Vitamin D is actually a steroid hormone with multiple physiologic roles in your body. Its effects on bone and calcium metabolism have been recognized for decades, and scientists are now beginning to unravel its actions in other tissues, according to 2010 information from Colorado State University. Cholecalciferol, an inactive form of vitamin D, is manufactured in your skin whenever you are exposed to sunlight. Cholecalciferol must undergo sequential chemical conversions in your liver and kidneys before it is fully activated. Vitamin D synthesis can be interrupted at any point in this pathway. Your doctor can help you determine if you need more vitamin D.
Skin
Ultraviolet light striking your skin interacts with a molecule called 7-dehydrocholesterol to form pre-vitamin D, which then spontaneously changes shape to form cholecalciferol. Cholecalciferol has no biologic activity, but it is the substrate for the further liver- and kidney-specific transformations that produce active vitamin D. According to Dr. Michael Holick at Boston University School of Medicine, any factor that decreases sun exposure reduces your ability to synthesize cholecalciferol. Thus, clouds, shorter days, clothing, sunscreens, increased skin pigmentation -- including tanning -- lower altitude, higher latitude and limiting sun exposure to early morning or late afternoon decrease vitamin D synthesis.
Liver
Once cholecalciferol enters your circulation, it travels to your liver, where it undergoes the first of two "hydroxylations." 25-hydroxylase, an enzyme found in your liver's cells, converts cholecalciferol to 25-hydroxyvitamin D, or calcidiol. Like cholecalciferol, calcidiol has no significant biological activity. An August 2010 "Gastroenterology and Hepatology" review reports that only severe liver disease can interfere with cholecalciferol's conversion to 25-hydroxyvitamin D. However, even people with mild liver disease can exhibit low serum vitamin D levels, presumably due to decreased intestinal absorption of dietary vitamin D.
Kidney
The final transformation of cholecalciferol to active vitamin D occurs in your kidneys. There, an enzyme called 1-alpha-hydroxylase converts 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D, or calcitriol. Calcitriol is biologically active and is responsible for all of the metabolic effects of vitamin D. The July-August 2007 issue of "Seminars in Dialysis" states that patients with kidney disease exhibit some degree of vitamin D deficiency even in the early stages of their illness, and those with severe disease lose the capacity to produce calcitriol and require supplementation with activated vitamin D.
Considerations
Vitamin D is a hormone-like vitamin whose synthesis and activation require sunlight exposure and adequate liver and kidney function. Limited sun exposure or liver or kidney disease can interfere with your ability to produce calcitriol, or activated vitamin D. Supplementation with cholecalciferol, 25-hydroxyvitamin D or calcitriol can help to overcome deficiencies stemming from impaired synthetic capabilities at any step of vitamin D's activation pathway. Ask your physician if you need additional vitamin D.
References
- Colorado State University; Pathophysiology of the Endocrine System: Vitamin D (Calcitriol); April 24, 2010
- "The American Journal of Clinical Nutrition"; Vitamin D: Importance in the Prevention of Cancers, Heart Disease, and Osteoporosis; M.F. Holick; March 2004
- "Gastroenterology and Hepatology"; Vitamin D Deficiency and Liver Disease; S. Nair; August 2010
- "Seminars in Dialysis"; Expanding Role for Vitamin D in Chronic Kidney Disease: Importance of Blood 25-OH-D Levels and Extra-renal 1alpha-hydroxylase in the Classical and Nonclassical Actions of 1alpha,25-dihydroxyvitamin D(3); G. Jones; July-August 2007
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