The term vitamin was introduced in 1913 by C. Funk and indicates substances that the body cannot produce by itself but that have to be introduced with food or by having certain behaviors such as, for example, sun exposure, as required by vitamin D, a fat soluble vitamin that is produced through the skin after the exposure to sunlight or thanks to special foods. Sun exposure is the main source of vitamin D, hands, arms and face should be exposed to the sun for about fifteen minutes, a little more in winter, it would be better without sunscreen, which, by filtering the rays, reduces the production of vitamin D, and in the open air, not behind glass, so a walk in the nature will be a good choice. In fact, in addition to ensuring vitamin D, helps also keep the body active. In a smaller quantity vitamin D can also be introduced through special foods such as blue fish, offal, egg yolks, milk and milk derivatives, dried fruits, mushrooms, beans and green leafy vegetables. Vitamin D guarantees the health of the skeleton by promoting the absorption of calcium and phosphorus and their attachment to the bones. It helps to prevent fractures and osteoporosis and, in children, is necessary for bone formation. When synthesized from the sun, vitamin D improves mood, stimulates well-being, supports the immune system and reduces hunger. Moreover, vitamin D benefits the brain as it is related to a lesser risk of developing diseases such as Alzheimer's, on the contrary, its severe lack would increase more than twice the risk of Alzheimer. This correlation between vitamin D and Alzheimer's disease has been demonstrated by scientific studies such as that published in 2014 on the Neurology magazine by a team at the University of Exeter Medical School, UK.