Vitamin D requirement

Daily calcium and vitamin D requirements are highest in postmenopausal women and elderly men 1500 mg elemental calcium and 400 to 800 IU vitamin D (see Table 53-4). When these requirements cannot be achieved by diet alone, appropriate calcium and/or vitamin D supplementation is recommended. 

For example, elderly patients living in musing homes will not be able to meet vitamin D requirements and will need supplementation. Vitamin D deficiency is common in elderly patients owing to decreased exposure to sunlight and subsequent decreased vitamin D synthesis in the skin, decreased gastrointestinal absorption of vitamin D, and reduction in vitamin D3 synthesis. Individuals living in northern climates also have decreased exposure to sunlight and are less likely to achieve vitamin D requirements. 

Effects on vitamin D Barbiturates may increase vitamin D requirements, possibly by increasing the metabolism of vitamin D via enzyme induction. 

Some 20% of the daily vitamin D requirement are supplied by food intake, while about 80% are produced via endogenous synthesis through transformation of 7-dehydrocholesterol under the influence of ultraviolet light. Witliin the liver, D3 (= cholecalciferol) is activated by 25-hydroxylation. It is excreted into the bile to become pmrt of an enterohepmtic circulation. In liver cirrhosis, and particularly in cholestasis, this circulation is impaired, so that vitamin D metabolites are activated in larger amounts and excreted in the faeces. The resulting clinical picture is usually a mixture of osteoporosis and osteomalacia. 

The normal physiological action of PTH on bone is blunted in patients with renal failure, because their renal cells are not synthesizing the 1,25 dfliydroxy vitamin D required for normal PTH action. It is typical for patients in renal failure to have high serum PTH values this represents secondary hyperparathyroidism, the normal physiological response to vitamin D deficit. Deposition of vM at the bone mineralization front and binding to parathyroid calcium receptors interferes with this physiologic process. The usual parathyroid response to these conditions decreases secretion of PTH. The result is lower-than-expected serum PTH concentration for the degree of renal disease present. 

Treatment of hypocalcemia associated with vitamin D-deficient states should be individualized. In patients with malabsorption, vitamin D requirements vary markedly, and large doses may be required. In contrast, vitamin D deficiency associated with anticonvulsant med-... 

Adverse effects of oral calcium and vitamin D supplementation include hypercalcemia and hypercalciuria, especially in the hy-poparathyroid patient, in whom the renal calcium-sparing effect of parathyroid hormone is absent. Hypercalciuria may increase the risk of calcium stone formation and nephrolithiasis in susceptible patients. One maneuver to help prevent calcium stones is to maintain the calcinm at a low normal concentration. Monitoring 24-hour urine collections for total calcium concentrations (goal <300 mg/24 h) may also minimize the occurrence of hypercalciuria. The addition of thiazide dinretics for patients at risk for stone formation may result in a reduc-tionof both urinary calcium excretion and vitamin D requirements." ... 

Another important hormone derived from cholesterol is vitamin D. This steroid-like hormone is involved in regulating calcium and phosphorus metabolism. The complete synthesis of vitamin D requires ultraviolet light to convert 7-dehydrocholesterol to previtamin Dj. The reaction scheme is shown in Figure 34-3. The active hormone 1,25-dihydroxycholecalciferol (calcitriol)... 

METABOLIC ACTIVATION Whether endogenously synthesized or acquired from diet, vitamin D requires modification to become biologically active. The primary active metabohte of vitamin D is calcitriol, which is formed by two successive hydroxylations (Figure 61 ). 

Vitamin D requirements are often expressed in terms of international units (iu). One iu of vitamin D is defined as the vitamin D activity of 0.025 pg of crystalline vitamin D3. 

Eish and shellfish are rich in vitamins. Varieties of fat fish are rich in vitamins A, D, and K. An average sewing of these fish gives 10% of the daily adult vitamin A requirement and 50% of the vitamin D requirement. The important B complex vitamins are present in seafoods, too. A serving of lean or fat fish or shellfish, yields about 10% of the thiamin, 15% of the riboflavin, and 50% of the niacin required each day. 

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