggs, and mushrooms. And fatty fish such as salmon and mackerel are also rich in vitamin D.
No need to worry about vitamin D deficiency if you often go outside and are exposed to the sun daily and eat fish or vitamin D-fortified foods.