What is Calorimetry. Explanation

Liebig, as a good mechanist, believed that carbohydrates and lipids were the fuels of the body just as they would be fuel for a bonfire if thrown into one. This marked an advance over Lavoisier's views of a half-century earlier (see page 47). Lavoisier had then been able to speak of carbon and hydrogen only, whereas now one could speak, more specifically, of the carbohydrates and lipids which were made up of carbon and hydrogen (plus oxygen)

Liebig's views naturally encouraged attempts to determine whether the amount of heat obtained from such fuel in the body was really the same as that obtained if the carbohydrates and fats were simply burned in ordinary fashion outside the body. Lavoisier's crude experiments had indicated the answer would be "yes," but techniques had been refined since his day and it was now necessary to put the question more rigorously.

Devices to measure the heat released by burning organic compounds were developed in the 1860s. Berthelot (see page 51) utilized such a device ("calorimeter") to measure the heat produced by hundreds of such reactions. In an ordinary calorimeter, such as that which Berthelot used, a combustible substance is mixed with oxygen in a closed chamber and the mixture is exploded by means of a heated electrified wire. The chamber is surrounded by a water bath. The water absorbs the heat produced in the combustion and from the rise in the temperature of the water, one can determine the amount of heat that has been released.

In order to measure the heat produced by organisms, a calorimeter must be built large enough to hold that organism. From the amount of oxygen the organism consumes and the amount of carbon dioxide it produces, the quantity of carbohydrate and lipid it "burns" can be calculated. The body heat produced can be measured, again by the rise in temperature of a surrounding water jacket. That heat can then be compared with the amount that would have been obtained by the ordinary burning of the same quantity of carbohydrate and lipid outside the body.

The German physiologist, Karl von Voit (1831-1908), a student of Liebig's, together with the German chemist, Max von Pettenkofer (1818-1901), designed calorimeters large enough to hold animals and even human beings. The measurements they made seemed to make it quite likely that living tissue had no ultimate energy source other than what was available in the inanimate universe.

Voit's pupil. Max Rubner (1854-1932), carried matters further and left no possibility of any remaining doubt. He measured the nitrogen content of urine and feces and carefully analyzed the food he fed his subjects in order that he might draw conclusions as to the proteins as well as the carbohydrates and lipids. By 1884, he was able to show that carbohydrates and lipids were not the only fuels of the body. Protein molecules could also serve as fuel after the nitrogen-containing portions were stripped away. Allowing for protein fuel, Rubner was able to make his measurements that much more accurate. By 1894, he was able to show that the energy produced from food-stuffs by the body was precisely the same in quantity as it would have been if those same foodstuffs had been consumed in a fire (once the energy content of urine and feces were allowed for). The law of conservation of energy held for the animate as well as the inanimate world, and in that respect at least there was no room for vitalism.

These new measurements were put to work on behalf of medicine. A German physiologist, Adolf Magnus-Levy (1865-1955), beginning in 1893, measured the minimum rate of energy production ("basal metabolic rate" or "BMR") in human beings and found significant changes in diseases involving the thyroid gland. Thereafter, measurements of BMR became an important diagnostic device.