Isodynamics, Law of

the possibility of substituting certain nutrients for others in the diet in energy-equivalent quantities.

The concept of isodynamics was introduced by the German physiologist M. Rubner in 1883 to designate the interchange ability of nutrient substances in ratios that correspond to their heat of combustion. Thus, 1 g of carbohydrates can be replaced with 1 g of protein (oxidation of proteins and carbohydrates in the body liberates 17.2 kilojoules [kJ] per g, or 4.1 kilocalories [Kcal] per g) and 2.27 g of protein corresponds to 1 g of fat (oxidation of 1 g of fat liberates 39 kJ/g, or 9.3 kcal/g). However, a number of limitations must be observed in replacing some substances with others, since the nutrients must satisfy not only the energy requirements but also the plastic expenditures of the body. This concerns protein above all, the content of which in food must not go under a certain level. Carbohydrates and fats may replace one another within the limits of the capabilities of the digestive organs. Excessive inclusion of proteins in the diet creates an excess load for the liver, where their deamination takes place; excess fat leads to the formation of unoxidized metabolic products, which in turn leads to acidosis. Carbohydrates are the principal substances that compensate for the energy expenditures of the organism. Thus, isodynamic substitution is possible for substances that serve as an energy source when there are sufficient amounts of protein, vitamins, and minerals in the diet.