CHAPTER 45: VITAMIN D

OBJECTIVES

  1. Identify normal sources of vitamin D2, vitamin D3 and DHT for animals, and discuss the hepatic metabolism of each.

Vitamin D2 is found in plants and yeast, while vitamin D3 comes from animal sources. The liver metabolizes vitamin D2 and vitamin D3 into 25-hydroxyvitamin D (25(OH)D), which is the major circulating form of vitamin D

  1. Discuss the intestinal absorption and plasma transport of vitamin D (see Chapters 62 & 64).

Vitamin D is absorbed in the small intestine. It is absorbed by passive diffusion and a mechanism involving membrane carriers, especially cholesterol transporters. Factors that modified cholesterol absorption also altered vitamin D absorption.

Vitamin D is transported in the blood by vitamin D-binding protein (DBP). DBP-bound vitamin D is transported to the liver where it is hydroxylated at the 25th position to form 25-hydroxyvitamin D [25(OH)D], which is the major circulating form of vitamin D. 25(OH)D is then transported to the kidneys where it is hydroxylated at the first position to form 1,25-dihydroxyvitamin D [1,25(OH)2D], which is the biologically active form of vitamin D

  1. Outline and explain the reasoning behind the renal endocrine control of vitamin D hydroxylation.

Vitamin D stimulates intestinal calcium absorption by increasing the expression of calcium transport proteins. Vitamin D is hydroxylated in the liver to form 25-hydroxyvitamin D (25(OH)D), which is the major circulating form of vitamin D.

  1. Identify and discuss steps involved in vitamin D-stimulated intestinal Ca2+ absorption.

Vitamin D is converted enzymatically in the liver to 25-hydroxyvitamin D (25(OH)D), which is the major circulating form of vitamin D. 25(OH)D is then converted in the kidney to 1,25-dihydroxyvitamin D (1,25(OH)2D), which is the active form of vitamin D

  1. Distinguish between the various active and inactive forms of vitamin D.

Vitamin D from the diet or dermal synthesis is biologically inactive and requires enzymatic conversion to active metabolites. Vitamin D plays a role in calcium and phosphorus homeostasis in the body. In the kidneys, vitamin D is hydroxylated to form 1,25-dihydroxyvitamin D (1,25(OH)2D), which is the active form of vitamin D.

  1. Identify and describe primary physiologic roles for vitamin D in bone and in the kidneys.

Vitamin D plays a crucial role in the normal function of the kidney and metabolism. The kidneys have vitamin D receptors and convert vitamin D from supplements or the sun to its active form. This helps balance calcium and phosphorus in the body by controlling absorption of these minerals from the food and regulating parathyroid hormone (PTH). Vitamin D deficiency can lead to kidney dysfunction and further renal disorder.

Vitamin D is essential for bone health. It helps absorb calcium in the gut, keeping calcium and phosphorus in balance to mineralize bones. Without enough vitamin D, bones can become thin, weak, brittle, or misshapen.

  1. Explain why vitamin D is used to treat patients with PTH deficiency.

Vitamin D is used to treat patients with PTH deficiency because it helps the body absorb calcium and eliminate excess phosphorus, so it acts similarly to PTH and can help compensate for the hormone deficiency.

  1. Identify the signs and symptoms of vitamin D toxicity, and explain why CT and glucocorticoids can be used to treat this condition.

Vitamin D toxicity can cause symptoms such as nausea, vomiting, muscle weakness, and loss of appetite. This happens because there is too much vitamin D in your body. The main consequence of vitamin D toxicity is a buildup of calcium in your blood (hypercalcemia), which can cause nausea and vomiting, weakness, and frequent urination. Vitamin D toxicity might progress to bone pain and kidney problems, such as the formation of calcium stones. CT and glucocorticoids can be used to treat vitamin D toxicity by reducing calcium levels in the blood.

  1. Explain the signs and symptoms of vitamin D deficiency, and also explain why 25(OH)DHT is active in the bone and intestine of nephrectomized rats.

Vitamin D deficiency can cause impairment in bodily functions including bone mineralization. Some of the common signs and symptoms of vitamin D deficiency include fatigue, muscle weakness, bone pain, depression, and hair loss23.

25(OH)DHT is active in the bone and intestine of nephrectomized rats because it is converted to 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) by 1-alpha-hydroxylase in these tissues.