The nitrogen balance

Dietary proteins contain an average of 16% nitrogen and it is this element that characterizes and distinguishes them from other nutrients.

Nitrogen removed from amino acids is incorporated into ammonia, a compound that is toxic to the body and is eliminated in the urine in the form of urea (10 ÷ 35 g); inside the same nitrogen is also present in ammonia (0.34 ÷ 1.20 g), in uric acid (0.25 ÷ 0.75 g) and in creatinine (1.2 ÷ 1.8 g). Further ways of elimination are represented by defecation, skin desquamation, hair and hair loss, menstruation, lactation and ejaculation.

To assess the protein needs of an individual, the nitrogen balance must be taken into account, or rather the value that tells us how much nitrogen has been retained by the body

Nitrogen balance = Ingested nitrogen - Nitrogen eliminated

In an adult individual in physiological conditions the nitrogen balance is always in equilibrium as the organism manages 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 body's demands, about 250 grams of these proteins are demolished and re-synthesized 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 statural and muscular growth. In these circumstances, due to increased nitrogen retention, the nitrogen balance becomes positive.

In case of reduced protein intake with the diet the body tends to decrease nitrogen losses instead. However, below a certain critical level of assumption this regulation is no longer valid and, by continuing to lose nitrogen, the balance becomes negative.

The nitrogen balance is positive during:

the accretion;

the pregnancy;

lactation;

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 with urine is initially reduced, then stabilizes and then after a certain period returns to increase.

In fasting protein, at first it reduces and then stabilizes around a minimum value.

In this case we talk about wear quota or minimum expenditure of nitrogen defined as the minimum urinary elimination of nitrogen when energy needs are guaranteed by carbohydrates and lipids.

Some hormones such as testosterone, GH and IGF-1 promote muscle mass gain and make the nitrogen balance positive; others, known as stress hormones (cortisol, ACTH and prolactin) tend to make it negative, favoring protein catabolism.