Only a few foods contain vitamin D. The main natural source of the vitamin is the synthesis of cholecalciferol in our skin, which comes from cholesterol through a chemical reaction that is dependent on pure sun exposure. Vitamin D is made up of a group of fat-soluble secosteroids, which are steroids with a “broken” ring. They are responsible for increasing abdominal absorption of calcium, magnesium, and phosphate, and many other organic effects. Within humans, the most important compounds in this group are vitamin D3, which are also known as cholecalciferol, and vitamin D2, also known as ergocalciferol. Cholecalciferol and ergocalciferol can be consumed through a diet or from different supplements. Dietary suggestions usually assume that all of ones vitamin D is taken by into the mouth.
Vitamin D that is from the diet or skin synthesis is biologically unused and hydroxylation in the liver and kidney is required for activation. As vitamin D can be synthesized in adequate amounts by most mammals exposed to sufficient sunlight, it is not an essential dietary factor, and so not really a vitamin. Instead it could be considered a hormone, with activation of the vitamin D prohormone resulting in the active form, calcitriol, which then produces effects via a nuclear receptor in multiple different locations. Cholecalciferol is converted in the liver to calcifediol 25-hydroxycholecalciferol and ergocalciferol is converted to 25-hydroxyergocalciferol.
These two vitamin D products, which are called 25-hydroxyvitamin D or 25(OH)D) are measured in an antibody to determine a person’s vitamin D condition. Calcifediol is further hydroxylated by the kidneys to and from calcitriol, which are also known as 1,25-dihydroxycholecalciferol and the biologically effective design of vitamin D. Calcitriol acts as a hormone within the blood, playing a large role in regulating the concentration of calcium and phosphate, and developing the healthy growth and strength of the bones. If one does not like the sun, suffer from milk allergies, or have a strict vegan diet, they could be at risk for vitamin D failure.
Known as the sunshine vitamin, vitamin D is produced by the body in response to skin being exposed to sunlight. It also occurs naturally in different foods, which include some fish, fish liver oils, egg yolks, and in fortified dairy and grain products. Vitamin D is very important for strong bones, because it benefits the body by using calcium from the diet. In the past, the lack of vitamin D has been associated with rickets, which is a disease in that bone tissue doesn’t properly mineralize and leads to soft bones and skeletal malformation. However, research is revealing the importance of vitamin D in protecting against many health problems.Vitamin D benefits calcium absorption in the gut and controls adequate serum calcium and phosphate concentrations to enable normal mineralization of bone and to prevent hypocalcemic tetany. It is also required for growth of bones and bone remodeling by osteoblasts and osteoclasts.
Without ample vitamin D, bones can become thin, brittle, or misshapen. Vitamin D prevents rickets in children and osteomalacia in adults. Together with calcium, vitamin D also helps protect older adults from osteoporosis. Vitamin D plays other large parts in the body, including modulation of cell growth, neuromuscular and immune function, and reduction of inflammation. Serum concentration of 25(OH)D is the best example of the vitamin D status. It collects foods and supplements and has a fairly long circulating half-life of 15 days. Serum 25(OH)D levels do not indicate the amount of vitamin D stored in body tissues. Protected foods provide most of the vitamin D in the American diet. For example, almost all of the U.S. milk supply is voluntarily fortified with 100 IU/cup.
In Canada, milk is fortified by law with 35–40 IU/100 mL, as is margarine at 530 IU/100 g.