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The main difference between vitamin D and vitamin D3 is that vitamin D3 is just one of the five types of vitamin D. The different forms include vitamin D1, D2, D3, D4 and D5. The two main forms of vitamin D, however, are vitamin D2, which is also known as ergocalciferol; and vitamin D3, also known as cholecalciferol. Vitamin D can refer to both of them, or to just one of them.
Vitamin D can obtained by consuming either plants or animal products, such as mushrooms, eggs, meat and fish. Vitamin D3, on the other hand, can be obtained only from animals or through exposure to ultraviolet light from the sun or from tanning beds. When the rays hit the skin, it reacts with 7-dehydrocholesterol, which is a precursor to cholesterol, and it is changed into vitamin D3.
Both vitamin D and vitamin D3 can be obtained from supplements. Vitamin D can also be obtained from many vitamin D-fortified foods such as cereal, dairy and juice. Vitamin D3, however, is not usually available from fortified foods. Instead, as a form of a supplement, it usually is available only in the form of a liquid or a pill.
A vitamin D deficiency can result in weak and brittle bones in the elderly, which is referred to as osteoporosis. In young children it can result in rickets, which is also a disease of the bones. The reason for this is that vitamin D helps the body to absorb calcium and balances the levels of calcium and phosphorus in the body. Both minerals are essential for strong bones, but when phosphorus levels rise too high, it can result in weak and brittle bones. Taking both vitamin D and vitamin D3 can also help to protect the body against cancer, a weak immune system, influenza, depression and anxiety.
One cannot overdose from vitamin D and vitamin D3 when they are consumed naturally from food or when they are produced by ultraviolet light. An overdose, however, can occur when it is taken in the form of supplements. The recommended daily intake is only 400 international units (IU) per day. Symptoms of an overdose can include dehydration, constipation, vomiting, loss of appetite and fatigue. The long-term side effects of taking vitamin D supplements might include heart disease and premature aging.