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Vitamin D

Vitamin D is one of the fat-soluble vitamins and is also a sterol that is considered a hormone because it can be synthesised endogenously.

Due to the small number of foods containing vitamin D, a small amount of this vitamin is taken with food. Fatty fish species such as salmon, mackerel, tuna, sardines, egg yolk, milk, broccoli, green onion, parsley are rich in vitamin D. However, no foodstuff contains enough vitamin D to meet the daily vitamin D requirement. The main source of vitamin D synthesis is sunlight. Direct sunlight contact with the skin is necessary for synthesis. This synthesis function is adversely affected by factors such as the latitude of the country, seasons, the angle at which sunlight reaches the earth, skin pigmentation, air pollution level, protective creams applied to the skin, and type of clothing.

Vitamin D made in the skin or taken by diet is not biologically active. It is first converted into 25 hydroxyvitamin D [25(OH)D] in the liver and then into the biologically active form 1,25 dihydroxyvitamin D [1,25(OH)2D] in the kidneys.

1,25 dihydroxyvitamin D controls nearly 200 genes by forming a complex with vitamin D receptors located in 37 different tissue cells such as pituitary, ovaries, skin, stomach, pancreas, thymus, breast, kidney, parathyroid glands and peripheral leukocytes.

Its most important effect is on calcium and phosphorus metabolism and bone mineralisation. However, in recent years, vitamin D deficiency and insufficiency have been found to be associated with many chronic diseases, including common cancers, cardiovascular diseases, metabolic syndrome, infectious and autoimmune diseases.

To assess vitamin D level in a person, 25 hydroxyvitamin D [25(OH)D] level, which indicates both vitamin D intake and endogenous production, should be analysed.  Vitamin D deficiency is considered vitamin D deficiency if the 25(OH)D level is lower than 20 ng/mL, vitamin D insufficiency if it is between 21 and 29 ng/mL, adequate level if it is higher than 30 ng/mL (preferred range 40-60 ng/mL) and vitamin D intoxication if it is higher than 150 ng/mL.

Adequate vitamin D intake and maintenance of optimum vitamin D levels in serum are very important not only for bone, calcium and phosphorus metabolism but also for general health.

Vitamin D deficiency is now recognised as a global epidemic. Studies conducted in our country and in the world show that vitamin D deficiency is widespread. Especially those who cannot benefit from sunlight sufficiently, dark-skinned people, the elderly, obese people, anticonvulsant and glucocorticoid drug users are in the high risk group for vitamin D deficiency and insufficiency.

Vitamin D has been reported to have beneficial effects on the regulation of the immune system, autoimmune diseases, some chronic diseases and the prevention of cancer development.

Vitamin D deficiency is a common problem in patients with generalised musculoskeletal pain. In a study of 14925 patients over 20 years of age with diffuse musculoskeletal pain, the prevalence of vitamin D deficiency was found to be 73.9%.

In a meta-analysis of 25 randomised controlled trials involving 10933 volunteers, vitamin D supplementation was shown to be effective in preventing acute respiratory infections. This effect was observed with daily or weekly use, but not with infrequent or high doses of supplementation.

According to the meta-analysis results of 11 prospective studies included in the EPIC-Norfolk study in which 3600 people were followed up, it was stated that there was a decreasing relationship between serum 25(OH) D level and the frequency of Type 2 DM and that the risk of Type 2 DM decreased with vitamin D administration.

In the meta-analysis of 11 studies with 16434 participants, it was stated that low serum (OH) D level was associated with a high risk of carotid atherosclerosis.

In a meta-analysis of 7 randomised controlled trials, vitamin D supplementation was reported to reduce the frequency of asthma exacerbations.

2000 IU vitamin D supplementation daily for 6 months significantly reduced the number of acute exacerbations in COPD patients.

Women with serum (OH) vitamin D levels above 40 ng/ml have a significantly lower cancer risk than those with levels below 20 ng/ml.

Vitamin D levels <30 ng/ml require treatment. The goal of treatment is to maintain serum 25(0H)vitamin D levels of 30-50 ng/ml. In adults, at least 1,500-2,000 IU vitamin D intake is required to maintain serum 25(0H)D levels above 30 ng/ml.



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