Vitamin D differs from other vitamins in that the body is able to synthesize it in the skin when exposed to sunlight from a compound that the body makes from cholesterol . Average exposure to the sun is sufficient for most people to make all their needs of vitamin D, and therefore it is known as the sunshine vitamin.
Vitamin D can be synthesized from two types of primary compounds, the first is (7-dehydrocholesterol), which is found in animal lipids and from which vitamin D is synthesized in the human body, and the second is (Ergosterol) which is present in plants, (2) It is used in the manufacture of vitamin D in food supplements.
as ultraviolet rays convert these two compounds into cholecalciferol, known as vitamin D3, and ergosterol, known as vitamin D2, respectively,  and both require another two steps in the liver and kidneys to convert them. To the active form of vitamin D,  which is (1,25-dihydroxy vitamin D3) or (1,25-dihydroxyvitamin D2), and with these formulas, vitamin D performs its functions in the body. 
Scientific research has found that exposure to sunlight for 10 to 15 minutes on sunny days two to three times a week is sufficient to meet the needs of vitamin D in most people,  and people with darker skin need more time than sun exposure to meet their needs. 1]
Vitamin D dietary sources
In general, food sources are not considered sufficient to obtain the daily needs of the body from vitamin D, which is found in animal foods, especially fish liver oil, in addition to its presence in butter, cream, egg yolks and liver in varying quantities,  and can also be obtained from food. It is fortified with it, such as breakfast cereals, juices, and milk.  In infants, vitamin D supplements should be given in breastfeeding cases with a prescription, and formula milk is usually fortified with it. 
In recent years, the phenomenon of the use of vitamin D pills has become widespread, and in this article we detail the reasons for the widespread use of and their most important uses.
Vitamin D deficiency rates
Despite the medical developments of our time, more than a billion people in the world suffer from vitamin D deficiency or insufficiency,  and among the reasons for the high deficiency of this vitamin is the frequent use of sunscreens, which doctors often advise to avoid damage to the skin. From exposure to the sun, such as skin cancer and its aging,  in addition to the fact that modern lifestyle at home and at work and modern building designs prevent many from adequate exposure to the sun. 
Despite the abundance of sunshine in our Arab region almost all year round, it suffers from high rates of deficiency, and even records among the highest rates of deficiency in the world, as in addition to the previous reasons, some traditional or religious practices (such as clothing It may play a role as well, as some studies found the rate of deficiency in Saudi Arabia between 50-80%,  and the percentage of people with levels less than 25nmol / L in Lebanon, Jordan, and Iran is 60-65%. And the prolonged breastfeeding that is common in the region without vitamin D supplementation, causes a deficiency in children. 
In view of the high rates of deficiency and its association with many health problems, which include softening and fragility of the bones in adults, and rickets in children,  in addition to its role in obesity, high blood pressure, depression, and fibromyalgia, And chronic fatigue syndrome, Alzheimer’s, and some types of cancer such as breast, prostate, and colon cancer  The use of vitamin D pills to treat this deficiency and prevent its complications, in addition to some other uses, has increased.
Cases of using vitamin D pills
Vitamin D deficiency
In cases of vitamin D deficiency, and when the blood level is less than 30nmol / L, high doses of it are given by prescription, and high doses of vitamin D should not be taken without the supervision of a doctor. 
Cases where the risk of developing a vitamin D deficiency increases
Vitamin D supplements are given in the following cases where the risk of deficiency increases according to age, as follows: 
- Infants (0-1 years): 400-1000 IU per day, and the supplement is not in the form of pills in children.
- From 1-18 years old: 600-1000 global units per day.
- Adults over 18 years of age: 1500-2000 global units per day.
- Pregnant and breastfeeding women under the age of 18: 600-1000 global units per day.
- Pregnant women over 18 years: 1500-2000 global units per day.
Pregnant women over 18 years: 1500-2000 global units per day.
It has been found that supplementation is effective in the following cases: 
- Low phosphate in the blood.
- Calcium deficiency due to parathyroid hormone deficiency.
- Renal osteodystrophy, which affects patients with kidney failure.
- Rickets, but in children, another form of vitamin D is used, not pills.
- Preventing bone loss in people taking corticosteroids.
- Reduced risk of falls in the elderly.
- tablets along with calcium help reduce the risk of falls due to lowering blood pressure and body bobbing.
- Reducing bone loss and fracture risk in cases of osteoporosis.
- Taking high doses of calcium, not vitamin D alone, lowers the risk of some types of cancer.
- Vitamin D intake reduces the risk of tooth decay in children and adults.
- Lowering the level of parathyroid hormone and bone loss in cases of elevated parathyroid hormone levels.
Some studies indicate the following roles, but these roles need more scientific proof: 
- Reducing the chance of an asthma attack in asthma patients.
- Reducing the risk of colon and rectal cancer, according to some studies, while others did not find a relationship between them.
- deficiency levels are high in some diseases, such as heart disease and chronic obstructive pulmonary disease, and therefore it is given in these cases.
- Reducing the risk of type 1 diabetes in children who are given daily vitamin D during the first year of life.
- Vitamin D deficiency is associated with an increased risk of developing type 2 diabetes, but there is no scientific evidence that taking vitamin D cures or prevents type 2 diabetes.
- Reducing the risk of developing high blood pressure and lowering the level of blood pressure in the infected and non-infected according to what some studies have found, while some other studies did not find a relationship between them, and vitamin D deficiency may be associated with a higher risk of developing high blood pressure, and in contrast, studies did not find Taking high doses of vitamin D reduces the risk of developing high blood pressure.
- The incidence of high cholesterol is much higher in people with vitamin D deficiency, and some limited studies suggest that taking vitamin D with calcium and following a low-calorie diet raises the level of good cholesterol (HDL) and reduces the level of bad cholesterol (LDL) in overweight women, While consuming vitamin D and calcium without dieting does not lower bad cholesterol, in contrast, some studies have found that vitamin D may raise bad cholesterol without affecting good cholesterol or total cholesterol and triglycerides.
- Some studies have found that taking vitamin D during pregnancy reduces the risk of having a low birth weight baby.
- deficiency is associated with a higher risk of death from any cause, and some studies have found that taking daily reduces the risk of death, while other studies have found that taking with only calcium is effective in reducing mortality rates.
- Reducing the risk of gum disease in people over 50.
- and calcium intake can reduce PMS symptoms.
- Help treat Proximal myopathy.
- Reducing muscle pain associated with taking statins used to lower blood cholesterol.
- Improve symptoms of seasonal depression.
- Some roles in bronchitis, respiratory disorders, and others.
Vitamin D toxicity
Taking vitamin D in the recommended doses is safe, and it does not cause side effects in most people, which only occur when very high doses of it are consumed. Symptoms include weakness, fatigue, drowsiness, headache, loss of appetite, dry mouth, vomiting, nausea, and taste. It is considered unsafe to take vitamin D in doses higher than 4000 IU daily for long periods.  Therefore.
care must be taken when taking vitamin D from dietary supplements, as it is a fat-soluble vitamin and has toxicity if taken in doses. It is high, and it is the most of the vitamins with the potential to cause toxicity when consumed in high quantities. Even though eating it in food and manufacturing it in the body with the help of sunlight does not cause toxicity, the doses taken in nutritional supplements may cause it, and therefore vitamin D supplements should not be taken without medical supervision 
High amounts of vitamin D raise the levels of calcium and phosphorous in the blood,  and this causes calcium to be deposited in soft tissues, such as the kidneys, heart, lungs, and tympanic membranes in the ear and this can cause deafness,  kidney stones, and calcification in the artery walls. It can cause hardening, and this is dangerous when it occurs in the main arteries in the heart and lungs and may cause death.  Also, poisoning in infants causes intestinal distress, developmental delay, and weakness in the bones. 
interacts with many drugs, and care and caution must be taken when taking it with drugs that interact with it,  therefore, you must consult a doctor and make sure that it does not interact with the drugs that a person takes.