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SECTION I-FUNCTIONS AND CONSTITUENTS OF FOOD.
WITHOUT entering into a discussion of the various chemico-physical changes which food undergoes in the living body, it may be broadly asserted that its ultimate destiny is the development of heat and other modes of motion, which together constitute the physiological phenomena of animal life. The potential energy with which the food is stored becomes converted into actual or dynamic energy, and is manifested in the body as heat, constructive power, nervo-muscular action, mechanical motion, and the like. But as food also supplies the materials which are requisite for the development and maintenance of the living fabric, as well as for the display of its various kinds of active energy, it may be inferred that inorganic and organic substances are both necessary. The organic alone are oxidisable, or capable of generating force, while the inorganic, though not oxidisable, are essential to the metamorphosis of organic matter which takes place in the animal economy.
The organic constituents of food are generally divided into nitrogenous, fatty, and saccharine compounds; and the inorganic into water and saline matters. Both classes of constituents are present in all ordinary articles
of diet, whether they be derived from the animal or vegetable kingdom.
1. Functions of the Nitrogenous Constituents.-The nitrogenous constituents consist of albumen in its various forms, fibrine, syntonin or muscle-fibrine, casein, gluten, legumin, and other allied substances, such as gelatine. Their chemical composition is remarkably uniform, and, as they seem all capable of being reduced by the digestive process to a like condition, they can replace each other in nutrition, though not to an equal
Up to a comparatively recent period, it was believed that nitrogenous constituents must first be converted into tissue before their dynamical energy can be elicited; in other words, that muscular force is entirely dependent on the metamorphosis of muscular tissue, and that urea, being the product of the change, ought to be regarded as a measure of the force. This was the doctrine taught by Professor Liebig, and it was generally accepted by physiologists until Drs. Fick and Wislicenus of Zurich published their famous experiments connected with their ascent of the Faulhorn. While these experiments proved that a non-nitrogenous diet will sustain the body during severe exercise for a short period, and without any notable increase in the amount of urea, the more carefully-conducted experiments subsequently made by Dr. Parkes showed that possibly the amount of urea is even lessened. If this view were confirmed, Dr. Parkes' inference would be rendered highly probable-the inference, namely, that muscle, instead of oxidising during labour, and becoming wasted by losing nitrogen, does in reality appropriate nitrogen, and grows, and that its exhaustion does not depend so
much on decay for the time being, as on an accumulation of the oxidised products of other food-constituents within its tissues. He takes care to point out, however, that in the long run some decay of muscle does take place, and that the amount of nitrogen must be increased as the work increases.
Judging from these and other experiments, it would therefore appear that, although the main functions of the nitrogenous constituents of food are the construction and repair of the tissues, they exercise other important functions of a regulative and dynamic nature not well defined. There is no doubt that a certain portion of them is directly decomposed in the blood, and so far they contribute to the maintenance of animal heat and the development of dynamic energy; but the experiments of Pettenkofer and Voit also tend to show that the nitrogenous substances composing the tissues determine the oxidation of the other constituents, or, in other words, that no manifestation of force is possible without their participation in the process.
2. Functions of the Fatty Constituents.-The fact that food containing a large proportion of fatty ingredients is invariably used by the inhabitants of cold countries, indicates that these constituents play an important part in the maintenance of animal heat. Indeed, it has been proved by experiment that the respiratory or heat-producing powers of fat are twice and a half as great as those of the other hydrocarbons, as starch or sugar. Fat also takes an active share in the conversion of food into tissue, and aids the removal of effete products from the system. The experiments already alluded to likewise show that its oxidation in the blood generates to a great extent the force which
is rendered apparent in locomotion or manual labour. Further, its distribution in the tissues gives rotundity to the form, serves to retain animal heat by its non-conducting properties, and greatly facilitates the working of the various parts of the living machine by lessening friction and preventing jarring by its elasticity.
3. Functions of the Saccharine Constituents or Hydrocarbons. These constituents comprise cellulose, starch, and sugar; and, like the fatty constituents, are directly subservient to the maintenance of animal heat and the production of animal force. Starch is for the most part converted into dextrine, and by a further oxidation generates carbonic acid, which is given off by the lungs. As already stated, the heat-producing powers of these constituents are much inferior to those of fat, but they are capable of being converted into fat in the system, and are largely concerned in carrying on the digestion of nitrogenous substances.
4. Functions of Water and Saline Matters.-The principal functions of water in the animal economy are the solution and conveyance of food to different parts of the system, the removal of effete products, the lubrication of the tissues, the equalising of the body temperature by evaporation, and the regulation of the chemical changes which take place in the processes of nutrition and decay. Saline matters, on the other hand, are the chief media for the transference of the organic constituents throughout the body. They are largely concerned in the consolidation of the tissues, and are supposed to convert unabsorbable colloids into highly diffusive crystalloids.
The functions of what are called the accessories of food, such as beverages, stimulants, etc., are still matters of speculation.
That all these four classes of constituents should be present in a well-arranged dietetic scheme is alike taught by experience and proved by experiment. No single class is capable of sustaining life by itself, although it is certain that health can be maintained for some time on a diet consisting of the nitrogenous, fatty, and saline matters.
The separate amounts and relative proportions of the several classes of constituents required in a standard diet for a healthy male European adult, of average size and weight, and performing a moderate amount of work, are given in the following table :—
Although no single standard will meet all cases, the above, which is given by Dr. Parkes, and quoted by him from Moleschott's numbers, is found to accord fairly with the observations of numerous other physiologists.
SECTION II.-NUTRITIVE VALUES OF FOOD.
As the phenomena of nutrition depend mainly on the chemical interchanges of nitrogen and carbon with oxygen, different articles of diet have been estimated according to the amount of nitrogen and carbon which they contain. But inasmuch as the actual value of