Vitamin D is known as a fat soluble vitamin. However, we have learned that it is actually a prehormone with extensive physiological effects. It is available in two forms: ergocalciferol, vitamin D2 and cholecalciferol, vitamin D3. Ergocalciferol (VitD2) comes from ergosterol, a plant sterol and yeast. Cholecalciferol VitD3, is synthesized in the skin via 7-dehydrocholesterol, a cholesterol precursor. Both forms are biologically inert (inactive) until they are hydroxylated (converted) in the body to Calcitriol.
Hydroxylation (conversion) of Vitamin D to calcitriol occurs in the kidneys. People with kidney failure may require Vitamin D in its active form. Vitamin D3 is significantly more potent and effective in humans.
Few foods naturally contain Vitamin D and the ones that do have been fortified. Brief exposure to sunlight is the best way to get Vitamin D. Skin exposure to the sun provides as much as 80% to 90% of the body's Vitamin D stores. In northern latitudes sun exposure particularly in the winter months may not be sufficient to cause Vitamin D synthesis in the skin. Sunlight intensity depends on latitude, altitude, season, cloud cover, ozone levels, etc. The capacity of UVB mediated Vitamin D synthesis is huge. Six days of casual sunlight exposure without sunscreen can make up for 49 days of no sunlight exposure. Skin pigmentation affects Vitamin D synthesis. A light-skinned person in northern latitudes wearing a bathing suit receives about 10-20,000 units of cholecaliferol in 10-12 minutes of peak July sun. A darker skinned person would require 30 minutes and an African-American 120 minutes for their skin to produce the same amount of Vitamin D.
African-American infants who are exclusively breast fed are at higher risk for Vitamin D deficiency and Rickets. Sun exposure three times a week for a period of time ( 25% of that which would cause a mild sunburn) will produce adequate amounts of Vitamin D.
Factors that affect Vitamin D levels:
Insufficient sun exposure
Reduced skin synthesis
Lower dietary intake
Impaired intestinal absorption
Decreased metabolism by the kidneys to active form of Vitamin D
(this condition increases with aging)
Vitamin D receptors decrease with age (serious concern for the elderly)
Obese people may have reduced serum Vitamin D levels and reduced
bioavailability. It seems the Vitamin D is sequestered into body fat
and unavailable.
Vitamin D regulates serum calcium and phosphorus concentrations. It enhances the efficiency of the intestinal absorption of calcium. Calcitriol (active form of Vitamin D), also has effects in the brain, heart pancreas mononuclear cells, activated lymphocytes, and skin.
Vitamin D increases muscle strength and neuromuscular function in addition to strengthening bone. It increase protein synthesis (700-800 units/day).
Vitamin D seems to have immunosuppressant activity, beneficial in autoimmune disorders like Rheumatoid Arthritis and Multiple Sclerosis. Evidence suggests that it improves respiratory disorders. There is an association between levels of 25-hydroxy vitamin D serum levels and pulmonary function. It might be involved in remodeling lung tissue. It could also decrease immune mediated inflammation in the airway.
Vitamin D deficiency is linked to an increase risk of falls. Taking Vitamin D supplements reduces falls by 22% in the elderly. Some studies showed that this benefit is independent of calcium supplementation.
A dose of 800units or higher is necessary for it to be effective. Interestingly, researchers believe that Vit D prevents falls by decreasing body sway and decreasing systolic blood pressure rather than increasing bone mass strength. Until now Vitamin D has been seen as a mere adjunct to calcium supplementation, but it is necessary and effective in it's own right.
Studies show that individuals with higher levels of vitamin D serum levels (25mol/L) concentrations have a significantly lower incidence of certin cancers, particularly gastrointestinal. In clinical trials, women taking calcium (400-1500mg/day with 1,100units of vit D3 had a 60% lower risk for developing cancers of various types.
Long term vit D supplementation decreases the risk of Multiple Sclerosis in women by 40%. Again the effect is dose dependent in this case a minimal dose of 400units/day. In addition, when vit D levels were measured in the general population the higher the level the lower the chances of developing Multiple Sclerosis (50nmol/L). Scientists suspect that this may be one of the reasons why the rate of MS is higher the further you are from the equator.
A large trial showed that women with lower levels of vit D have a higher risk of being obese, yet another reason to take vit D. When combined with calcium it helps maintain a healthy weight baseline. Effective doses are between 800-2000units/day.
Cautions: If you are using topical vitamin D creams you must consult your physician before taking vitamin D orally. Topical vitamin D is absorbed. Cimetidine (Tagamet) can decrease active vitamin D levels, particularly in patients with liver or renal insufficiency.
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