This blog post analyzes why Koreans are vulnerable to vitamin D deficiency and introduces effective lifestyle habits and supplementation methods to prevent it.
Vitamins are essential nutrients that regulate metabolism and physiological functions in very small amounts. While similar to hormones in that they regulate bodily functions in small quantities, unlike hormones synthesized by the body’s endocrine glands, vitamins must be obtained externally. For this reason, we frequently hear about the importance of vitamins throughout our lives. Vitamin D is the only vitamin synthesized within the human body, but its synthesis does not mean it can be ignored. Koreans have a higher prevalence of vitamin D deficiency compared to other countries due to their indoor-centric lifestyle and busy daily routines. Research on vitamin D deficiency among Koreans analyzed the correlation between demographic and residential factors, health-related characteristics, and 25(OH)D sufficiency levels. The results revealed that only 34.2% of men and 22.4% of women had normal levels, with the majority showing deficiency or insufficiency, indicating that many people are not getting enough vitamin D. This blog post explores the forms of vitamin D, its synthesis and metabolism, its impact on calcium homeostasis, why our bodies need vitamin D, and the reasons for vitamin D deficiency among Koreans.
Vitamin D is a fat-soluble vitamin, and there are two forms that significantly impact our bodies. One is vitamin D2 (ergocalciferol), found in yeast and plants. The second is vitamin D3 (cholecalciferol), produced within the human body when 7-dehydrocholesterol in skin cells is activated by UVB rays from sunlight. Vitamin D3 plays a crucial role in the human body.
How is vitamin D3 produced in our bodies? Sunlight’s ultraviolet rays are divided into long-wavelength UVA, mid-wavelength UVB, and short-wavelength UVC. The wavelength needed to produce vitamin D is UVB, which falls in the mid-range (290–315 nm). First, 7-dehydrocholesterol present in skin cells is converted into pre-vitamin D3 (the precursor to vitamin D3) with the help of ultraviolet rays from sunlight. Approximately 50% of this pre-vitamin D3 is then converted into vitamin D3 within two hours due to heat. The synthesized vitamin D3 is then stored in the body’s adipose tissue. This stored vitamin D3 is processed in the liver and converted into a hormone precursor called 25-hydroxyvitamin D3. 25-hydroxyvitamin D3 (25-OH D3) travels through the blood to the kidneys, where it is converted into the final active form, calcitriol (1,25-(OH)2 D3). This is then delivered to the necessary tissues and cells to perform its role as vitamin D.
Vitamin D contributes to all sites where calcium is metabolized, which can both help and harm our bones. Calcitriol converted in the kidneys promotes the formation of calbindin, a calcium-binding protein in intestinal cells, thereby increasing calcium absorption. Increased serum calcium lowers the concentration of parathyroid hormone (PTH), a hormone that breaks down bone, indirectly suppressing the increased bone resorption caused by PTH. Furthermore, when the body experiences hypocalcemia, vitamin D stimulates parathyroid hormone secretion, promoting the conversion of 25-OH D3 to 1,25-(OH)2 D3 in the kidneys.
Subsequently, 1,25-(OH)₂D₃ acts like a hormone: it promotes calcium absorption in the digestive tract, works with parathyroid hormone to release calcium stored in bones into the bloodstream, and increases calcium reabsorption in the kidneys to maintain calcium homeostasis. Because of these actions, taking vitamin D without calcium is considered inadvisable.
So why is vitamin D important for our bodies? As mentioned earlier, vitamin D is an essential element in calcium metabolism, crucial not only for bone health but also because deficiency can lead to numerous diseases. Vitamin D deficiency disorders include diabetes, breast cancer, prostate cancer, lung cancer, colon cancer, osteoporosis, hypertension, skin diseases (psoriasis, eczema, melanoma, etc.), obesity, rickets, muscle pain, tooth decay, asthma, fatigue, autism, vision loss, hearing loss, insomnia, migraines, schizophrenia, depression, memory loss, and dementia. Research also indicates vitamin D has cancer-preventive effects. Epidemiological studies indicate that people living in regions with the least natural sunlight exposure had higher rates of colorectal cancer. A 19-year follow-up study also found that insufficient vitamin D and calcium levels increase the likelihood of developing colorectal cancer. Furthermore, the fact that 1,25-(OH)2 D3 levels decrease with age also points to a link between vitamin D and cancer. Research also indicates that 1,25-(OH)2 D3 inhibits the growth of various cancer cells and reduces the formation, growth, and metastasis of cancer within the body.
Many Koreans appear deficient in vitamin D due to high rates of indoor living and sunscreen use. Examining differences in 25(OH)D concentrations based on characteristics of Korean study participants revealed higher concentrations among residents of rural (eup/myeon) areas compared to urban (dong) areas for both men and women, and higher concentrations among residents of general houses compared to apartment dwellers. According to occupational reclassification and unemployment/non-economic activity status codes, both men and women had the highest 25(OH)D concentrations among skilled agricultural, forestry, and fishery workers. Conversely, the unemployed group—including managers/professionals, office workers, students, and homemakers—had lower 25(OH)D concentrations, suggesting that occupational type and lifestyle significantly influence 25(OH)D levels. The daily recommended intake of vitamin D in Korea is approximately 400 IU. However, experts argue that the optimal intake should be significantly increased. For adults, 1000 IU is commonly recommended, and in cases where deficiency-related problems have occurred, 2000 to 5000 IU may be advised. Nevertheless, like vitamin A, excessive intake can cause side effects. It can cause loss of appetite, vomiting, diarrhea, and may lead to hypercalcemia, so it is essential to consume it in appropriate amounts.
We have now explored the synthesis and metabolic processes of vitamin D, the reasons why it is necessary, and the current state of vitamin D deficiency in Korea. Although vitamin D is synthesized within the body, its role is extremely important. Methods for absorbing vitamin D include not only going outside to get sunlight, but also consuming foods rich in vitamin D such as eggs, fatty fish, and mushrooms, or using vitamin D supplements when necessary. Since the proportion of the population deficient in vitamin D is relatively high in Korea compared to other countries, it is important to understand vitamin D well and make efforts to maintain adequate levels in the body.
