Biochemistry - The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

Bioenergetics of Biochemical Reactions
Thermodynamics
Heat of Combustion and Caloric Value of "Physiological Fuel"

Enthalpy changes can be determined by directly measuring The amount of heat absorbed by a system at constant pressure. Consequently, the enthalpy changes accompanying The oxidation of nutrients have long attracted the attention of both chemists and physiologists. The heat of combustion (—∆Н_С) is usually determined from ∆E_с measured calorimetrically. Since ∆E_v and ∆Е_Р are nearly identical, ∆H_p = ∆AE_v + P∆V, where ∆V is the volume change that would occur if the reaction proceeded at constant pressure—a quantity that is easy to calculate. Because the parameter of interest is the ∆Н of the process in which the combustion products are carbon dioxide, Water, elemental nitrogen (N₂), and sulfur, a correction must be made for The formation of oxides of the latter two elements. Ultimately, values of ∆H_C can be obtained with high precision for both biochemical compounds and multi-component food products. In recent years, determining relatively small ∆Н values for various chemical or biochemical reactions using microcalorimeters has become a routine Procedure [6, 7].

In discussions related to Nutrition, the value — ∆Н_с is sometimes referred to as gross energy. The corresponding values are typically expressed in kilocalories; in biochemical literature, kilocalories are abbreviated as kcal, whereas in nutritional literature they are denoted as Cal.

Caloric value refers to the adjusted enthalpy of combustion (∆H_C) taken with the opposite sign. Because the enthalpy of combustion of food is always negative, the caloric value is expressed as a positive number. The caloric values of certain substances are given in the table below.

^a Nitrogen is converted into urea.

For proteins, the calculation is based on the assumption that nitrogen is converted not into N₂, but into urea (the primary nitrogenous waste product excreted by mammals).

Can a human or animal Organism be viewed—from a strictly thermochemical standpoint—as a catalyst for the combustion of foodstuffs? To answer this question, large calorimeters capable of housing a human or an animal were constructed. If an individual inside the calorimeter neither loses nor gains weight, the heat they dissipate should exactly equal the — ∆Н of combustion of the foods converted into CO₂, water, and urea. While the fact that this principle has been experimentally verified no longer surprises us, at the beginning of this century, when such experiments were first conducted, many scientists doubted whether The First Law of Thermodynamics applied to living organisms.

In practice, calorimetric measurements on animals involve numerous difficulties associated with the impossibility of precisely measuring the amount of heat released, uncertainties regarding stored nutrient reserves, and the necessity of applying corrections to ∆H_C due to the formation of Metabolic waste products destined for excretion. Nevertheless, such METABOLISM/26.html">Energy Metabolism measurements are crucial for nutritional science and medicine. Indirect calorimetry Methods have been developed to measure basal metabolic rates in humans. These topics are thoroughly covered in the book by White et al. [8].

Basal metabolic rate (BMR) refers to the rate at which an organism releases heat at rest, a significant amount of time¹ after a meal. Under these conditions, The Human Body derives energy from previously stored nutrients at an approximately constant rate of consumption. BMR is generally proportional to body surface area; for young women, this value averages ~154 kJ∙h^-1∙m^-2, and for young men, ~172 kJ∙h^-1∙m^-2. For a 70 kg individual, this corresponds to roughly ~320–360 kJ∙h^-1. Notably, 360 kJ∙h^-1 is equivalent to the power output of a 100-watt light bulb. Although metabolism varies significantly among individuals, excessive deviations in either direction indicate underlying disorders, such as a deficiency or excess of the thyroid hormone thyroxine. Metabolic rate drops slightly during Sleep and rises sharply during strenuous physical exertion. In some cases, human metabolic rates can reach up to 2500 kJ∙h^-1 (600 kcal∙h^-1). At a metabolic rate of 320 kJ∙h^-1 (76 kcal∙h^-1), a person requires 7680 kJ (1835 kcal) of energy every 24 hours to maintain basal metabolism, plus additional energy for muscular work. Routine office work or light housework approximately doubles the metabolic rate.