Vitamin D3 helps the kidneys eliminate phosphate to maintain the blood's pH, improves overall health and is used to treat underactive parathyroid glands, osteoporosis and low levels of phosphate in the blood, notes Everyday Health. Children with vitamin D3 deficiency are at risk of developing rickets, and the bones of adults with chronic deficiency tend to become softer.
People with genetic conditions that prevent them from processing parathyroid hormones also take vitamin D3, reports Everyday Health. People with certain digestive diseases, including some involving the liver, often have deficiencies of vitamin D3, although the deficiency itself is not necessarily a cause of the condition.
Vitamin D3 is made by the body when the skin is exposed to sunlight, and the other type of vitamin D, vitamin D2, is found in food, according to Everyday Health. Vitamin D3 in supplement form is thought to be more natural and easier to absorb than vitamin D2. People who spend most of their time inside without being exposed to sunlight often suffer from vitamin D2 deficiency. People with darker skin require more sunlight to produce vitamin D3. Vitamin D is found in foods fortified with it, such as milk and cereal, and fish like salmon and cod.