Vitamin D is essential for maintaining the calcium properties and a healthy body operation, because of its actions in the intestine, kidney, bone and glands. Receptors for this hormone can be found in almost all body tissues.

The main source of vitamin D is activated by means of the exposure to ultraviolet B, diet (which accounts for only 20% of the source of the needs, but mainly helps elderly people, institutionalized people and those living in temperate climates). Inadequate levels of vitamin D nowadays are considered a public health problem and because of this, the exact measurement of vitamin D in the body is of a great clinical importance, as this helps in the diagnosis of diseases related to the lack of this component.

Diet is a way to balance the nutrient levels. Meet food abundantly offering this vitamin:

• Fish liver oils (mainly of tuna and cod);

• Eatable parts of high fat fish (sardines, mackerel and tuna);

• Salmon;

• Mammalian liver;

• Eggs;

• Dairy products;

• Mushrooms;

• Fortified cereals.

Vitamin D acts differently in every part of the body, playing key roles in the body:

• Homeostasis of calcium: homeostasis is the ability to maintain the internal environment in constant equilibrium, regardless of changes occurring in the external environment. Calcium homeostasis, a role of vitamin D, contributes to bone growth and maintain bone density;

• Bone metabolism: Vitamin D deficiency is characterized by mineralization or demineralization of the skeleton, which can cause greater risks of developing osteoporosis and other bone health issues;

• Proliferation and cell differentiation: uncontrolled cell proliferation associated with specific mutations can lead to diseases such as cancer;

• Immune system: increased immunity, which hinders the development of autoimmune diseases such as insulin-dependent diabetes mellitus, multiple sclerosis, rheumatoid arthritis, systemic lupus erythematosus and inflammatory bowel disease.

• Neoplasms: Vitamin D deficiency in the body can help in the development of some types of cancers (breast, colorectal and prostate);

• Neuromuscular system: there is evidence that vitamin D participates in two important aspects, namely muscular strength and balance. It exercises roles that involve functions from the protein synthesis to the kinetics of muscular contraction, which help in the accomplishment of fast movements, avoiding falls. Moreover, lack of vitamin D can cause weakness and muscle pain in children and adults;

• Insulin secretion: it may decrease the risk of type-2 mellitus diabetes;

• Cardiovascular system: it has a say in the control of the heart function and blood pressure;

• Brain: it presents actions that boost the neural growth and modulators of the brain development, exerting, therefore, actions of regulation in the development and in brain functions.