Isodynamia

ISODYNAMIA (from Greek isos—equal and dynamis—force), equivalence of force, a term first introduced by Rubner in 1883 when formulating the so-called “law of isodynamia” of food substances. Considering the process of metabolism from an energetic point of view, Rubner first provided in his calorimetric studies on animals proof of the applicability of the law of conservation of energy to the animal organism. To establish an energy balance, he had to calculate its income and expenditure, expressing both in identical units, and as such, a calorie was naturally taken. Since complex organic substances of the body (proteins, fats, and carbohydrates) undergo complete oxidation (carbohydrates, fats) or incomplete oxidation (proteins) in the animal organism, consequently, the reserve of energy contained in these substances in the form of potential chemical energy is released in the organism. The reserve of potential energy is easily determined calorimetrically by burning substances in a Berthelot bomb (see Calorimetry). Thus, knowing the quantity of proteins, fats, and carbohydrates destroyed in the organism, and knowing their heats of combustion, it is easy to determine the energy income. The energy expenditure in an animal is determined with sufficient accuracy by accounting for its heat loss in a respiration calorimeter, which was first carried out by Rubner on dogs. In doing so, he established the equality of energy income and expenditure. Subsequently, similar studies with the same result were conducted by Atwater (USA) on humans in a correspondingly constructed respiration calorimeter (see Calorimetry), wherein not only heat loss was taken into account, but also the expenditure for mechanical work performed in the apparatus. Proceeding from the energetic conception of the role of food in the organism, Rubner, in his work on the laws of energy expenditure in nutrition (1902), proved that food substances in rations replace one another in quantities equal in the energy contained within them—"isodynamic"; thus, if 100 g of carbohydrate contains 410 calories (latent chemical energy), then the isodynamic quantity of fat (1 g of which contains 9.3 calories) equals 44 g (410:9.3=44). The mentioned studies by Atwater seemed to also confirm Rubner's idea. Thus, in one of the experiments, a subject over two periods, of 22 days each, showed the following energy balance (on average per day): Energy produced (in calories): in the form of muscular work—543 (I period), 550 (II period); heat formation—4,593 (I period), 4,555 (II period); Sum—5,136 (I period), 5,105 (II period). Energy income: proteins—434 (I period), 489 (II period); fats—1,288 (I period), 3,190 (II period); carbohydrates—3,371 (I period), 1,465 (II period); Sum—5,093 (I period), 5,144 (II period).

From this example, it is evident that under the same conditions (work), the same amount of energy is consumed, regardless of whether this energy is drawn mainly from fats, as in the second case, or from carbohydrates, as in the first. However, from the very beginning, an exception had to be made in the law of isodynamia with respect to proteins, which must always be present in food in a certain definite quantity to cover protein losses (the wear-and-tear coefficient—Abnutzungsquote—of Rubner). This quantity of proteins could not be replaced by any calorically equivalent combination of other food substances, and consequently, the law of isodynamia is valid only with respect to the nitrogen-free parts of food—fats and carbohydrates—and to the excess quantity of proteins. Furthermore, more detailed studies of metabolism discovered that the replacement of food carbohydrates with fats immediately causes an increased breakdown of proteins in the organism; this indicates that the organism cannot utilize fats with the same result as carbohydrates: the former supply energy significantly more slowly than the latter, which forces the organism to cover its energy needs at the expense of the compensatory breakdown of the proteins of its own body. An illustration of what has been said is the following experiment by Kayser with a person weighing 67 kg. Thus, the law of isodynamia, as well as the evaluation of food solely by calories in general, cannot at the present time be considered well-founded. Calorically equivalent substances can produce different "physiological" effects in tissues due to differences in their physical-chemical states and chemical properties or by virtue of differences in intermediate products formed during metabolic reactions. Furthermore, it should always be remembered that the organism's need for food substances is determined not exclusively by the need for energy, and furthermore, in relation to energy, the organism in different cases clearly prefers the consumption of certain food substances over others: for example, when performing work, carbohydrates are consumed first (the respiratory quotient rapidly approaches 1), although from the point of view of the law of isodynamia, strictly speaking, it would be indifferent to the organism from which substances it draws energy.