Although vitamin D is an essential fat-soluble vitamin for humans, excessive consumption of it can cause physical and developmental problems. High doses of vitamins do not always yield better health.
Storage
Unlike water-soluble vitamins (C, B6, folate), which are excreted in the kidneys, excess fat-soluble vitamins (A, D, E, K) are stored in the liver for future use. So if you did not consume enough fat-soluble vitamins in a day or two, your body uses the reserves to maintain normal bodily functions.
Toxicity Amount
Adults would have to consume 10 times the daily intake (RDA recommendation: 5-10 mcg a day) for six months to a year to experience toxicity, while infants and toddlers would have to consume five times the daily recommendation for the same effect. Such toxicity is caused by excess supplementation, not from sun exposure or milk consumption.
Hypercalcemia
This is a condition in which there is too much calcium in the blood. Symptoms include fatigue, anorexia, nausea, vomiting, pancreatitis and increased urination. Uncommon but severe cases of hypercalcemia (above 15-16 mg/dL) can lead to coma and cardiac arrest.
Calcinosis
Kidney failure is another effect of overconsuming vitamin D. That can cause calcinosis, a condition in which calcium deposits build up in the soft tissues, such as arteries and skin.
Prevention/Solution
Vitamin D toxicity is uncommon, because you would have to take megadoses of the vitamin over for many months before the symptoms show. Avoid megadoses of vitamin D in supplement form.
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