The term "vitamin D" actually refers to two forms of vitamin D, called ergocalciferol, or D2 and cholecalciferol, or D3. Sun exposure activates production of vitamin D3 in the skin, while D2 comes only from food and supplements. Both D2 and D3 are inactive in their original forms and must be converted to active forms in the liver and kidney. It's impossible to have too much vitamin D3 from sunlight. Most overdoses of vitamin D3 occur because of taking too much D3 in supplements, the Mayo Medical Laboratories states.
Bones
Vitamin D3 deficiency alone doesn't cause symptoms, as long as vitamin D2 levels are adequate, because vitamin D testing measures D2 and D3 levels and adds them together. The total vitamin D level, called 25-hydroxyvitamin D, determines whether a person has a vitamin D deficiency. If both D2 and D3 are low, the total will be low. Symptoms of total vitamin D deficiency in the bones including bone softening and deformities, called rickets, in children and bone softening or osteomalacia in adults. Children with rickets sit, crawl and walk late, and may develop bowlegs or knock-knees. Bone pain can occur. Ironically, high levels of total vitamin D can also lead to bone loss from hypercalcemia, an increase in calcium levels in your blood.
Muscles
Muscle pain and weakness often occur if you have total vitamin D deficiency. Weakness increases the risk of falls in vitamin D deficiency; giving supplemental vitamin D decreased falls in elderly women by 50 percent over giving calcium alone, the Linus Pauling Institute at Oregon States University reports. Weakness and fatigue can also occur if you have excessively high levels of vitamin D3 that lead to elevated total vitamin D levels. Ataxia, a lack of muscle coordination, can also occur if you have elevated vitamin D levels.
Kidney Damage
Vitamin D toxicity from overdose can cause calcifications in the kidney and other organs, which can lead to severe kidney damage. Excessive thirst and urine output, nausea and protein in the urine can lead to decreased urine output and renal failure.
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