VITAMIN D

Also Known As
Calciferol, cholecalciferol, ergocalciferol, irradiated ergosterol

Physiology and Clinical Effects
Vitamin D’s most important role is maintaining blood levels of calcium, which it accomplished by increasing absorption of calcium from food and reducing urinary calcium loss. Both effects keep calcium in the body and therefore spare the calcium that is stored in the bones. When necessary, vitamin D transfers calcium from the bone into the bloodstream, which does not benefit bones. Although the overall effect of vitamin D on the bones is complicated, some vitamin D is necessary for healthy bones and teeth.

From animal and human population studies, researchers from the University of Wisconsin have hypothesized that vitamin D may protect people from multiple sclerosis.¹

Vitamin D plays a role in immunity and blood cell formation. Vitamin D also helps cells “differentiate”—a process that may reduce the risk of cancer. Vitamin D is also needed for adequate blood levels of insulin² and has been reported to help the body process sugar.³

Food Sources
Cod liver oil is an excellent dietary source of vitamin D, as are vitamin D–fortified foods. Traces of vitamin D are found in egg yolks and butter. However, the majority of vitamin D in the body is created during a chemical reaction that starts with sunlight exposure to the skin. Colecalciferol (vitamin D3) is the animal form of this vitamin.

Deficiency Risk and Symptoms
Vitamin D deficiency, which causes abnormal bone formation, is more common after the winter due to restricted sunlight exposure in that season. Deficiencies are also more common in strict vegetarians (who avoid vitamin D–fortified dairy), dark-skinned individuals, people with malabsorption conditions, liver disease, or kidney disease, and alcoholics. People with liver and kidney disease can make vitamin D but cannot activate it.

One in seven adults has been reported to be vitamin D deficient.⁴ In hospitalized patients under age sixty-five, 42% were reported to be vitamin D deficient.⁵ This same report found that 37% of the people studied were vitamin D deficient despite the fact that they were eating the currently recommended amount of this nutrient. Vitamin D deficiency is particularly common in the elderly.

Recommended Dosage
People who get plenty of sun exposure don’t require supplemental vitamin D. (Sun light increases vitamin D synthesis when it strikes bare skin.) Otherwise, 400 IU per day is a safe adult dose.

Contraindications
Individuals with sarcoidosis who have elevated blood levels of calcium, and people with hyperparathyroidism should not take vitamin D without consulting a physician. Too much vitamin D taken for long periods of time can lead to headaches, weight loss, and kidney stones; and rarely deafness, blindness, increased thirst, increased urination, diarrhea, irritability, failure to gain weight in children, and even death. Most people take more than 400 IU per day (a safe amount for adults), although one study showed that 800 IU per day prevented bone loss more effectively than 200 IU per day in postmenopausal women.⁶ Anyone wishing to take more than 1,000 IU per day for long periods of time should consult a physician. People should remember that the total daily intake of vitamin D includes vitamin D from fortified milk and other fortified foods, cod liver oil, and supplements that contain vitamin D. In addition, people who receive adequate sunlight exposure do not need as much vitamin D in their diet as do people who receive minimal sunlight exposure.

Vitamin D increases both calcium and phosphorus absorption. Vitamin D has also been reported to increase absorption of aluminum. Increased blood levels of calcium (which can be a marker for vitamin D status) have been linked to heart disease.⁷ Some,⁸ but not all⁹ research suggests that vitamin D may slightly raise blood levels of cholesterol in humans.

References:

1. Hayes CE, Cantorna MT, Deluca HF. Vitamin D and multiple sclerosis. Proc Soc Exper Biol Med 1997;216:21–7 [review].
2. Labriji-Mestaghanmi H, Billaudel B, Garnier PE, Sutter BCJ. Vitamin D and pancreatic islet function. 2. Time course for changes in insulin secretion and content during vitamin deprivation and repletion. J Endocrine Invest 1988;11:577–87.
3. Boucher BJ. Inadequate vitamin D status: does it contribute to the disorders comprising syndrome ‘X’? Br J Nutr 1998;79:315–27.
4. Chapuy M-C, Preziosi P, Maamer M, et al. Prevalence of vitamin D insufficiency in an adult normal population. Osteoporosis Int 1997;7:439–43.
5. Thomas MK, Lloyd-Jones DM, Thadhani RI, et al. Hypovitaminosis D in medical inpatients. N Engl J Med 1998;338:777–83.
6. Dawson-Hughes B, Harris SS, Krall EA, et al. Rates of bone loss in postmenopausal women randomly assigned to one of two dosages of vitamin D. Am J Clin Nutr 1995;61:1140–5.
7. Lind L, Skarfors E, Berglund L, et al. Serum calcium: A new, independent prospective risk factor for myocardial infarction in middle-aged men followed for 18 years. J Clin Epidemio 1997;50:967–73.
8. Heikkinen A-M, Tuppurainen MT, Komulainen M, et al. Long-term vitamin D3 supplementation may have adverse effects on serum lipids during postmenopausal hormone replacement therapy. Eur J Endocrinol 1997;137:495–502.
9. Scragg R, Khaw K-T, Murphy S. Effect of winter oral vitamin D3 supplementation on cardiovascular risk factors in elderly adults. Eur J Clin Nutr 1995;49:640–6.

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