Nitrogen balance and protein requirements

In the course of the discussion, describing the qualitative characteristics that the initial diet should have, we have estimated the protein requirement by multiplying a coefficient, which varies according to the type of athletic discipline practised, by the body weight. The value thus obtained, however, is, as the product of an estimate, only approximate. There is a way to obtain a more accurate value for one’s protein requirements, although, let’s be clear, it is not immediately usable. The methodology by which it is possible to obtain the correct value, or at least a much more accurate estimate of protein requirements, is based on the concept of the nitrogen balance. Let us now look at this method in detail. In dealing with protein, it has been clearly explained that the need for this nutrient is linked not only to energy requirements, but, above all, to plastic and transport needs. The need for regular dietary intake is in itself explanatory of the fact that the proteins used for these purposes need to be periodically replaced. This replacement process is called protein turnover. Since the proteins to be disposed of are mainly eliminated through the urine, knowing the amount of urinary proteins makes it possible to know how much protein to take in with the diet so that the balance between eliminated and introduced proteins is zero, negative or positive. A slightly negative balance will indicate prevailing catabolic phenomena, while a positive one, on the contrary, will mean greater anabolism; therefore those who wish to lose muscle mass (e.g. marathon runners) will have to keep a slightly negative balance, while those who wish to increase it will have to look for a slightly positive one. Well, to know the amount of protein disposed of through urine, it will be necessary to measure the amount of nitrogen in it. Nitrogen is in fact an element that makes up 16% of protein. In other words, in 100g grams of protein there is 16g of nitrogen. Knowing, therefore, the amount of nitrogen present in urine, it is possible, by means of a simple calculation, to find out how much protein has been disposed of via this channel. The mathematical formula, based on the above proportion, is as follows:

$Proteins\;eliminated\;through\;urine\;(g)\;=\;Urinary\;nitrogen\;(g)\;\times\;6,25$

Now, as has been said, most nitrogen is excreted via the urine, but there is also a very small amount that is eliminated via other routes (skin, hair, nails, menstrual blood, faeces, semen, breast milk, etc.) and which, for a more accurate calculation, should be taken into account. For this reason, in order to obtain the total protein loss, the number 3 is usually added to the measured urinary nitrogen when calculating. This number is an estimate of the amount, expressed in grams, of nitrogen disposed of by secondary routes. The complete formula therefore results:

$Total\;protein\;removed\;(g)\;=\;(Urinary\;nitrogen\;(g)\;+\;3)\;\times\;6,25$

Once the calculations are complete, all that remains is to make one’s own considerations about the desired nitrogen balance, whether positive, zero or negative, and manipulate the diet accordingly.