The Recommended Dietary Allowance -- RDA -- of vitamin D for healthy adults is a controversial subject among dietitians and physicians. Vitamin D synthesis in the human body, however, has been well studied. MayoClinic.com notes that spending as little as 10 minutes in the sun can allow your body to manufacture enough vitamin D for the day.
Cholesterol
The body itself produces cholesterol for use as a building block for other necessary compounds. Cholesterol is converted into 7-dehydrocholesterol, a vitamin D precursor. The 7-dehydrocholesterol present in the skin is ready for the next step in making vitamin D.
Sunlight
When ultraviolet rays from the sun hit the skin, the 7-dehydrocholesterol is converted to pre-vitamin D-3. This intermediate works its way into the body and is slowly converted to the active form -- vitamin D-3 -- using heat from the body. The biological activity of vitamin D-3 is 500- to 1,000-fold higher than pre-vitamin D-3. These steps take about 36 hours.
Liver
Vitamin D-3 from any source -- the body's synthesis or from food or supplements -- is transported to the liver. A hydroxyl group is added to provide the next intermediate -- 25-hydroxyvitamin D or calcidiol. This precursor to fully active vitamin D is the major circulating form in the body.
Kidneys
The 25-hydroxyvitamin D is sent to the kidneys where the enzyme 25-hydroxyvitamin D-3-1-hydroxylase adds a second hydroxyl group. Active vitamin D -- 1,25-dihydroxyvitamin D or calcitriol -- is then carried to the intestines, kidneys and bones by a protein escort.
Uses
Vitamin D is part of a cooperative group of nutrients required for bone maintenance and bone synthesis. It maintains the levels of calcium and phosphorus in the blood to regulate bone growth. Vitamin D is also part of cells in the immune system, brain, pancreas, skin, muscles, cartilage and reproductive organs.
References
- "Understanding Normal and Clinical Nutrition"; Sharon Rady Rolfes, R.D., Kathryn Pinna, Ph.D., R.D., Ellie Whitney, Ph.D.; 2009
- Linus Pauling Institute; Vitamin D; Jane Higdon; March 2004
- MayoClinic.com: Vitamin D
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