Forced feeding, therefore, may be defined as any caloric excess over the ordinary nutritive requirements of the body, and is always associated with a retention of nitrogen, not necessarily in the form of muscular tissue. Every gram of retained nitrogen is equal to 625 grams of albumin, which quantity is contained in 29.4 grams of muscle tissue. But it is known that the retained nitrogen is not incorporated with the active protoplasm of the body, because it does not contribute to the consumption of oxygen, nor on the other hand can it be found in blood. For this reason von Noorden has promulgated the doctrine that it is stored as a reserve stock of albumin in the cells, just as fat and glycogen are, and it may be that it has a different construction from the ordinary protoplasm of the cells. As a point of practical importance, it is worth noting that casein supplies proportionately more of this retained nitrogen than the protein of eggs, meat, vegetables or fruits, and as much as from 2 to 6 grams may be accumulated daily, although after cessation of the overfeeding the unutilised excess is rapidly dissipated as urea.

There is not a single scrap of evidence in support of the conception that "flesh" or muscular tissue is increased by overfeeding, but it is certain that a reserve stock of protein, fat, and glycogen must first be accumulated in the body before any attempt can be made to promote the actual growth of muscle. This can only be effected by active use of the muscles, and Zuntz has shown that systematic exercise is accompanied by a loss of weight, chiefly due to the consumption of fat, but the muscles are increased in size, and as a concomitant there is a greater increase in the consumption of energy.

Flesh Formation

Actual tissue development or flesh formation is known to take place in the body during growth, during pregnancy and lactation, during convalescence from such disease or illness as has occasioned loss of flesh, and, as a result of everyday experience, in those who are performing more muscular work than usual. Overfeeding per se is unable to accomplish this. Actual exercise of the muscles is necessary to stimulate their growth, and this may even occur when the protein ingested is less than the nutritive requirements of the body would warrant. But a certain excess of nutriment must be at the disposal of the body to enable it to develop in the most efficient manner, both as to its muscles and for the accumulation of adipose tissue.

Development or even efficient maintenance may be inhibited by various toxins as well as by a deficiency in the supply of reserve material, and many asthenic conditions may be caused simply by the removal of those toxins without supplying an excess of nutriment. It is a mere accident that the administration of iron should cure anaemia, and certainly the amelioration produced is not due to the addition of some missing factor in the diet, but because of the stimulation of the haematogenic properties of bone marrow, an event which may be equally well accomplished by arsenic. This plainly shows that anaemia is not due to a deficiency of iron in the food so much as to an inability of the tissues for some reason to utilise it.

Tissue-building is really a potentiality inherent in the cells. It may be diminished by lessening the supplies of nutriment, but it is doubtful whether it can really be increased by over-nutrition any more than by providing a maintenance diet.

It is a very different matter with the adipose tissue, which can be increased within limits in any degree that is considered advisable. It is only a question of augmenting the quantity of food beyond the nutritive requirements of the body, and after certain deductions have been made, the surplus becomes fat. Eight per cent. of the energy of the surplus food is utilised in the processes of digestion and absorption, 4 per cent. remains unabsorbed, 10 per cent. is stored in the cells as reserve protein, and an unascertainable but small quantity becomes glycogen. All the rest of the surplus food becomes fat, and should it amount to 930 calories, would mean a deposit of 100 grams of fat.