Vitamin D made in the skin lasts twice as long in the blood as vitamin D ingested from the diet... Read more..
Vitamin D is a fat-soluble vitamin that is naturally present in very few foods, is added to some foods, is available as a nutritional supplement, and is made when ultraviolet light, specifically UVB rays from the sun, strike the skin and stimulate its synthesis. Both the vitamin D that is absorbed from food and supplements, and that is made by the skin is biologically inert.
It must be hydroxylated twice in the body – first into 25(OH) vitamin D (calcidiol) by the liver, and again into 1,25(OH)2
vitamin D (calcitriol.) by the kidney. Vitamin D is needed for the absorption of calcium from the small intestine and for the functioning of calcium in the body. Vitamin D also acts like a hormone and has many functions unrelated to its co-functions with calcium absorption and bone growth and remodeling. Besides being in bone, receptors for vitamin D have been identified in the gastrointestinal tract, brain, breast, nerve, and many other tissues.
Vitamin D maintains adequate serum calcium and phosphate concentrations to prevent hypocalcemia tetany. It also modulates cell growth, neuromuscular and immune function, and reduction of inflammation. Many genes that encode for the regulation of cell proliferation, differentiation and apoptosis are modulated by vitamin D
VITAMIN D SYNTHESIZED FROM SUNLIGHT EXPOSURE
Vitamin D made in the skin lasts twice as long in the blood as vitamin D ingested from the diet. The skin not only makes vitamin D upon exposure to UVB rays, but also makes other photoproducts that cannot be obtained from food or sun is unknown if any of these products have unique benefits to health, but research continues in this area. UVB light cannot pass through glass; exposure of the skin to sunlight through glass will not result in vitamin D synthesis.
Another deterrent to vitamin D synthesis by the skin is sunscreen. A sunscreen with an SPF 15 reduces skin synthesis of vitamin D by 95%, and an SPF30 reduces it by 99%. How much sun exposure is the correct amount to maintain optimal vitamin D levels in the body? A person sunbathing in a bathing suit will have received a dose of between 10,000 and 25,000 IU of vitamin D when he or she has sunbathed long enough to be slightly pink 24 hours later (technically called a minimal erythema dose or “1 MED.”) Exposing 25% of the body (arms and legs) for ¼ to ½ the time it takes to get slightly pink will allow the body to make 2,000 to 4,000 IU of vitamin D with each exposure.
The amount of time necessary for sunlight exposure to produce adequate vitamin D depends on the person’s skin type (pale skin requires less time than dark skin with a lot of the burn protecting pigment melanin), season of the year (the lower the sun on the horizon in the winter, the greater the time needed), the latitude (within 1 or – 35 degrees from the equator, the most vitamin D can be produced when skin is exposed to UVB rays), and the time of day (more vitamin D is synthesized by the skin when the sun is directly overhead between 11:00 am and 3:00 pm. A person should be exposed to sunlight 2 to 3 times per week from March through October in northern climates to accumulate enough vitamin D to get through the winter with adequate vitamin D.
VITAMIN D IN FOODS
Vitamin D is measured in international units (IUs). 1 mcg 5 40 IU of vitamin D or calciferol. IUs are used on food and
supplement labels and both are used in the 2011 RDAs for vitamin D. The vitamin D in foods is measured as calciferol.
There are only a few food sources of vitamin calciferol. Fish such as salmon, tuna and mackerel and fish oils are one of the few natural sources of vitamin D. Beef liver, cheese and egg yolks contain small amounts of vitamin D3, a metabolite of vitamin D which appears to be approximately five times more potent than the parent vitamin (calciferol) in raising serum 25(OH)D concentrations. At the present time, the USDA’s Nutrient Database does not include this vitamin D metabolite when reporting the vitamin D content of foods. Actual vitamin D intakes in the U.S. population may be underestimated for this reason.
Mushrooms are the only plant food known to contain vitamin D, and the amount varies widely depending on the type
of mushroom and amount of sunlight exposure during growth. Commercially raised mushrooms are now being grown with controlled UVB exposure so that they synthesize and thus contain much more vitamin D than if grown in the wild. In fact, grown with lots of UVB exposure, 4-5 button or cremini mushroom’s may contain as much as 400 IU of vitamin D. Good sources of vitamin D are fortified foods and beverages such as milk, fortified soy, rice and nut beverages, some yogurts and margarine fortified breakfast cereals, fortified orange juice and other juices, and fortified products. These fortified foods supply most of the calcium in the American diet. Cow’s milk in the US is voluntarily fortified to 100 IU/cup and in Canada is fortified by law to 35-40 IU/100 ml (84 – 96 IU/cup). Yogurt, cheese, cottage cheese, quark and other milk products, unless made with vitamin D fortified milk (which is not required), or are fortified with vitamin D during production, are not good sources of vitamin D.
An Article by a Nutrition Student ‘Syeda Ruhina Raushan’