Harnessing the Sun
Cholecalciferol, also known as vitamin D3, is the most common and absorbable form of vitamin D. Human beings naturally absorb D3 through sunlight exposure, but sedentary, indoor lifestyles prevent millions of people from getting adequate levels of this essential compound. Dr. William Cannel writes, "Humans make thousands of units of vitamin D within minutes of whole-body exposure to sunlight." Nevertheless, deficiency is alarmingly common. To create a form of D3 that can be consumed orally, biologists must harness the biological process that takes place inside skin cells.
A Chemical Reaction
Vitamin D3 is formed as a steroid-like compound within human skin. The skin of mammals contains 7-dehydrocholesterol, a cholesterol-like precursor to cholecalciferol. When the skin comes into contact with ultraviolet B (UVB) rays found in sunlight, a chemical reaction occurs that converts 7-dehydrocholesterol into vitamin D3. The vitamin is then converted by the kidneys and liver into two other forms, calcidiol and calcitriol. The body can also form these compounds-- the "end" products of vitamin D synthesis--from vitamin D2 (ergosterol). However, D2 is generally considered to be less absorbable than D3.
The Role of Lanolin
Fortunately for people who get too little exposure to sunlight, humans are not the only mammals who produce 7-dehydrocholesterol, the chemical precursor to vitamin D3. According to the National Institutes of Health, cholecalciferol "is manufactured by the irradiation of 7-dehydrocholesterol from lanolin and the chemical conversion of cholesterol." Lanolin, the oil found on sheepskin, is a concentrated source of 7-dehdrocholesterol. When it is irradiated using UVB lamps, it creates a chemical process nearly identical to that which takes place in human skin.
Manufacture and Distribution
After lanolin is irradiated, manufacturers extract cholecalciferol from it using mechanical or chemical processes. The producers then apply D3 to fortified foods, including milk, yogurt, cheese and other dairy products. Cholecaliferol is also sold in an isolated form as a dietary supplement. Because it is a fat-soluble vitamin, it is frequently sold in a softgel form that enables faster absorption. Other manufacturers may sell D3 as a tablet or dry capsule. Concentrations range tremendously between products: each capsule may contain a dose of between 50 and 10,000 international units per serving.