Vitamin D source: The Sun

Vitamin D is fat soluble vitamin and it is naturally made by our body when the skin is exposed to the sun. Sunlight is the best and only natural source of vitamin D. So, it is commonly called the sunshine vitamin. Chemically it is called calciferol and it is a fat soluble vitamin.

Vitamin D exists in several forms. The two major forms are vitamin D₂ or ergocalciferol, and vitamin D₃ or cholecalciferol; these are known collectively as calciferol. Vitamin D is essential for the  maintenance of healthy bones and teeth. It also increases absorption of calcium, iron, magnesium, phosphate, and zinc from intestine. Its deficiency results in impaired bone mineralization and bone damage which leads to bone-softening diseases, such as rickets and osteomalacia.

Sunlight is composed of about 1,500 wavelengths. Only a very small portion of the total radiation from the sun reaches the earth's surface. Much is filtered out by the atmosphere. Among all these UVB is the only wavelength that will produce vitamin D when hits unexposed skin. UVA's have a longer wavelength than UVB and can more easily penetrate the ozone layer and other obstacles (like clouds and pollution) on their way from the sun to the earth. UVB rays, will only penetrate the atmosphere when the sun is above an angle of about 50° from the horizon. When the sun is lower than 50°, the ozone layer reflects the UVB-rays but let through the longer UVA-rays.

UVA increases the risk of skin cancer and causes photoaging of the skin. UVA destroys vitamin D3 and also increases oxidative stress. Therefore, it is important to determine the ideal times of year for safe and effective sun exposure to avoid exposure to UVA.

Most people can make enough vitamin D from being out in the sun daily for short periods with their forearms, hands or lower legs uncovered and without sunscreen.

Humans make 90 percent of their vitamin D naturally from sunlight exposure.  Ultraviolet B of sunlight naturally initiates the conversion of cholesterol in the skin to vitamin D3. Vitamin D synthesis in sunlight depends upon various factors, such as skin colour, duration of skin, exposure to sunlight, season and intensity of the UVB in the sun.
Normally, in outdoor, sun exposure to UVA and UVB at the same time is not a problem. But when a person is in indoors and exposes himself to sunlight filtered through window glass, he or she may be increasing risk of a variety of conditions, primarily skin cancer, because the UVA's are effectively destroying vitamin D3 levels.

Earth’s atmosphere and window glass will effectively filter out the majority of UVB radiation, but only minimally filters out UVAs. So, when exposed to sunlight through windows -- in office, home or car -- persons get the UVA but not the beneficial UVB. This is one of the reasons why people who drive long hours in their cars develop skin cancer on the arm.

If a person is sitting indoors by a sunny window, can't make vitamin D because ultraviolet B (UVB) rays (the ones our body needs to make vitamin D) can't get through the glass.

Sunlight is strongest at the equator, where the sunlight comes from directly overhead rather than at an angle. The UV radiation is about four times as strong at the equator as it is at the Arctic and Antarctic circles.

UV radiation is also more intense at higher altitudes, because there is less atmosphere to absorb it.

If clouds are more, the less UV radiation will reach the earth's surface. However, UV can penetrate cloud cover to some extent, so it is still possible to get sunburned on a cloudy day. Light clouds, which can block infrared radiation but not UV radiation, leaving the day cool.

Ground cover such as sand, snow, and water reflect UV radiation, increasing its intensity even in shaded areas.

The optimal time to be in the sun for vitamin D production is between roughly 10:00 am and 2:00 pm. During this time, the shortest exposure time is needed to produce vitamin D because UVB rays are most intense at this time.  When the sun goes down toward the horizon, the UVB is filtered out much more than the dangerous UVA.

Dark-skinned people living in temperate climates have lower vitamin D levels. Dark-skinned people may be less efficient at making vitamin D because melanin in the skin obstructs vitamin D synthesis.

Melanin protects skin against damage from too much exposure to UVB. So, darker skins with more melanin allow less UVB to enter the skin. With less UVB getting through the skin, less vitamin D is produced each minute. That is why dark skinned persons need more sun exposure to make vitamin D than fair skinned persons. People with dark skin will need to spend longer period in the sun to produce the same amount of vitamin D as someone with lighter skin.

But one should be careful not to stay longer periods till skin starts to turn red or burn in the sun.

The longer stay in the sun, for prolonged periods without sun protection, may cause greater risk of skin cancer.

During winter UV levels are lower and most people cover up their body and spend more time indoors. So, vitamin D levels also will be lower. In summer, UV levels are high and people are more likely to be active outdoors, so vitamin D levels will also increase.