Vitamin D

What Is Vitamin D?

Vitamin D is a fat-soluble vitamin that is naturally present in very few foods, added to others, and available as a dietary supplement.

It is also produced by your body when ultraviolet rays from sunlight strike the skin and trigger vitamin D synthesis. Vitamin D obtained from sun exposure, food, and supplements is biologically inert and must undergo two chemical processes in the body for activation.

The first occurs in the liver and converts vitamin D to 25-hydroxyvitamin D [25(OH)D], also known as calcidiol. The second occurs primarily in the kidney and forms the physiologically active 1,25- dihydroxyvitamin D [1,25(OH)2D], also known as calcitriol.

Vitamin D is essential for promoting calcium absorption in the gut and maintaining adequate serum calcium and phosphate concentrations to enable normal mineralization of bone and prevent hypocalcemic tetany.

It is also needed for bone growth and bone remodeling by osteoblasts and osteoclasts. Without sufficient vitamin D, bones can become thin, brittle, or misshapen.

Vitamin D sufficiency prevents rickets in children and osteomalacia in adults. Together with calcium, vitamin D also helps protect older adults from osteoporosis. Vitamin D has other roles in human health, including modulation of neuromuscular and immune function and reduction of inflammation.

Many genes encoding proteins that regulate cell proliferation, differentiation, and apoptosis are modulated in part by vitamin D.

Sources of Vitamin D In Food

Very few foods in nature contain vitamin D. The flesh of fish (such as salmon, tuna, and mackerel) and fish liver oils are among the best sources. Small amounts of vitamin D are found in beef liver, cheese, and egg yolks.

Vitamin D in these foods is primarily in the form of vitamin D3 (cholecalciferol) and its metabolite 25(OH)D3. Some mushrooms provide vitamin D2 (ergocalciferol) in variable amounts.

Fortified foods provide most of the vitamin D in the American diet. For example, almost all of the U.S. milk supply is fortified with 100 IU/cup of vitamin D (25% of the Daily Value or 50% of the AI level for ages 14-50 years). In the 1930s, a milk fortification program was implemented in the United States to combat rickets, then a major public health problem. This program virtually eliminated the disorder at that time.

Other dairy products made from milk, such as cheese and ice cream, are generally not fortified. Ready-to-eat breakfast cereals often contain added vitamin D, as do some brands of orange juice, yogurt, and margarine. In the United States, foods allowed to be fortified with vitamin D include cereal flours and related products, milk and products made from milk, and calcium-fortified fruit juices and drinks. Maximum levels of added vitamin D are specified by law.

Sun Exposure And Vitmain D

Ultraviolet (UV) B radiation with a wavelength of 290-315 nanometers penetrates uncovered skin and converts cutaneous 7-dehydrocholesterol to previtamin D3, which in turn becomes vitamin D3. In other words, sunlight is a great way to get your Vitamin D.

Unfortunately, there are many factors that regulate how much vitamin D you will be able to produce naturally. Season, geographic latitude, time of day, cloud cover, smog, skin melanin content, and sunscreen are among the factors that affect UV radiation exposure and vitamin D synthesis.

The UV energy above 42 degrees north latitude (a line approximately between the northern border of California and Boston) is insufficient for cutaneous vitamin D synthesis from November through February; in far northern latitudes, this reduced intensity lasts for up to 6 months.

Latitudes below 34 degrees north (a line between Los Angeles and Columbia, South Carolina) allow for cutaneous production of vitamin D throughout the year. However, during the summer months American�s are staying out of the heat and not getting enough sun.

Complete cloud cover reduces UV energy by 50%; shade (including that produced by severe pollution) reduces it by 60%. UVB radiation does not penetrate glass, so exposure to sunshine indoors through a window does not produce vitamin D.

Sunscreens with a sun protection factor of 8 or more appear to block vitamin D-producing UV rays, although in practice people generally do not apply sufficient amounts, cover all sun-exposed skin, or reapply sunscreen regularly. Skin likely synthesizes some vitamin D even when it is protected by sunscreen as typically applied.

The factors that affect UV radiation exposure and research to date on the amount of sun exposure needed to maintain adequate vitamin D levels make it difficult to provide general guidelines.

It has been suggested by some vitamin D researchers, for example, that approximately 5-30 minutes of sun exposure between 10 AM and 3 PM at least twice a week to the face, arms, legs, or back without sunscreen usually lead to sufficient vitamin D synthesis and that the moderate use of commercial tanning beds that emit 2-6% UVB radiation is also effective. Individuals with limited sun exposure need to include good sources of vitamin D in their diet or take a supplement.

Two Forms Of Vitamin D In Dietary supplements

In supplements and fortified foods, vitamin D is available in two forms, D2 (ergocalciferol) and D3 (cholecalciferol). Vitamin D2 is manufactured by the UV irradiation of ergosterol in yeast, and vitamin D3 is manufactured by the irradiation of 7-dehydrocholesterol from lanolin and the chemical conversion of cholesterol.

The two forms have traditionally been regarded as equivalent based on their ability to cure rickets, but evidence has been offered that they are metabolized differently.

Vitamin D3 could be more than three times as effective as vitamin D2 in raising serum 25(OH)D concentrations and maintaining those levels for a longer time, and its metabolites have superior affinity for vitamin D-binding proteins in plasma. Many supplements are being reformulated to contain vitamin D3 instead of vitamin D2.