Factors Which Influence Normal Metabolism

FACTORS WHICH INFLUENCE NORMAL METABOLISM Fasting.—During fasting the body draws upon its own re serve of stored material for its requirements in the production of energy, and the rate of breakdown varies with the energy require ments. An individual kept warm in bed, therefore, stands fasting longer than one who is compelled to take exercise in a cold place. The breakdown of tissue during the early days of a fast is much greater than later, for as the fast progresses the body becomes more economical in its utilization of tissue. During a fast the tissues do not waste at an equal rate ; those which are not essen tial are utilized at a much greater rate than those which are essen tial to the maintenance of the organism. For instance, it has been shown that during a fast the skeletal muscles may lose over 4o% of their weight, whereas an essential organ like the heart loses only some 3%.

The essential tissues obtain their nourishment from the less essential by ferment action, a process which has been termed auto digestion or autolysis. The autolytic products of tissue digestion are practically identical with those which arise during the ordinary gastrointestinal digestion.

Muscular Work.

The muscular tissue plays the most im portant part in general metabolism. Not only is muscle the most abundant tissue present, but it is constantly active and is the great energy-liberating machine of the body. Formerly it was be lieved on the authority of Liebig that muscular work was done at the expense of the protein material, but it has been conclusively shown that the real source of energy in moderate work is the non protein material, carbohydrates and fats ; of these the former plays the greater part in a man on ordinary diet. That there is some influence on protein is shown by the fact that there is a definite but small increase in the output of nitrogen in the urine as the result of muscular work, but it is in no sense proportionate to the work done; whereas the output of carbon dioxide produced by the combustion of the carbohydrates and of the fats is in creased proportionately to the work done.

Internal Secretions.

Evidence is accumulating to show that the activities of the various tissues of the body are presided over and controlled not merely by the action of the nervous system but also by certain chemical substances elaborated in special glands known as endocrine glands or glands of internal secretion. The

active secretions of these glands are poured out into the blood stream. We have direct evidence that some of these secretions like thyroxin from the thyroid gland and insulin from the pancreas exercise a very active influence on metabolism. As regards the other secretions we know more about their action on various tis sues than of their direct influence on metabolism.

While we know comparatively little of the intermediate stages in the breakdown of the food constituents, and more particularly of the protein moiety, our knowledge of the final products of the metabolic changes excreted is fairly full. The urine is the main channel of excretion for the nitrogenous waste products. Water is excreted by the lungs, the kidneys and the skin.

The urine is a yellowish fluid which varies greatly in its depth of colour, from pale amber to a deep brown. It has a specific gravity of about 1020, varying with the percentage of solids in solution, and it usually has an acid reaction. Among the principal organic substances present are urea, ammonia, purins (uric acid and the so-called purin bases, xanthin, etc.), creatinine, conju gated sulphates, various aromatic bodies and many other sub stances in small amount, together with the water and inorganic salts.

The following table from Folin gives a good idea of the aver age composition of the urine as regards the nitrogen-containing constituents, and its variation according to the nature of the diet when this is free of creatin, creatinine and the precursors of the purins : Urea, which forms the chief nitrogenous constituent, amount ing on an ordinary diet to about 3og. per diem, is for the most part formed in the liver, from ammonia obtained either directly from the blood after absorption from the intestine, or resulting from the deaminization of the amino acids. There is at present a controversy as to the true chemical nature of this substance.