Vitamin D occurs naturally in fatty fish, liver, and egg yolk. Milk, unless it is artificially fortified, is not a good source. The RDA for individuals ages 1–70 years is 15 µg/day and 20 µg/day if over age 70 years. Experts disagree, however, on the optimal level of vitamin D needed to maintain health. [Note: 1 µg vitamin D = 40 international units (IU).] Because breast milk is a poor source of vitamin D, supplementation is recommended for breastfed babies.
E. Clinical indications for vitamin D 1. Nutritional rickets: Vitamin D deficiency causes a net demineralization of bone, resulting in rickets in children and osteomalacia in adults (Fig. 28.25). Rickets is characterized by the continued formation of the collagen matrix of bone, but incomplete mineralization results in soft, pliable bones. In osteomalacia, demineralization of preexisting bones increases their susceptibility to fracture. Insufficient exposure to daylight and/or deficiencies in vitamin D consumption occur predominantly in infants and the elderly. Vitamin D deficiency is more common in the northern latitudes, because less vitamin D synthesis occurs in the skin as a result of reduced exposure to ultraviolet light. [Note: Loss-of-function mutations in the vitamin D receptor result in hereditary vitamin D– deficient rickets.] 2.
Renal osteodystrophy: Chronic kidney disease causes decreased ability to form active vitamin D as well as increased retention of PO43− , resulting in hyperphosphatemia and hypocalcemia. The low blood Ca2+ causes a rise in PTH and associated bone demineralization with release of Ca2+ and PO43− . Supplementation with vitamin D is an effective therapy.
However, supplementation must be accompanied by PO43− reduction therapy to prevent further bone loss and precipitation of calcium phosphate crystals.
3.
Hypoparathyroidism: Lack of PTH causes hypocalcemia and hyperphosphatemia. [Note: PTH increases phosphate excretion.] Patients may be treated with vitamin D and calcium supplementation.
F. Toxicity
Like all fat-soluble vitamins, vitamin D can be stored in the body and is only slowly metabolized. High doses (100,000 IU for weeks or months) can cause loss of appetite, nausea, thirst, and weakness. Enhanced Ca2+ absorption and bone resorption results in hypercalcemia, which can lead to deposition of calcium salts in soft tissue (metastatic calcification). The UL is 100 µg/day (4,000 IU/day) for individuals ages 9 years or older, with a lower level for those under age 9 years. [Note: Toxicity is only seen with use of supplements. Excess vitamin D produced in the skin is converted to inactive forms.]
XIII. VITAMIN K
The principal role of vitamin K is in the posttranslational modification of a number of proteins (most of which are involved with blood clotting), in which it serves as a coenzyme in the carboxylation of certain glutamic acid residues in these proteins. Vitamin K exists in several active forms, for example, in plants as phylloquinone (or vitamin K1), and in intestinal bacteria as menaquinone (or vitamin K2). A synthetic form of vitamin K, menadione, is able to be converted to K2.