Vitamin D is a group of biologically active substances that enter the body with food (D-2) or are produced by exposure to sunlight (D-3). 

The main role of the vitamin is to enhance the absorption of trace elements in the intestine: calcium to a greater extent, iron, magnesium, phosphorus and zinc to a lesser extent. 
In other words, without the help of vitamin D, calcium from food will simply be removed from the body. 25-OH vitamin D is the substance that is formed from all forms of vitamin D (D2,3, etc.), so the determination of this metabolite is the most accurate representation of the total amount of vitamin D in the body.

To understand the importance of vitamin D, it is worth mentioning the functions that calcium performs: 

• Neuromuscular impulse transmission;
• Blood clotting processes (important for stopping bleeding); 
  
• Heart muscle function; 
• Production and activation of a huge number of enzymes involved in almost all biologically significant processes of the whole organism;
• Formation of strong bony tissue. 

Vitamin D is especially important in early childhood, as it is during this period that bone structures are formed. 
If the child does not receive an adequate amount of vitamin, rachitis develops.