Introduction
The term vitamin D actually refers to a pair of biologically inactive precursors of a critical micronutrient. They are vitamin D3, also known as cholecalciferol, and vitamin D2 also known as ergocalciferol. Cholecalciferol (D3) is produced in the skin by a photoreaction on exposure to ultraviolet B light from the sun (wavelength 290 to 320 nanometers). Erogcalciferol (D2) is produced in plants and enters the human diet through consumption of plant sources. Once present in the circulation, both D2 and D3 enter the liver and kidneys where they are hydroxylated to form both 25-hydroxyvitamin D and 1, 25 dihydroxyvitamin D. 25-Hydroxyvitamin vitamin D3 is the major circulating form of vitamin D3 in human blood, and therefore, it is the form measured by physicians to evaluate vitamin D status in people worldwide. It has recently been shown that there is no difference in circulating blood levels of Vitamin D when people take D3 as compared to D2 despite some current published opinions. Knowledge of the role of vitamin D metabolic activity, its role in human health and identification of the forms and metabolic pathways for vitamin D had been building for many decades but only became fully elucidated during the 1970s. Thanks in part to Dr. Holick from Boston University Medical School, who has been leading this research scince that time. While nutrition is fundamental in human health, understanding of nutritional metabolism has generally lagged behind the pace of medical investigation and practice focusing on factors external to the host such as infectious micro-organisms.
The first major functions of vitamin D to be recognized were (1) enhancement of calcium absorption from the diet through the intestine and (2) mobilization and re-absorption of calcium from bone which represents the major store of calcium (or “calcium bank”) in the body. Calcium in turn is critical for cellular metabolism and membrane actions, enzymatic reactions, muscle function, skeletal structure and a host of activities needed to sustain life and maintain homeostasis. Since vitamin D has long been recognized for its role in calcium metabolism it has long been used to treat patients with renal failure and bone diseases. It is also important in postmenopausal osteoporosis for the current epidemic of bone fractures in the elderly
However, in 1979 DeLuca found that vitamin D is actually recognized by every tissue in the body. Every cell has receptors for vitamin D. Since then it has been used to treat hyperproliferative skin diseases such as psoriasis.
For the immune system, the large white blood cell macrophages activate vitamin D. The activated vitamin D in turn causes macrophages to make a peptide that specifically kills infective agents such as tuberculosis mycobacteria. Vitamin D also has a role in autoimmune diseases such as multiple sclerosis, rheumatoid arthritis and diabetes type. Given vitamin D’s long recognized activity in the kidney it was also found to effect renin/angiontensin production in the kidney, the major regulators of blood pressure. There is a direct correlation between higher (more northern) latitudes and higher blood pressure (where both sunlight and vitamin D levels are lower) in both northern and southern hemispheres of the earth. People at high latitudes with high blood pressure experience a return to normal blood pressure levels following UVB light exposure, and restoration of active vitamin D levels, in a tanning bed three times per week for three months (and you thought it only worked if the sunlight was captured on a beach in the Bahamas!). Multiple sclerosis also shows a marked association with higher latitudes worldwide and there may be a similar role for protection by vitamin D.
Vitamin D is also thought to have an important role in cancer. As early as the 1940s it was noted that living at higher latitudes is associated with a higher incidence of several cancers (while only skin cancer specifically has a lower incidence at higher latitudes). Recent epidemiologic observations have continued to bear out this association.
Benefits of Sunlight/Vitamin D
• Improves Bone Health
• Improves Mental Health
• Improves Heart Health
• Prevents Many Common Cancers
• Alleviates Skin Disorders
• Decreses Risk of Autoimmune DIsorders
• Decreases Risk of Multiple Sclerosis
• Decreses Risk of Diabetes
There is essentially little or no active vitamin D available from regular dietary sources. It is principally found in fish oils, sun-dried mushrooms, and fortified foods like milk and orange juice. However, many countries worldwide forbid the fortification of foods. There is potentially plenty of vitamin D in the food chain because both phytoplankton and zooplankton exposed to sunlight make vitamin D. Wild caught salmon, which feeds on natural food sources, for example, has available vitamin D. However, farmed salmon fed food pellets with little nutritional value have only 10% of the vitamin D of normal fish. The “perfect storm” of photophobia, lack of exposure to sunlight, and insufficiency of available dietary vitamin D has led to a national and worldwide epidemic of vitamin D deficiency.
It is estimated that at least 30% and as much as 80% of the US population is vitamin D deficient. In the US, at latitudes north of Atlanta, the skin does not make (photoconvert) any vitamin D from November through March (essentially outside of “daylight saving time”; so while we shift the clock around, it does not salvage vitamin D synthesis). During this season the angle of the sun in the sky is too low to allow ultraviolet B light to penetrate the atmosphere and it is absorbed by the ozone layer. Even in the late spring, summer and early fall, most vitamin D is made between 10 AM and 3 PM when UVB from the sun penetrates the atmosphere and reaches the earth’s surface. It might be expected that vitamin D deficiency would be a problem limited to northern latitudes.
However, vitamin D deficiency is also a national problem. The US Centers for Disease Control and Prevention completed a national survey at the end of winter and found that nearly 50% of African-American women ages 15 to 49 years were deficient. These represent the critical childbearing years. A growing fetus must receive adequate vitamin D from the mother, especially since breast milk does not provide adequate Vitamin D. A study of pregnant women in Boston found that in 40 mother-infant pairs at the time of labor and delivery, over 75% of mothers and 80% of newborns were deficient. This observation was made despite the fact that pregnant women were instructed to take a prenatal vitamin which included 400 IU vitamin D and to drink two glasses of milk per day.
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