A New Light on Vitamin D
Vitamins are a crucial part of maintaining biological stability. Yet, it has only been within the past few years that researchers and physicians alike have realized their potential not only to respond to deficiency, but to promote better general health.1 One of the most essential-yet oddly often overlooked- nutrients is vitamin D.
Although written accounts of diseases such as rickets and osteoporosis date back centuries, it wasn't until 1919 that researchers first began to realize how important vitamin D was to the prevention and correction of bone disorders.2 Since then, research into this important nutrient has concluded that not only is it more than a simple vitamin-actually functioning as a hormone-but that it also plays a significant role in bone formation, the promotion of nerve function and may even help prevent certain forms of cancer.
While today the word vitamin is a household word, it wasn't until the early 20th century that it first began making its rounds in the common vernacular. First appearing in dictionaries in 1912, the term vitamin was originally coined to describe the organic substances in food that are essential for most of the biochemical processes in the body.3
Since then, scientists have identified 13 vitamins that are considered essential for good health-essential because they are required for life and because the body does not manufacture these nutrients itself. In other words, they must be obtained from external sources, such as food or supplements. These "essential vitamins" are divided into two groups, fat soluble (stored in the fat cells) and water soluble (used and excreted through urine). Fat-soluble vitamins include A, D, E and K. The water-soluble essential vitamins are C (ascorbic acid), B1 (thiamin), B2 (riboflavin), B3 (niacin), B5 (pantothenic acid), B6 (pyridoxine), B12, folic acid and biotin.4
What is vitamin D?
Most people think of bones as static, rigid, unchanging structures that form the internal framework of their bodies. In reality, however, bones are dynamic living tissue composed of a variety of cell types that co-exist in a constant battle between two conflicting processes-formation and resorption. Through these two actions, bone is continually remodeled to maintain stability and preserve proper functioning. To achieve this, our bodies require a sufficient and reliable supply of vital minerals-most notably calcium and phosphorus-and a mechanism to efficiently absorb them. Enter vitamin D.
Even though it's not one of the most talked-about vitamins, vitamin D's importance to maintaining health cannot be overstated. Years of painstaking research has proven conclusively that vitamin D is necessary for both the efficient absorption of dietary calcium and phosphate, as well as their metabolism once ingested.5 In addition, vitamin D has been implicated in playing an important role in a wide variety of actions elsewhere in our physiology-everything from improving immune function and blood cell formation to fostering normal muscle contractions, stimulating neural growth and regulating cardiac action.6
Sources of vitamin D
Although vitamin D enters our bodies through a variety a different mechanisms, such as diet and nutritional supplementation, the primary resource is via exposure to sunlight. Research has shown that when exposed to the sun's ultraviolet light (UVB), a cholesterol compound in the skin is transformed into a precursor of vitamin D (called vitamin D3) which then enters into our circulation.7 As a result, most of our physiological requirements for vitamin D are actually provided for by casual exposure to sunlight. In fact, researchers have estimated that as much as 80% to 100% of an individual's daily requirement for vitamin D comes from simple exposure to sunlight.8
Our secondary source of vitamin D is dietary. While some foods have a small amount of vitamin D naturally (egg yolks, fatty fish such as salmon and fatty fish oils including cod liver oil) fortified foods, such as milk, typically offer the highest level of dietary vitamin D, about 100 IU or 25% of the daily requirements.9
How does vitamin D work?
While researchers still strive to uncover all of the mysteries surrounding the mechanism behind vitamin D's control over mineral metabolism, there are three primary target tissues for its biological actions: intestine, bone and kidney.
The small intestine is an integral part of the human body's ability to stimulate both the absorption and active transport of dietary calcium. Research has shown that in the small intestine, up to 90% of the calcium absorption is vitamin D-dependent.10 Furthermore, according to studies performed at the University of Pennsylvania's School of Medicine, the proximal duodenum holds the greatest concentration of vitamin D receptors in the body. These receptors, once joined with ingested vitamin D, facilitate increased serum calcium levels that are used throughout the body. Without this potent role contributed by vitamin D, calcium serum levels would be seriously compromised.11
In the case of bone, the principal actions of mineral metabolism are a byproduct of vitamin D's effect on the intestine. Because the action of the intestine increases biophysical calcium absorption, increased availability of minerals for incorporation into bone is derived from vitamin D. When there is inadequate calcium in the diet to satisfy the body's calcium requirement, vitamin D communicates to the osteoblasts (bone producing cells) that signal osteoclast (bone resorbing cell) precursors to mature and dissolve the calcium stored in the bone. As such, vitamin D plays a key role in regulating the proliferation and differentiation of both types of bone remodeling cells-those responsible for bone breakdown and those responsible for bone reformation.8
The kidneys represent the third prominent tissue affected by vitamin D. According to researchers at the Boston University Medical Center once vitamin D is in circulation, it is metabolized in part by the kidney to become 1,25-dihydroxyvitamin D, the receptors for which are found not only in the intestine and bone, but in a wide variety of other tissues, including the brain, heart, stomach, pancreas, activated T and B lymphocytes, skin and gonads. In addition, the kidneys function to enhance re-absorption of calcium and other minerals from the renal fluid. As bones are being remodeled on a continual basis, there is a constant release or mobilization of calcium stores from these sites-and the kidneys aid in retaining the minerals for recycling back to bone.12
Factors influencing vitamin D Levels
While there are many factors that affect the level of vitamin D in the blood, most relate to the cutaneous synthesis of vitamin D3. Location, time of day, season, skin pigmentation, use of sunscreen, age and dietary intake all play an important role in the circulating level of vitamin D.13
Location, for example can dramatically affect the production of vitamin D3 in the skin. Researchers have found that individuals living in higher latitudes, such as Boston, Massachusetts (42o N latitude) produce little or no vitamin D from November to February, while 10o farther north in Edmonton, Canada, this period is extended from October to March.14 To maintain minimal levels of vitamin D during these months, reserves of vitamin D produced in the summer and stored in the fat cells become liberated from their reservoirs. However, researchers in Canada have recently shown that during winter months at least 34% of the northern population are vitamin D deficient despite the reserves and require supplementation.15
Reducing the skin's UVB exposure via sunscreen or clothing can cause similar vitamin insufficiencies since they retard the skin's ability to absorb radiation.16 According to a recent study of veiled women in Turkey, for example, out of 51 women examined (ages 14 to 63), 82% were found to be severely vitamin D deficient, while another 8% were moderately deficient. In addition, about half of the deficient women complained of muscle pain, weakness or fatigue-symptoms consistent with profound vitamin deficiency. These results confirm that reduced exposure to sunlight compromises the endogenous levels of vitamin D.17
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