Dealing with Vitamin D

  Since our bodies can produce vitamin D by the action of sunlight on the skin, it is considered both a hormone and a vitamin. "It is quite easy to get the vitamin D needed from sunlight exposure, which is one of the best sources," states Mark Meskin, Ph.D., R.D., associate professor of food, nutrition and consumer sciences at California State Polytechnic University, Pomona, CA. "All it takes is several 10-to-20 minute sessions of sunlight exposure each week on a small patch of skin anywhere on your body."

  When the deep layer of the skin absorbs ultraviolet radiation, synthesis and activation of vitamin D begins with the conversion of 7-dehydrocholesterol, a precursor to vitamin D, into cholecalciferol. The cholecalciferol is then transported to the liver, where an enzyme converts it to 25-hydroxycholecalciferol. An enzyme in the kidneys then converts the 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol - which, at a potency 10 times that of cholecalciferol, is the most potent form of vitamin D3.

  Liver or kidney disorders result in impaired conversion of cholecalciferol to more-potent vitamin D compounds. In fact, many individuals with osteoporosis have high levels of 25-hydroxycholecalciferol, while their levels of 1,25-dihydroxycholecalciferol are very low.

  Vitamin D inhibits the development and growth of many different cancers, including breast, colon and prostate, and a deficiency is associated with many chronic diseases, making it vital to consume the RDA via dietary or supplement form.

  "There are some reports which show a positive impact using 600 IUs of vitamin D in association to osteoarthritis," says Ram Chaudhari, Ph.D., C.N.S., executive vice president research and development of Fortitech, Inc., Schenectady, NY. It appears that vitamin D helps maintain integrity of the cartilage in joints (e.g., the knee joint). Without this cartilage, the joint loses its shock-absorbing capacity.

  Vitamin D deficiency causes rickets in children. In adults, it causes osteomalacia, a condition in which new bone tissue fails to harden due to lack of calcium.

  Vitamin D deficiency also exacerbates osteoporosis via direct and indirect effects on existing bone tissue. As people age, they lose bone mass. Women tend to lose more than men, because menopause accelerates bone-mass loss. In fact, bone loss during the immediate postmenopausal period may be as much as 2% annually. With a vitamin D deficiency, only 10% to 15% of all calcium is absorbed. In fact, the body removes the calcium from bones, so a vitamin D deficiency actually increases bone-mass loss.

  "Over 65 years old, extra vitamin D, approximately 200 IUs extra, may be necessary, since as you age, skin loses its ability to synthesize vitamin D from sunlight," says Chaudhari. For instance, the skin of an 80-year-old is able to produce only half as much vitamin D as the skin of a 20-year-old.

  Those living in the northern third of the United States might not synthesize sufficient quantities of vitamin D from sunlight in the winter because the sun's rays are too weak; they may require supplements to maintain bone health. Strict vegans may also require vitamin D supplements to sustain adequate levels.

  Insufficient levels of vitamin D appear to be more commonplace in the 50-and-older population, particularly if they live in the northern hemisphere. Adults over 50 should consider getting a serum blood test for vitamin D - the 25-hydroxycholecalciferol levels of this age group should not be lower than 20 nanograms.

  High dosages of vitamin D, even for a short period, can easily lead to toxicity. Metastatic calcification, which can lead to increased blood concentrations of calcium, deposits calcium into internal organs and can cause kidney-stone development. Additionally, studies have shown that a diet low in magnesium, with high levels of vitamin D, can lead to an increase of atherosclerosis and heart disease.

  Andrea D. Platzman is a registered dietitian who is a consultant to the food industry, and regularly writes for nutrition publications. She earned a master's degree in nutrition from New York University, and has a culinary and business background.