It is only comparatively recently—during the last quarter of a century—that physi ologists have satisfied themselves, that in order to supply the wants of the system, food must consist of a combination of these groups; and that animals which are fed exclusively on food belonging to one of the groups—as, for example, albumen or fibrin—perish under symptoms of inanition, as certainly as if they had been deprived of all nourishment. As in the infant's milk we find the albuminous, oleaginous, sac charine, and saline groups represented by the caseine, butter, sugar, and salts, so, for other periods of life, the food, whether derived from the animal or vegetable kingdom, must contain a due (although not necessarily the same) proportion of the different types. Again, in judging of the nutritive value of any kind of food, we must take into consideration its digestibility. Thus, the experiments made by Dr. Beaumont on Alexis St. Martin, a Canadian, in whom there was, in consequence of a gun-shot wound, a fistulous opening leading from the exterior to the interior of the stomach, and recent experiments made by Busch on a woman with a fistulous opening into the jejunum (see ALIMENTARY CANAL), show, for example, that bard-boiled eggs, meat that has been boiled for a long time, and hard cheese, which is poor in fat, are less easily and rapidly digested than soft-boiled Of fresh eggs, meat steeped in vinegar, or moist, fat cheese, and that starch is much more readily converted into sugar when boiled than in the raw form.
As the nitrogenous constituents of the food (articles containing albumen, fibrin, etc.) are chiefly employed in the formation of the blood and the reproduction of the tissues, it was at one time thought that the quantity of nitrogen which any kind of food con tained might be taken as a measure of its nutritive value; but this test is not altogether to be relied on, since the nitrogen in part depends upon the gluten-yielding matters, which probably contribute little or nothing to the formation of textures.
Both daily experience and chemico-physiological observations show us that the best kinds of food contain both fat and carbo-hydrates, in addition to albuminous mat ters. Instinct teaches us to combine highly amylaceous foods with fats; as, for example, bread and butter, beans and fat bacon; and the increased digestibility of such mixtures proves, no less than the simultaneous occurrence of fat and sugar in the milk and in the ego-fluids, that both substances are necessary, as independent ingredients of food, although, perhaps, one may temporarily serve as a substitute for the other. Unfortunately, we have no trustworthy data for enabling us to determine the proportion in which the different nutrient groups should be combined, so as to form the food best suited to the general wants of the organism. The proportions occurring in human milk are the following: 10 parts of plastic or blood-forming matter (caseine), 10 parts of fat (butter), 20 parts of a carbohydrate (sugar), and 0.6 of a part of salts. For the wants of adult life, a lesser ratio of plastic matter (albumen, etc.) would probably suffice.
The absolute quantity of food required for the maintenance of the human body in health varies so much with the age, sex, and habits of the individual, and with the cir cumstances in which he may be placed, that it is impossible to fix a standard applicable to every case; but from the consumption of food by large bodies of healthy men, such as our soldiers and sailors, we can determine with tolerable accuracy the average daily quantity. We extract the following remarks from Todd tk, Bowman's Physi ological Anatomy and Physiology of Man. "Each seaman in the British naval ser vice is allowed from 31 to 351 ounces of dry nutritious food daily, of which 26 ounces are vegetable, and the rest animal; the latter consisting of 9 ounces of salt meat, or 41 of fresh. Sugar and cocoa are also given. The soldier is allowed a pound of bread, and three quarters of a pound of meat. In most of the London hospitals, full diet, which is given to convalescent patients who need a liberal diet, consists generally of half a pound of meat, with from 12 to 14 ounces of bread, half a pound of potatoes, a pint of milk, and sometimes beer or porter—a pint of the former, or half a pint of the latter. The former dietary is destined for men who must be in readiness for the most active athletic exercises, requiring not only great muscular strength, but also considerable power of enduring fatigue. The latter is intended to recruit the powers of those who have been suffering from disease. If, now, we compare with these a dietary which has been found sufficient for the support of health in a state of more or less confinement, with a moderate amount of daily labor, we may fairly infer that the proper allowance for persons not engaged in actual manual labor lies between these extremes. In the union workhouses of England, able-bodied men obtain about 25 ounces of solid food daily, of which the quantity of meat does not exceed 5 or 6 ounces. In prisons, it has been found necessary to give a certain amount of animal food to prisoners who are sub ject to hard labor. Each of such prisoners, if confined to a term exceeding three months, and kept at hard labor, has a daily allowance of about 36 ounces of food, of which meat constitutes only a very small portion—namely, about 16 ounces in the week. The prisoner has obviously the advantage of the poor man, whose only crime is poverty; but there is doubtless sufficient justification for this in the fact, that the labor of the prisoner, and the mental depression which long-continued restraint and confinement induce, render a greater amount of nutriment necessary than the indigent would require, who seek in the workhouse a shelter from absolute want." For further details on this subject, which is one of great practical importance, we may refer to a paper by Dr. Lyon Playfair, " On the Food of Man under Different Conditions of Age and Employment," published in 1854, in the 56th volume of the Edinburgh New Philo sophical Journal, and to a memoir by Dr. E. Smith upon "Prison and other Dietaries." See also Dr. Pavy's Treatise on Food and Dietetics (211 ed. 185).