Food proteins contain on average 16% nitrogen and it is this element that characterizes them and distinguishes them from other nutrients.
To assess the protein needs of an individual, the nitrogen balance must be taken into consideration, that is the value that tells us how much nitrogen has been retained by the "organism.
Nitrogen balance = Nitrogen ingested - Nitrogen eliminated
In an adult individual in physiological conditions the nitrogen balance is always in equilibrium as the organism is able to regulate the elimination according to the introduction. The more nitrogen is ingested, the more it is eliminated.
In an adult man the proteins contained in the body tissues amount to about 5 kilograms. Every day to meet the demands of the organism about 250 grams of these proteins are demolished and resynthesized according to a process called protein turnover.
In particular moments of life, the protein synthesis of the human body increases to cope with anabolic processes such as body and muscle increase. Under these circumstances, due to the increased nitrogen retention, the nitrogen balance becomes positive.
In the event of a reduced intake of protein with the diet, the body tends to decrease nitrogen losses. However, below a certain critical intake level, this regulation is no longer valid and, continuing to lose nitrogen, the balance becomes negative. .
The nitrogen balance is positive during:
the "growth;
the pregnancy;
breastfeeding;
intense physical activity.
The nitrogen balance is negative during:
absolute and / or protein fasting
the presence of pathologies
In absolute fasting, the elimination of nitrogen in the urine is initially reduced, then it stabilizes and then after a certain period it returns to increase.
In fasting protein at first it reduces and then stabilizes around a minimum value.
In this case we speak of the amount of wear and tear or minimum expenditure of nitrogen defined as the minimum urinary elimination of nitrogen when the energy needs are ensured by carbohydrates and lipids.
Some hormones such as testosterone, GH and IGF-1 favor muscle mass gain and make nitrogen balance positive; others, known as stress hormones (cortisol, ACTH and prolactin) tend to make it negative by promoting protein catabolism.
