Vitamin D is a fat-soluble vitamin that is crucial for maintaining bone health, functioning of our immune system, and warding off several chronic conditions. It is popularly known as the sunshine vitamin as the body synthesises it from cholesterol when our skin is exposed to sunlight. Vitamin D is absorbed in the small intestine and stored in the liver, skin, bone, and brain. It acts as both a nutrient as well as a hormone. It helps in maintaining healthy bones by supporting the absorption of calcium and phosphorus.

Vitamin D can be acquired through sun exposure, diet, and supplements and is found in its inactive form. It must undergo two hydroxylations in the body for  activation. Initial hydroxylation happens inside the liver, where vitamin D is converted to 25-hydroxyvitamin D [25(OH)D], also known as calcidiol. The second hydroxylation takes place in the kidneys, where it forms the physiologically active form 1, 25-dihydroxyvitamin D [1,25(OH)2D], also known as calcitriol.

People who are at a higher risk of developing vitamin D deficiency include, the elderly, individuals with high BMI (body mass index), exclusively breastfed infants, and patients with cystic fibrosis or inflammatory bowel disease like Crohn’s disease. In addition, you are likely to develop vitamin D deficiency if you follow a strict vegan diet, as most of the natural sources of this nutrient are derived from animal-based foods. One can also be  vitamin D-deficient if the exposure to sunlight is limited because the body produces vitamin D when our skin is exposed to the sun's UVB rays. Vitamin D deficiency is more likely to be prevalent in regions experiencing winters almost throughout the year, where people get no or very little sunlight exposure. Having  dark skin means more melatonin (a skin pigment), which reduces the skin's ability to synthesise vitamin D in response to the sun’s  UVB rays exposure. Kidney diseases or impaired kidney function with age could be another factor for vitamin D deficiency as our kidneys help convert vitamin D into its active form. 

Calcium is considered to be the building block of bones- Vitamin D helps absorb calcium in the body. A deficiency of vitamin D can lead to bone diseases such as osteoporosis and osteomalacia in adults and rickets in children, poor immunity, and weaker nerves and muscles. Vitamin D deficiency can be difficult to notice as there may not be any visible symptoms for several months or years. Therefore one must get tested done after every 6 months to determine the current vitamin D levels in the body. Symptoms such as too much fatigue or tiredness, pain in bones, muscles, and joints, low energy levels, bones getting brittle or weak, depression, impaired immunity, reduced wound healing capacity, muscle cramps due to weak muscles, and difficulty in losing weight can suggest a deficiency of this important vitamin.  

Vitamin D significantly improves the function of white blood cells such as T cells and macrophages which fight against disease-causing pathogens. Thus, a deficiency of vitamin D in the body can lower the efficacy of immune system which can make us more susceptible to infections and diseases. 

Vitamin D can be obtained through  3 major sources- sunlight, diet, and supplements. Sunlight is a natural source of vitamin D. Our body makes vitamin D on exposure to sunlight. However, one can also get vitamin D through diet and supplements to maintain its adequate levels in the body. Only handful of foods contain vitamin D, including fatty fish, eggs, cheese, mushroom, yogurt and beef liver. Apart from these, one can also include vitamin D-fortified foods such as dairy products, cereals, orange juice, and tofu, among others. In case of severe nutritional deficiency amongst the vegan/vegetarian population, supplementation of vitamin D as prescribed by a healthcare provider can help improve the situation. 

Vitamin D supplements are available in two forms- vitamin D2 (ergocalciferol or pre-vitamin D), and vitamin D3 (cholecalciferol). Both are naturally occurring forms of vitamin D. Vitamin D2 is present in plants and fungi, whereas D3 comes from animals, including humans. The Recommended Dietary Allowance (RDA) given by NIN, ICMR for vitamin D for adults aged 19 years and more is 600 IU (15 mcg) daily, and for adults >70 years it is 800 IU (20 mcg) daily.

As per FSSAI, people with less than <20 ng/ml are called vitamin D deficient, between 20 -39 ng/ml are called vitamin D insufficient, and >40 ng/ml are called vitamin D sufficient. If levels of vitamin D exceed more than 150 ng/dl, people can develop vitamin D toxicity in the body.

Vitamin D gets converted into 25-hydroxyvitamin D, also called 25(OH)D, in the liver. The most commonly performed test to assess vitamin D levels in the body is the 25-hydroxyvitamin D test, as 25 (OH)D is the main form of vitamin D circulating in the bloodstream. 


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