The albumen which is taken with the food undergoes, after digestion, and the assimilation of the elements necessary for tissue construction, retrogressive changes, and it is finally thrown into the blood in the form of urea. This urea must be excreted by the kidneys, and therefore if an excess of albuminous food is taken, a great burden is thrown upon the kidneys, which may result in producing disease in these organs. In youth, when growth is rapid, a much larger amount of nitrogenous food is required than in old age, when tissue change is slow. Also, those engaged in hard labour require and can dispose of without detriment to their health, a larger amount of nitrogenous food than those who live sedentary lives.

Gelatine is derived from bone and fibrous tissues by boiling. It has the property of solidifying into a jelly on cooling. It approaches an albuminate in chemical composition, and is rich in nitrogen. It cannot, however, replace albumen in the repair of tissue waste, as it rapidly undergoes change in the body and is eliminated as urea. But just because it undergoes changes so rapidly and easily in the body it may be taken as a substitute for albuminates when stronger foods containing albumen cannot be tolerated. This is the rationale of the use of beef-tea - which is simply gelatine - in cases of acute sickness, when meat cannot be digested. It also contributes to force production.