Vitamin D3, as well as vitamin D2, helps regulate calcium and phosphorus homeostasis by increasing nutrient absorption from the intestines and mobilization from bone. Vitamin D3 also decreases renal excretion of calcium and phosphate. Vitamin D3 is naturally produced in the skin through UVB exposure found in sunlight. Fortified dairy, especially milk, is also a source of vitamin D3.
Dietary sources of vitamin D include shiitake mushrooms and oily fish such as salmon and tuna. Since natural foods have little vitamin D, foods including cereals, milk, cheese, and orange juice are vitamin D-fortified. Since dietary sources of vitamin D are limited, a dietary supplement may be necessary since many people do not get enough vitamin D through diet and sun exposure.
Vitamin D3 supplementation is used for prophylaxis and treatment of vitamin D deficiency. Symptoms of vitamin D deficiency include bone and muscle weakness and pain. In individuals with vitamin D deficiency, children experience rickets while adults develop osteomalacia. Vitamin D deficiency may contribute to bone loss, kidney disease, cancer, and diabetes. Other consequences of vitamin D deficiency include heart and lung disorders, rheumatoid arthritis, and overall mortality.
Excessive amounts of vitamin D3 may cause fatigue and muscle weakness. Too much vitamin D may cause hypercalcemia (refer to the Picmonic on "Hypercalcemia"). Excessive levels of serum calcium affects the central nervous system and may cause confusion and lethargy. Weakened bone related to calcium leaching from the bones to the blood may cause bone pain.
Vitamin D toxicity may present with constipation, vomiting, and nausea. Excessive amounts of serum calcium related to vitamin D toxicity slows down peristalsis and the digestive symptom resulting in stomach upset and constipation.
Vitamin D3 toxicity commonly occurs secondary to hypercalcemia. Excessive amounts of vitamin D3 may lead to hypercalcemia if the intestines are unable to absorb enough calcium. Despite high serum calcium concentrations, bone is decalcified and results in osteoporosis.
Renal function is affected by excessive amounts of ingested vitamin D3. Persistent hypercalcemia may cause polyuria, nocturia, and proteinuria. Calcium deposition into the kidneys leads to nephrolithiasis or kidney stones.
Cholecalciferol (vitamin D3) is more effective than ergocalciferol (vitamin D2) at raising total serum levels of vitamin D. Vitamin D3 has increased binding to tissue vitamin D receptors, an increased half-life, and increased potency when compared to vitamin D2.
Excessive amounts of vitamin D3 may lead to hypervitaminosis D, a toxicity from exceeding recommended doses or accidental poisoning. Vitamin D doses exceeding 1000 IU/day in infants and 50,000 IU/day in adults results in hypervitaminosis D. Symptoms include weakness, fatigue, nausea, vomiting, and constipation. Excess calcium causes tissue damage after deposition in the heart, blood vessels, and lungs. Vitamin D poisoning in children may suppress growth for at least 6 months. Treatment of hypervitaminosis D includes ceasing vitamin D ingestion, increasing fluid intake, and initiating a low-calcium diet.
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