Vitamin D is a group of fat-soluble prohormones, which provide the absorption and metabolism of calcium and phosphorous. It is not actually an essential dietary vitamin in the strict sense, as it can be synthesized in adequate amounts by all mammals from sunlight exposure (an organic chemical compound is scientifically called a vitamin when it cannot be synthesized in sufficient quantities by an organism, and must be obtained from their diet).
Vitamin D is a nutrient needed for health and to maintain strong bones. It helps the body absorb calcium from food and supplements. That's the main reason that calcium supplements often contain vitamin D
Vitamin D is important to the body. The immune system needs vitamin D to fight off invading bacteria and viruses, muscles need it to move, nerves need it to carry messages between the brain and other body part. Together with calcium, vitamin D helps protect older adults from osteoporosis.
We take vitamin D into our body in foods and supplements and our body produce it also after sunlight exposure. Very few foods naturally have vitamin D. Mushrooms provide some vitamin D. Most of the natural sources are animal-based like fish such as salmon, tuna and mackerel, beef liver, cheese and egg yolks provide small amounts.
People who get too little vitamin D may develop soft, thin, and brittle bones, a condition known as rickets in children and osteomalacia in adults. Yet even without symptoms of bone pain and muscle weakness, too little vitamin D can pose health risks like increased risk of death from cardiovascular disease and severe asthma in children. Higher blood levels of vitamin D are associated with a lower risk of colon and breast cancer in some age groups.