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The main symptoms of vitamin D overdose which are those of hypercalcemia including anorexia , nausea, and vomiting. These may be followed by polyuria , polydipsia , weakness, insomnia, nervousness, pruritus and ultimately renal failure . Furthermore, proteinuria , urinary casts , azotemia , and metastatic calcification (especially in the kidneys) may develop.  Other symptoms of vitamin D toxicity include mental retardation in young children, abnormal bone growth and formation, diarrhea, irritability, weight loss, and severe depression.  
The best source of Vitamin D is safe, regular exposure to sunlight. Doctors recommend exposing the face and hands to direct sunlight at 10 minutes at a time at least once a day to boost Vitamin D production. Through regular sun exposure (10-15 minutes a day), humans can make their own Vitamin D in the form of D3. People should use caution, however, as sunscreen blocks the production of Vitamin D (since the skin does not get direct exposure to ultraviolet B light).
Sun exposure can cause skin care and other issues, so moderation is key. Exposure to sunlight for 20-30 minutes is estimated to result int he production of approximately 10,000 IU of vitamin D, although this will vary based on location, heritage, and other factors. Sunlight is the easiest method to acquire vitamin D; by contrast, a person would have to drink roughly 25 glasses of 8oz milk to get the same 10,000 IU of vitamin D.