Your body needs 13 vitamins to survive and thrive. Vitamin D is unique among these vitamins in that your body can synthesize it -- with a little help from the sun. Exposure to UVB radiation from sunlight causes a reaction in your skin that stimulates vitamin D production. For those unable to get enough sun, dietary sources, including vitamin-D fortified foods, can help make up for the shortfall. Consult your doctor before taking vitamin D supplements. Vitamin D works to help promote physical activity and enhance survival in several ways.
Muscle Contraction
Muscle cells have special receptors designed to recognize and react to vitamin D, according to the international review journal "Progress in Biophysics and Molecular Biology." Vitamin D is required for muscle contraction, according to the Office of Dietary Supplements, and vitamin D deficiency can result in muscle pain and weakness.
Calcium Metabolism
Vitamin D assists in calcium metabolism by increasing the amount of calcium you absorb through your intestines from the foods you eat. It also reduces the amount of calcium that you excrete in urine. You need calcium, not just as part of the matrix of your skeletal system, but as part of the composition of the fluids that circulate in your bloodstream. When blood levels of calcium fall too low, vitamin D assists in transferring calcium from your bones to your blood in order to maintain normal levels.
Immune System Regulation
Vitamin D helps modulate the activity of your immune system, the physiological defense mechanism of your body. Your immune system produces cells that contain receptors specifically designed to attract vitamin D. According to the Linus Pauling Institute, vitamin D is believed to enhance your resistance to disease and inhibit disorders wherein your immune system mistakenly attacks healthy tissue.
Considerations
Direct sun exposure is required for your body to make vitamin D. Sunlight through a glass window will not have the same effect. Those with fair skin synthesize vitamin D readily and require only about 45 minutes of direct sun exposure weekly to get enough vitamin D. Darker-skinned people are less efficient at creating vitamin D, according to the Office of Dietary Supplements, and may need supplementation to make up for the shortfall. Those who live in climates that do not receive enough sunlight or who are prevented due to personal circumstance from getting outdoors may also need vitamin D supplementation.
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