Vitamin D is an essential fat-soluble vitamin that helps the body to metabolize calcium. Humans synthesize vitamin D by exposing the skin to the sun’s ultraviolet-B radiations, or by obtaining it from the diet. Unlike water-soluble vitamins, excess fat-soluble vitamins are stored in the liver and fatty tissue. Most healthy people do not need vitamin D supplements. Moreover, those who may require supplemental amounts of vitamin D should note that too much of it could be toxic.
Forms of Vitamin D
The term vitamin D refers to one of several steroid molecules. The skin generates vitamin D-3, also called cholecalciferol, when you stay in the sun. Foods such as fish oil, various plants and egg yolk also contain vitamin D. Vitamin D-2, or ergosterol, is the form of vitamin D you get from plants.
How Your Body Makes Vitamin D
You can get more than 80 percent of the vitamin D you need from the sun. The ultraviolet B radiations of the sun convert 7-dehydrocholesterol -- a compound found in your skin’s epidermis -- to previtamin D-3. The body then converts previtamin D-3 to vitamin D-3 in a process that consumes heat. Your body’s production of previtamin D-3 depends on factors like your skin pigment, seasons, time of the day, sunscreen use and clothing.
Vitamin D Storage
According to a 2010 journal article published by "Pediatric Nephrology," vitamin D-2 and vitamin D-3 circulate in the blood for about 24 hours. Afterward, these vitamins are stored in the fat tissue for approximately two months. When the body needs more vitamins, vitamin D-2 and vitamin D-3 are converted to their active form called 25-hydroxyvitamin D. The active vitamin is then released into the blood. The released 25-hydroxyvitamin D can circulate in the body for approximately three weeks. After the body is replenished, the biologically active form is stored in fat tissues for months; 25-hydroxyvitamin D is released from the storage irregularly depending on the body’s need. By the time the physician detects your vitamin D deficiency, your serum concentration of 25-hydroxyvitamin D is less than 20ng/ml. At this point, your vitamin D stores have been depleted.
Too much supplemental vitamin D can cause weight loss, nausea, weakness, vomiting, constipation and poor appetite. An overdose of the vitamin also raises the level of serum calcium and can cause anomalies in heart rhythms and mental confusion. To avoid these side effects, University of Rochester Medical Center recommends that the daily vitamin D supplement limit should be between 1,000 and 1,500 IU for infants; 2,500 and 3,000 IU for children between 1 and 8 years; and 4,000 IU for anyone older than 9.