Vitamin D is one of the fat-soluble vitamins. It is found in milk, salmon, eggs, cheese, fortified cereals, beef, and tuna. It can also be made in the body. When a person is exposed to sunlight, vitamin D is produced in the skin. Once it is produced in the skin, it must be converted to its most active form in the body by the liver and kidneys. Once activated, it stimulates the absorption of calcium and phosphorus in the gut. This vitamin is essential for the maintenance of adequate calcium levels in the body. Calcium is a mineral which is necessary for maintaining strong bones. Vitamin D may also have a role in cancer prevention. Some studies have shown that people with lung and colon cancer are often vitamin D deficient. However, it is still unclear if low vitamin D levels have a causal relationship with cancer. More studies are needed to fully evaluate the relationship between vitamin D and cancer. The recommended daily intake of vitamin D for adults aged 51 years and older is 400 units per day. Taking too much vitamin D can cause nausea, vomiting, constipation, weakness, loss of appetite, and weight loss.