NUTRITION SCIENCE. — It is the study of the nutritional needs of humans and, in a broader sense, also of animals and plants, and of the ways in which they may be adequately satisfied.
I. HISTORICAL PREMISES
From the moment of man’s appearance on earth, the determining force behind human actions must have been the reproduction and preservation of the species and the search for daily sustenance; hence an uninterrupted succession of migrations of peoples, wars, revolutions, and so forth. The principal features of primitive civilizations were determined by the activities undertaken by man to procure food: first hunting, fishing, and the gathering of wild plant foods; subsequently, pastoralism and agriculture. Many years were nevertheless required before his nutritional needs were scientifically defined.The oldest evidence demonstrating man’s interest in the nutritive qualities of foods comes from Egypt and Babylonia and dates back 4,000 and 5,000 years. Later, in 460 B.C., in Greece, Hippocrates was the first to associate the study of medicine with that of nutrition; it was only after the beginning of the Christian era, however, with Galen, that this association produced practical results.
The earliest information concerning the transformations that food undergoes in the organism is due to Fernel (1543), van Helmont (1577), and Sanctorius (1614); but major progress came after the discovery of oxygen. It was Lavoisier (1777) who explained the importance of this gas for living organisms and measured the degree of combustion in human subjects, thus laying the foundations of animal calorimetry. The relations between combustion and foodstuffs could subsequently be established more precisely through an exact knowledge of the chemical composition of the latter; Beccari (1728) and Liebig (1842), together with his school, contributed to this development. One of the latter’s pupils, Voit (1866–73), established that protein requirements depend on organized mass, whereas lipid and carbohydrate requirements depend on the activity of the organism. The foundations of the science of nutrition had thus been laid; scientists such as Rubner, Atwater, Müller, Lusk, Chittenden, Mendel, Sherman, Mc Collum, Albertoni, Rose, and others contributed to its further development.
II. CURRENT CONCEPTIONS
The following investigations aim at achieving the objectives of the science of nutrition: 1) those concerning the constitution and function of foods; 2) those concerning the energy and material transformations (metabolism) that take place in the organism in dependence on its various functional states.1. The chemical and functional study of foods
This has made it possible to identify and classify the following nutrients in foods:a) energy-yielding nutrients, which supply chemical energy usable by the organism and transformable principally into heat and work (they include carbohydrates and lipids; proteins only secondarily); b) plastic nutrients, which supply the material indispensable for the growth and maintenance of tissues (they are represented by proteins and their amino acids, but carbohydrates, lipids, and mineral elements also serve the same purpose, albeit in limited quantities); c) nutrients that maintain the integrity and efficiency of vital processes as a whole (vitamins and mineral salts belong to this group). d) Water and oxygen, indispensable for the occurrence of all reactions, must be added to these nutrients.
2. The study of energy and material metabolism
This has made it possible to consider the alimentary reaction under two aspects which, although for convenience treated separately, must nevertheless be considered as a unit:a) energy aspect. Every vital activity requires an expenditure of energy, which takes place at the expense of the chemical energy of foods; this corresponds to that released by them in an organism that utilizes them, and amounts to 4.1–9.3–4.1 calories, respectively, for each gram of carbohydrates, lipids, and proteins. It is this energy that, during vital activity, is converted into internal work, defined as basal metabolism, and into external or muscular work which, added to the former, expresses total energy metabolism. The values of this metabolism have been determined in the various functional states; they are expressed in calories and define the energy aspect of dietary requirements, which varies with age, sex, work, etc.
b) Qualitative aspect. Equal energy value does not, however, correspond to equal biological value in a ration, which is conditioned by various indispensable factors linked to the law of the minimum.
A minimum amount of proteins, or rather of certain amino acids essential to the organism, which lacks the capacity to synthesize them, must always be introduced with food. To assess the importance of the protein aspect of nutrition, it is enough to recall the highly variable structure of proteins (which accounts for their high specificity), and the fact that enzymes, numerous hormones, viruses, antibodies, etc., are protein in nature. Lipids and carbohydrates likewise belong among the factors for which a minimum cannot be exceeded below, since below that level metabolism is altered.
Since numerous inorganic elements are found in the organism in the form of soluble and insoluble salts, they must be introduced into it from outside.
There is scientific justification for maintaining that, in order to ensure the organism’s mineral requirements (including those of elements which, in trace amounts, perform important functions), it is sufficient to ensure the intake, in adequate quantities, of four elements—calcium, phosphorus, iron, and iodine—which, when deficient, alter many vital manifestations.
Vitamins are substances of the most disparate structures which, even in extremely small quantities, regulate the course of highly delicate physiological processes; many of them form part of enzymes (complex substances that catalyze the countless chemical transformations taking place in the organism). Not all vitamins that have been isolated and chemically identified are recognized as important for human nutrition; the best known are: vitamin A or axerophthol, vitamin B₁ or aneurin, vitamin B₂ or riboflavin, vitamin PP or niacin, vitamin C or ascorbic acid, the antianemic vitamin or folic acid, vitamin D or the antirachitic factor, vitamin E or tocopherol, vitamin K or the antihemorrhagic factor, vitamin P or the permeability factor.
The human organism does not synthesize them; they must therefore be supplied to it with food. As a result of the chronic deficiency of one or more of these limiting factors, the organism is subject to particular morbid conditions caused by nutritional deficiency, designated as deficiency diseases.
It has now been fully demonstrated how, and to what extent, developmental anomalies, growth defects, and intellectual delays are connected with quantitatively and qualitatively inadequate diets. It is a diet balanced in all its constituents that permits the organism to function at the highest level of efficiency and performance. Advances in the field of the science of nutrition also influence the agricultural and commercial activities of every country with regard to the production, distribution, and preservation of foodstuffs, with consequences no less profound than those produced in the fields of medicine, individual hygiene, and social hygiene.