Vitamin D is important to bone health. Most milk products are supplemented with Vitamin D, and experts urge to get ten minutes of sun exposure daily so that their body will produce this material. While there are many forms of Vitamin D, only two are critical to the proper functioning of the human body.
Vitamin D2, or ergocalciferol, is synthesized by plants. It is the compound used in most over-the-counter and prescription Vitamin D supplements. The problem with Vitamin D2 is that it is not absorbed very well by the human body. Many people incorrectly believe that they are receiving an adequate amount of this nutrient to protect their bone health when they supplement their diet with a pill that is reported to contain the recommended daily allowance (RDA) of Vitamin D. (The parathyroid glands and vitamin D.)
On the other hand, Vitamin D3, or cholecalciferol, is the substance that is produced by the human body when it is exposed to ultraviolet-B (UVB) rays. Studies show that each person should be able to produce enough Vitamin D3 if they expose their unprotected skin to the sun for about ten minutes each day. While this isn't a problem for most people, some are not able to do so because of their local weather, their health condition, or some other reason. Skin color can also affect natural Vitamin D3 levels; darker skin does not produce Vitamin D3 as easily as fairer skin. Because this is the type of Vitamin D that the human body prefers, it is readily absorbed even when taken as a supplement.
Although called a vitamin, Vitamin D does not meet the current formal definition of a vitamin — it can be generated in the body unlike a true vitamin which must be acquired through diet. Vitamin D is sometimes called a hormone or a steroid hormone. If you look up the chemical structure of vitamin D, you can see it is similar to cholesterol Others say Vitamin D is not really a hormone either
Both Vitamin D2 and Vitamin D3 can be used to fortify food products, although the current labeling requirements do not indicate which one has been added. Vitamin D is typically added to milk, soymilk, some orange juice, and most cereals. It is naturally part of fatty fish, egg yolks, and beef liver. Due to the limited types of food that contain Vitamin D, it is essential that humans receive an adequate supply from sunlight or from high-quality supplements that can be readily absorbed. It has been called a "pleiotropic hormone" meaning it influences more than one physiological system. Recent research has suggested vitamin D may help prevent the onset of Parkinson's Disease; the mechanism is not clear, but it is an interesting relationship given the higher risk that Parkinson's patients have for osteoporosis and the connection between low vitamin D levels and low bone density.
As far as its impact on the skeletal system, vitamin D in the body is converted to Calcifediol and then to Calcitriol. (The liver and kidney are instrumental in these conversions.) It is the calcitriol form that is a lynchpin in the calcium blood concentration mechanism. It affects the rate of absorption of calcium in the digestive system, as well as rates of blood serum levels of calcium and phosphorus.
Bone cells have receptors for calcitriol and the level of calcitriol affects mineral desorption.
Note that calcitriol is not the same as calcitonin. Calcitonin is a polypeptide hormone - more like a protein than like a steroid. It is also involved in calcium biochemistry and bone mineralizations.
Calcitonin is a peptide hormone that limits calcium levels inhibiting intestinal uptake and bone release. It is not a significant physiological factor in humans.
Calcitriol (1,25-Dihydroxycholecalciferol) is a form of vitamin D which stimulates intestinal absorption of calcium, release of calclum from bone tissue, and re-uptake by the kidney.
For prevention of osteoporosis, the National Osteoporosis Foundation (NOF) recommends that each person under 50 years of age include 400 to 800 international units (IUs) of Vitamin D in their diet daily, while those over 50 should have 800 to 1,000 IUs. A recent study of nursing home residents found Vitamin D successful in lowering risk of hip fractures. A 2009 review indicated that three-quarters of Americans are deficient in vitamin D, which is someone at odds with other recommendations.
To treat severe bone disease, the current recommendations are either 2,800 or 5,600 IUs daily depending on the extent of bone loss. A doctor should be consulted before consuming such high levels. Excessive levels of Vitamin D can cause kidney problems.
A rule of thumb is that blood Vitamin D levels increase approximately 0.5–1 ng/mL for every 100 IUs of the vitamin taken orally as a supplement. The optimal Vitamin D level in the blood is 30 ng/mL (or 75 nm/mol) or higher. A serum level of 20-30 ng/ml is borderline and a level below 20 ng/ml is deficient.
Although you can buy Vitamin D without a prescription, your doctor should know if you are taking it on a regular basis, especially if you are on other medications. It can interact with other medicines, including diuratics, heart medicine, anti-fungals, and calcium supplements.
In 2010 the Institute of Medicine made headlines by recommending a tripling of the recommended daily allowance of calcium. However, the IOM also said most people get enough Vitamin D and do not need supplements. Indeed, no credible organization seems to recomment supplements for bone health, unless the individual has a low Vitamin D level for some reason. The website TheNNT.com lists the idea of Vitamin D supplements for older people living in the community (outside an institituion) as No benefit. Vitamin D for people living in nursing homes was found to be worth it, with an average of 36 patients getting the supplement to prevent one fracture.
In 2012 the US Preventative Services Task Force issued a draft recommendation against postmenopausal women taking Vitamin D supplements. The group said there was not enough evidence to support this practice. Vitamin D supplements are often paired with calcium, and the USPSTF also said calcium supplements to prevent bone problems were not needed in most cases.
In Feb 2013 the USPSTF finalized their conclusion that "current evidence is insufficient" to recommend vitamin D supplements. Extra calcium and vitamin D beyond the recommended daily allowances derived from a healthy diet; older women do not need to take supplements. In response to the USPSTF announcement the National Osteoporosis Foundation (NOF) issued a statement recommending higher levels of calcium and vitamin D for older people and stating that supplements are useful.
Looking into the disparity in results of vitamin D tests on long-term health. one scientists speculated it might be due to genetic differences in people in the studies. And that "specific 25-(OH)D metabolism genotypes may be particularly susceptible to, or protected from, the potential adverse health effects of low vitamin D"