The normal diet

Our body is made up of 5 main elements: water, proteins, fats, minerals, carbohydrates and vitamins which play an essential role. We need all these elements to live. They are mainly provided by food.


When we eat, food is digested, i.e. gradually transformed and reduced into substances that can be absorbed and used by the cells of our body: these are the nutrients.

Certain nutrients will produce energy, which is essential for all vital functions. These are: sugars (or carbohydrates) and fats (or lipids).

Proteins (protides) are mainly involved in tissue synthesis (growth), maintenance and renewal. “Excess protein can be broken down to produce energy. Other nutrients do not provide energy but are essential to our body: these are water, minerals and vitamins.

What is metabolism?

It is the set of chemical transformations of nutrients, which are necessary to produce energy and synthesise the body’s components. These chemical reactions are carried out by enzymes (a kind of “tool”). There are many different enzymes, one for each chemical reaction. The normal functioning of an enzyme consumes energy, and sometimes requires the presence of a cofactor (a substance that increases the functioning of the enzyme and is often a vitamin).

Protein metabolism

Proteins are made up of amino acids (like links in a chain). There are 20 different amino acids that make up the proteins of the human body. Some of these amino acids must be provided by the diet (essential amino acids), as the human body is unable to produce them. Proteins differ in their composition, i.e. the length of the chain and the order in which the amino acids are assembled.

Origin of amino acids in the blood

Most foods contain proteins. The digestion of proteins will result in the release of amino acids into the bloodstream (fig. 1). Amino acids circulating in the blood also come from the breakdown of proteins in the body, which must be constantly renewed when they are “used up”. Amino acids therefore have two origins, one exogenous (food) and the other endogenous (breakdown of body proteins).

Synthesis of the body’s proteins

Amino acids circulating in the blood (precursor amino acids) are mainly used for the synthesis of body proteins, also known as “anabolism”. This synthesis (which consumes energy) is essential for the growth and renewal of the body’s proteins.

Amino acid catabolism

Our daily diet is often “too rich” in protein and far exceeds the amino acid requirements for protein synthesis in the body. The “excess” amino acids are broken down to produce some energy. This is known as the “catabolism” of amino acids. This catabolism requires the intervention of different enzymes, usually with a specific enzyme for each amino acid, sometimes for a few amino acids. The intermediate products of the degradation of each amino acid are organic acids, the degradation of which will ultimately produce some energy.