A number of factors influence the metabolism of vitamin D. These include season, geographic location, race, body habitus, pregnancy, lactation, age, and drugs. Individuals with pigmented skin make less vitamin D and, as a consequence, may have low serum vlaues for vitamin D and 25-hydroxyvitamin D. Obese subjects also may have low serum values for vitamin D and 25-hydroxyvitamin D, presumably because the vitamin is fat soluble and is stored in fat. Some subjects in these groups may have mild secondary hyperparathyroidsm with increases in serum immunoreactive parathyroid hormone and urinary cyclic adenosine 3',5'-monophosphate and decreases in urinary calcium. It is important to distinguish vitamin D depletion from vitamin D deficiency because subjects with vitamin D depletion do not have osteomalacia or rickets and do not require treatment. Before determining whether a given subject has vitamin D deficiency and requires treatment, it is important to determine whether factors that may alter metabolism of the vitamin account for the abserved changes. Additional studies, including serum calcium, phosphorus, alkaline phosphatase, and X-rays, may be required to determine whether rickets or osteomalacia are present.
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