Voit and M. Pettenkofer were leaders in the new studies of respiration and energy balance, at first using small chambers for animals. A much larger apparatus was then constructed in which both the total carbon excretion (breath, urine, and feces) could be measured and the quantity of individual foods actually burned in the human body could be determined. This was a remarkable technological achievement and a significant scientific advance. On the death of Pet-tenkofer in 1901,Voit (quoted in Kinney 1988) wrote:
Imagine our sensations as the picture of the remarkable process of the metabolism unrolled before our eyes and the mass of new facts became known to us. We found that in starvation protein and fat alone were burned, that during work more fat was burned and that less fat was consumed during rest, especially during sleep; that the carnivorous dog could maintain himself upon an exclusive protein diet, and if to such a protein diet fat were added, the fat was almost always entirely deposited in the body; that carbohydrates, on the contrary, were burned no matter how much was given and that they, like the fat of the food, protected the body from fat loss, although more carbohydrates than fat had to be given to effect this purpose; that the metabolism in the body was not proportional to the combustibility of the substances outside the body, but the protein which burns with difficulty outside, metabolizes with the greatest ease, then carbohydrates, while fat, which readily burns outside, is the most difficult to combust in the organism (Kinney 1988: 524).
Max Rubner (1854-1932), who was trained in the laboratories of Voit, determined standard food energy (caloric) values for the major foodstuffs. He continued to demonstrate, until well into the twentieth century, that the animal body followed the law of the conservation of energy.
W. O. Atwater (1844-1907), an American who had studied under Voit in Germany, returned to the United States to work with a physicist at Wesleyan University in developing a large calorimeter capable of measuring precisely the amount of heat given off by a person living in it.1 This calorimeter confirmed the earlier experimentation of Voit, Pettenkofer, and Rubner and demonstrated that the energy expended by a person in doing any work, such as static bicycle riding, was equal to the heat released by the metabolism of food in the body. The physiologist F G. Benedict extended Atwater’s work by constructing special equipment at the Carnegie Institute in Boston for the simultaneous measurement of gaseous metabolism and heat production. An example from one of their studies is shown in Table IV. C.3.1. On a daily basis, the difference between heat determined and heat estimated was only -1.6 percent. The precision obtained in these experiments was remarkable and was obtained without computer assistance!
The finding from the calorimeter work that received the most attention in Atwater’s lifetime concerned alcohol. In 1899 he reported that if a subject drank alcohol in small portions over the course of the day, it was almost fully oxidized and replaced the food energy equivalent of either fat or carbohydrate. At this level, therefore, it acted as a food (Atwater and Benedict 1899). This finding was advertised by the liquor trade, and Atwater was subsequently attacked by temperance advocates as a disgrace to his church and to his profession (Pauly 1990). He defended himself vigorously and, while agreeing that abstinence was certainly the safest way to avoid addiction, argued nevertheless that for most people,
Moderate consumption was possible and seemed to improve their well-being rather than damage it (Carpenter 1994b). (We have heard recent echoes of this debate following recommendations in the Dietary Guidelines for the United States concerning alcohol consumption.)
Further related studies undertaken by Atwater (1899) and his associates (Atwater and Bryant 1900), which overlapped those of Rubner, made determinations of the fuel value of foods by the use of a bomb calorimeter and by comparing those determinations with the heat produced in the body. A bomb calorimeter permits the complete combustion of the sample in the presence of oxygen under pressure, with the direct determination of the heat produced by measurement of the temperature rise in the surrounding water. A diagram of Atwater’s bomb calorimeter is shown in Figure lV. C.3.1.
It was found that the total (gross) energy produced by combustion of food with oxygen in a bomb calorimeter was greater than the metabolizable energy of the same food when utilized in the body. However, when account was taken of digestibility (that is, not all the food consumed was absorbed into the body) and of the energy lost in the urine (that is, the urea produced from protein), then there was essential agreement. Fuel values of foods were compared with their analytical composition, and thus were born the 4:9:4 Atwater factors, where the metabolizable energy of foods, in kilocalories per gram, could be readily calculated from the contents of carbohydrate, fat, and protein. These concepts were reported, essentially in their present form, in the texts of R. Hutchison (1903) and H. C. Sherman (1914).
Figure IV. C.31- The Atwater bomb calorimeter: (A) heavy steel bomb filled with oxygen under pressure; (B) capsule inside bomb containing weighted sample; (C) weighed amount of water at known initial temperature; (D) thermometer of high accuracy; (E) electric ignition wire. (From Sherman 1952.)
The unit used today for food energy, deriving from these and other basic physical studies, is the kilocalorie (kcal).This is the amount of heat necessary to raise 1 kilogram of water from 15° to 16° Celsius. The international unit of energy is the more scientifically fundamental joule (J). Although not greatly used in the United States in a nutritional context, the joule is widely employed in Europe. To convert from kilocalories to kilojoules (kJ), the rounded value 4.2 may be used (1 kcal = 4.184 kJ).The classical Atwater metabolizable energy conversion factors of 4 kcal/gram (g) of food protein and carbohydrate and 9 kcal/g of food fat have been verified and are adequate for computation of the energy content of customary diets (Miles, Webb, and Bodwell 1986). Alcohol (ethanol) has a computed food energy value of 7 kcal/g or 5,600 kcal/liter (L). It should be noted that in the original Atwater calculations, carbohydrate was measured by difference (that is, only protein, fat, and minerals were determined in the food, and the residue after allowing for water content was considered to be mainly carbo-hydrate).This, therefore, included fiber.
World History
