In adult life, when the processes of digestion and assimilation are active, the amount of food may without harm, be in excess of the actual needs of the body. This is true, however, only so long as active muscular exercise is taken.
In advanced life the tissue changes are slow, digestion is less active, and the ability to assimilate food is greatly diminished. Growth has ceased, the energy which induced activity is gone, and the proteids are no longer required to build up worn-out tissues. Hence, as old age approaches, the quantity of nitrogenous foods should be steadily diminished.
Experiment 64. Obtain a sheep's bladder and pour into it a heavy solution of sugar or some colored simple elixir, found at any drug store. Tie the bladder carefully and place it in a vessel containing water. After a while it will be found that an interchange has occurred, water having passed into the bladder and the water outside having become sweet.
Experiment 65. Make a hole about as big as a five-cent piece in the large end of an egg. That is, break the shell carefully and snip the outer shell membrane, thus opening the space between the outer and inner membranes. Now put the egg into a glass of water, keeping it in an upright position by resting on a napkin-ring. There is only the inner shell membrane between the liquid white of the egg (albumen) and the water.
An interchange takes place, and the water passes towards the albumen. As the albumen does not pass out freely towards the water, the membrane becomes distended, like a little bag at the top of the egg.
162. Ill Effects of a too Generous Diet. A generous diet, even of those who take active muscular exercise, should be indulged in only with vigilance and discretion. Frequent sick or nervous headaches, a sense of fullness, bilious attacks, and dyspepsia are some of the after-effects of eating more food than the body actually requires. The excess of food is not properly acted upon by the digestive juices, and is liable to undergo fermentation, and thus to become a source of irritation to the stomach and the intestines. If too much and too rich food be persistently indulged in, the complexion is apt to become muddy, the skin, especially of the face, pale and sallow, and more or less covered with blotches and pimples; the breath has an unpleasant odor, and the general appearance of the body is unwholesome.