Review of Measuring Metabolic Rates: A Manual for Scientists

Abstract

Animal calorimetry quantifies the heat resulting from the intricate metabolic combustion process termed “the fire of life” by pioneering nutritionistMax Kleiber (1975). This fire is expressed in joules, watts, or calories, with the latter being preferred by us rebarbative old-school types who resist taking the “calorie” out of calorimetry, so to speak, because “the calorie defined in terms of mass and difference in temperature is the most directly understandable and therefore most useful unit” (Kleiber 1972, p. 309). But typically, the respiratory gases oxygen (O2) and carbondioxide (CO2) are actually measured, as each liter of O2 consumption represents∼4.7–5.0 kcal ofmetabolic energy transfer depending on how much CO2 is released per liter of O2 uptake. Accordingly, O2 consumption and CO2 production are the respirometric signals of indirect calorimetry, the predominant approach to measuring metabolic rates, which is the title of John Lighton’s book, now in its second edition (Lighton 2019). The book’s emphasis is on the technology, techniques, equations, and data analytic methods used in respirometry, matters to which Lighton brings the hard-won knowledge of one who has been there, done that, translated his knowledge into a thriving calorimetry business, and successfully distilled his knowledge into the book that I wish had existed when I first entered the field. The equations for converting gas exchange into heat energy (Weir 1949; Kaiyala et al. 2019) owe their biophysical validity to Hess’s law of constant heat summation, the intuitively elusive but enormously powerful concept that the total heat energy released in a chemical reaction does not depend on its number of steps or temperature. Hence, the coupling of O2 uptake and CO2 release with the resulting heat yields in the low-temperature catalytic combustions thatpoweranimal life can literallybeworked outbyequating thegas exchangesandcaloricyieldswhensuddenly combusting carbohydrates, fats, and proteins in high-temperature bomb calorimeters (Kleiber 1975).