Vitamin D is a fat-soluble vitamin essential for human beings, which not only plays the role of regulating calcium and phosphorus metabolism and maintaining bone health, but also participates in the differentiation, proliferation and immune regulation of tissue cells.
Human vitamin D is mainly derived from vitamin D synthesized by the skin through sunlight, and the other source is vitamin D in food (such as mushrooms after sunlight, wild marine fish, etc.), which exist in two forms after being metabolized by the liver, namely 25 (OH) D and 25 (OH) D. Among them, 25(OH)D is the main form of vitamin D in the blood (a table of vitamin D content in common foods is attached at the end of the article).
What conditions might vitamin D deficiency indicate?
- rickets, osteoporosis, cancer, etc
1. Bone-related diseases
Vitamin D deficiency is a bone disease called rickets in children, and in adults it can cause osteoporosis or osteomalacia, depending on the degree of deficiency. Vitamin D deficiency is a common cause of secondary hyperparathyroidism. Increasing PTH levels, especially in older adults with vitamin D deficiency, can lead to osteomalacia, increased bone turnover, decreased bone mass, and a risk of fractures. Low levels of 25-hydroxyvitamin D are also associated with low bone density. Combined with other clinical data, the test results can help determine bone metabolism.
2. Other diseases
In addition to being clearly associated with bone diseases, vitamin D may also be associated with diseases such as cancer, cardiovascular disease, diabetes, chronic kidney disease, and autoimmune diseases.
The association of vitamin D with cancer remains controversial. Laboratory and animal studies suggest that vitamin D may inhibit carcinogenesis and slow tumor progression by promoting cell differentiation and inhibiting metastasis, and that vitamin D may have anti-inflammatory, immunomodulatory, pro-apoptotic and antiangiogenic effects.
➤ Cancer: The association between vitamin D and cancer is still controversial. Laboratory and animal studies suggest that vitamin D may inhibit carcinogenesis and slow tumor progression by promoting cell differentiation and inhibiting metastasis, and that vitamin D may have anti-inflammatory, immunomodulatory, pro-apoptotic and antiangiogenic effects.
➤ Cardiovascular disease: A number of studies have shown that vitamin D is associated with hypertension, and vitamin D supplementation in hypertensive patients can reduce the occurrence of cardiovascular and cerebrovascular complications. Low levels of serum 25(OH)D3 can lead to an increased incidence of metabolic syndrome, hyperlipidemia, insulin resistance, and hypertension, which are all risk factors for cardiovascular disease. A 25 nanomol/L (10 micrograms/L) increase in serum 25(OH)D is associated with a 10% reduction in the risk of cardiovascular disease events.
➤ Diabetes: Vitamin D has a certain regulatory effect on pancreatic β cells, which also contain vitamin D receptors, and pancreatic β cells express 1-α hydroxylase like kidneys. Large-scale randomized clinical trials have shown that vitamin D supplementation at a dose of 1 600~4 000 U/d can control blood sugar or reduce the incidence of diabetes. Vitamin D insufficiency or even deficiency is common in patients with diabetes and kidney disease, and vitamin D supplementation or active vitamin D supplementation can protect kidney function. However, there is controversy about the types and exact dosages of vitamin D supplements.
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➤ Chronic kidney disease: Vitamin D deficiency is prevalent in people with chronic kidney disease. Vitamin D status is inversely associated with the progression of chronic kidney disease, so guidelines for the diagnosis and treatment of chronic kidney disease recommend maintaining vitamin D levels of 30 micrograms/L or higher.
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➤ Autoimmune diseases: The level of vitamin D content in the body is closely related to the development of rheumatoid arthritis, and there is a specific vitamin D receptor expressed on macrophages and articular chondrocytes at the site of rheumatoid arthritis. Active vitamin D binds to intracellular vitamin D receptors, which can not only improve human osteoporosis, but also act as a catalyst to promote the synthesis of inflammatory cytokines such as interleukin-17 and interferon-γ in human T, B lymphocytes and dendritic cells, promote the secretion of anti-inflammatory cytokines, enhance the immunomodulatory effect of rheumatoid arthritis patients, and improve the condition of patients. Vitamin D supplementation can reduce the risk of autoimmune diseases by 22%, and long-term vitamin D supplementation can help prevent autoimmune diseases, especially in people ≥ 50 years of age.
Diagnostic criteria for vitamin D deficiency
——血清25-羟基维生素D＜30nmol/L
There are about 40 kinds of vitamin D metabolites that can be detected in the body, among which 25-hydroxyvitamin D is the most present metabolite in the circulation, which can reflect the nutritional level of vitamin D in the body. Serum 25-hydroxyvitamin D levels have been recognized as the most reasonable indicator of vitamin D status.
The following table shows the reference judgment value of vitamin D nutritional status of the population given by the national standard WS/T 677-2020.
Table 1 Reference values of vitamin D nutritional status in the population
1 Table summary: Who should be tested for vitamin D?
There are many factors that affect vitamin D levels, including age, skin color, season, geographic latitude, altitude, sunshine hours, dress habits, sun protection measures, dietary habits, air pollution, obesity, and drugs that affect vitamin D metabolism.
Although vitamin D deficiency or insufficiency is very common in the global population, it does not advocate universal screening of blood vitamin D levels in the population, and the "Consensus on the Clinical Application of Vitamin D and its Analogues" recommends screening blood vitamin D levels in people who are at risk of vitamin D deficiency and need to maintain a reasonable vitamin D nutritional status.
Table 2 Populations with blood vitamin D levels recommended for screening
Supplementation measures for vitamin D deficiency
- Sunshine, diet, and supplements
Increasing sun exposure and intake of vitamin D-rich foods is a cost-effective way to prevent vitamin D deficiency/insufficiency.
The human body can produce vitamin D after being irradiated by ultraviolet rays, and it is necessary to ensure adequate UV exposure when exposed to the sun regularly. It is also important to note that basking in the sun through glass has no effect, because the glass blocks almost all UVB rays. Wearing sunscreen can also hinder the skin's ability to synthesize vitamin D.
Vitamin D can also be obtained through ingestion of food or vitamin D supplements. Vitamin D is also found in animal liver and lean meats, eggs and fish, cereals and milk, as well as mushrooms and cod liver oil, which can be supplemented by moderate consumption.
In addition, supplementing with vitamin D through oral dietary supplements is also a safe, effective, and fast way.
表3 常见食物中维生素D含量［μg（U）/100 g可食部］
Source: Expert Consensus on Evaluation and Improvement of Vitamin D Nutritional Status
Resources
[1] Fan Lieying, Wang Weiling, Hu Min, Chen Falin.Standardized operating procedures for clinical chemistry testing[M].Shanghai:Shanghai Science and Technology Press.
[2] Shang Hong, Wang Yusan, Shen Ziyu.National Clinical Laboratory Operation Procedures[M].4 Edition.Beijing:People's Medical Publishing House,2015.
[3] Chinese Society of Osteoporosis and Bone Mineral Diseases. Consensus on the clinical application of vitamin D and its analogues[J]. Chin J Osteoporosis and Bone Mineral Diseases, 2018, 11(1):19.
[4] Health Management Branch of Chinese Nutrition Society. Expert consensus on the evaluation and improvement of vitamin D nutritional status [J]. Chinese Journal of Health Management,2023,17(04):245-252.
[5] Screening method for vitamin D deficiency in population, Health Industry Standard of the People's Republic of China.WS/T 677—2020.
[6] Zhou Tingyin, Wang Hualiang. Standardized operating procedures for clinical chemistry testing[M]. Shanghai: Science and Technology Press, 2020.
Source: Laboratory Medicine
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