Vitamin D 2 Vs Vitamin D 3 Supplements: How Much Is Enough? The 24-hydroxylase gene is under the transcriptional control of calcitriol, thereby providing tight negative feedback. Renal 24-hydroxylase activity further limits the availability of calcitriol by creating inert metabolites of both calcitriol (1,24,25-trihydroxyvitamin D) and calcidiol (24,25-dihydroxyvitamin D). Increased calcitriol levels inhibit PTH both directly (through the vitamin D response element on the PTH gene) and indirectly (by increasing intestinal calcium absorption), causing calcitriol production in the kidney to decrease. The rarity of reports of vitamin D toxicity can be explained in part by the kidney's ability to limit production of active calcitriol. That fear is generally unmerited given the dearth of reports of vitamin D toxicity compared with the expansive literature on vitamin D deficiency. What Is an Optimal 25(OH)D Level?īut this intake level may still be inadequate to reach optimal levels in many people, especially those at risk and (3) physicians may be uncomfortable recommending larger doses of vitamin D. Doing so can lead to an erroneous interpretation of vitamin D status because calcitriol levels are often normal or even elevated in patients with vitamin D deficiency as a result of elevated PTH levels. Clinicians should not measure 1,25(OH) 2D levels to diagnose hypovitaminosis D. Patients with severe and long-standing vitamin D deficiency may present with overt hypocalcemia and/or hypophosphatemia, but this is the exception. Thus, vitamin D deficiency is usually accompanied by normal blood levels for calcium and phosphorus, high-normal or elevated levels of PTH, normal to elevated levels of total alkaline phosphatase, a low 24-hour urine calcium excretion rate, and low levels of total 25(OH)D. In the setting of vitamin D deficiency, secondary HPT causes both release of calcium stored in bone and resorption of calcium by the kidney to maintain normal serum calcium and phosphorus levels. In people with healthy kidneys and bones, normal serum levels of calcium and phosphorus are maintained predominantly through the interaction of 2 hormones: parathyroid hormone (PTH) and calcitriol. TESTING AND INTERPRETING VITAMIN D STATUS How Prevalent Is Vitamin D Deficiency And Who Is At Risk? The predominant effects of vitamin D are exerted through the endocrine and autocrine actions of calcitriol via activation of the vitamin D receptor in cells. Once absorbed from the intestine, they are metabolized in the liver to 25-hydroxyvitamin D, composed of 25(OH)D 2 and 25(OH)D 3 25(OH)D (also called calcidiol) is subsequently converted to 1,25-dihydroxyvitamin D, also known as calcitriol, in the kidney and select other tissues by the action of the 1α-hydroxylase enzyme. This value varies greatly by region of the world because fortification markedly improves the availability of vitamin D through diet. Aside from rich sources such as oily fish, the vitamin D content of most foods is between 50 and 200 IU per serving. Vitamin D 3 (D 3), also known as cholecalciferol, is obtained primarily from skin exposure to ultraviolet B (UVB) radiation in sunlight, ingestion of food sources such as oily fish and variably fortified foods (milk, juices, margarines, yogurts, cereals, and soy), and oral supplements. Vitamin D 2 (D 2), also known as ergocalciferol, is obtained from dietary vegetable sources and oral supplements. Vitamin D consists of 2 bioequivalent forms.
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