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Calcitriol

Vitamin D is a fat-soluble steroid hormone precursor that contributes to the maintenance of normal levels of calcium and phosphorus in the bloodstream. Strictly speaking, it is not a vitamin since human skin can manufacture it, but it is referred to as one for historical reasons. It is often known as calciferol. more...

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Forms of Vitamin D

  • Vitamin D1: molecular compound of ergocalciferol with lumisterol, 1:1
  • Vitamin D2: ergocalciferol or calciferol (made from ergosterol) also called
  • Vitamin D3: cholecalciferol (made from 7-dehydrocholesterol)
  • Vitamin D4: 22,23-dihydroergocalciferol
  • Vitamin D5: sitocalciferol (made from 7-dehydrositosterol)

Overview

Vitamin D3, also known as cholecalciferol, is the natural human form of vitamin D. It is made in the skin when cholesterol via 7-dehydrocholesterol reacts with ultraviolet light in the skin. Ultraviolet light (UVB, which is wavelengths 290 to 315 nm), found in sunlight when the sun is high enough above the horizon for UVB to penetrate the atmosphere, is responsible for the production of cholecalciferol. Up to 20,000 IU can be made in the skin after one minimal erythemal dose of exposure, or until the skin just begins to turn pink. Vitamin D2 is derived by irradiating fungi to produce ergocalciferol. Ergocalciferol does not naturally occur in the human body unless it is added by supplementation.

In certain parts of the world, particularly at higher latitudes, total vitamin D input is usually not sufficient, especially in the winter, thus the recent concern about widespread vitamin D deficiency. To help prevent this possibility, foods such as milk may be fortified with vitamin D2 or vitamin D3, but milk only contains 100 IU per glass, 1/200 as much as is made after 15 minutes of sunbathing at solar noon in the summer. A severe deficiency of vitamin D leads to rickets in children, which is a softening of the bones owing to faulty mineralization, and a similar condition in adults, osteomalacia. Recent medical studies also associate vitamin D deficiency with everything from most forms of cancer, to heart disease, depression, diabetes, hypertension, autoimmune diseases, periodontal disease, and even obesity.

Cholecalciferol is transported to the liver where it is hydroxylated to calcidiol or 25-hydroxy-vitamin D, the storage form of the vitamin. A blood calcidiol level is the only way to determine vitamin D deficiency; levels should be between 40 and 60 ng/ml (100 to 150 nMol/L) for optimum health.

The most active form of the vitamin is calcitriol, a potent steroid hormone. Calcitriol is synthesized from calcidiol in the kidneys to perform its endocrine function of maintaining the calcium economy. Calcitriol binds to a transcription factor which then regulates gene expression. The outcome is the maintenance of calcium and phosphorus levels in the bone and blood with the assistance of parathyroid hormone and calcitonin.

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Calcitriol in postmenopausal women with osteoporosis - Tips from Other Journals
From American Family Physician, 6/1/92

Estrogen and calcium are commonly used to treat postmenopausal osteoporosis. Calcitriol has been considered for the treatment of osteoporosis because it increases intestinal calcium absorption, stimulates bone synthesis and decreases bone resorption. However, the results of studies of its efficacy are conflicting. One study found that calcitriol was ineffective in the treatment of established osteoporosis. Another found that it reduced bone loss by increasing calcium absorption and reducing bone resorption. A third study found that it increased spinal bone density and total body calcium. To compare the effects of calcium and calcitriol on the rate of new vertebral fractures in women with postmenopausal osteoporosis, Tilyard and colleagues conducted a prospective, randomized study of calcitriol therapy in women who had one or more vertebral compression fractures.

The study included 622 women. The 314 women in the calcitriol group received 0.25 [mu]g of calcitriol twice daily, and the 308 women in the calcium group received 1 g of elemental calcium as 5.2 g of calcium gluconate twice daily. The rate at which new fractures developed, as measured by lateral roentgenography, was used to determine the efficacy of treatment. A fracture was defined as a decrease of 15 percent or more in the anterior or posterior height of the body of each vertebra from T4 through L4.

After the second year of treatment, the calcitriol group had a significantly lower rate of new vertebral fractures than the calcium group (nine fractures versus 25 fractures per 100 patient-years). After the third year, 10 fractures per 100 patient-years had occurred in the calcitriol group, compared with 32 fractures per 100 patient-years in the calcium group. In addition, 11 women in the calcitriol group had 11 peripheral fractures, while 22 women in the calcium group had 24 peripheral fractures. Among women initially classified as having mild to moderate osteoporosis (five or fewer fractures), four fractures occurred in the calcitriol group and 31 occurred in the calcium group after the third year of treatment. The incidence of side effects was similar in both groups.

The study results indicate that calcitriol therapy in women with postmenopausal osteoporosis is safe and effective in reducing the incidence of new vertebral fractures. (New England Journal of Medicine, February 6, 1992, vol. 326, p. 357.)

COPYRIGHT 1992 American Academy of Family Physicians
COPYRIGHT 2004 Gale Group

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