Energy

We return now to the role of carbohydrates and fats. After the realization that carbon, hydrogen, oxygen, and nitrogen together make up the great bulk of food materials, and the development of reasonably good analytic methods for their determination by about

1830, attempts were made in England to assess what quantities of carbon and nitrogen were required for an adequate diet. (Hydrogen and oxygen could be supplied in the form of water.) There was a demand at this time for objective, scientific answers to the problem of providing food for prisoners and inmates of poorhouses that would support health without being superior to what was within reach of honest, working laborers. The first estimate was that the average man required at least 300 grams of carbon and 15 grams of nitrogen per day.

It had been demonstrated as early as the 1780s, by Antoine Laurent Lavoisier and others, that respiration and combustion were similar processes, with carbon dioxide being an end product of each. By 1870 it had been shown that the heat produced by the metabolism of food in the body was the same as that produced by the combustion of food in the laboratory. This made it possible to transfer the physicists' new thinking about the conservation of energy and the mechanical equivalent of heat to nutrition. The contribution of carbohydrates and fats to the diet was then expressed not as so many grams of carbon, but as so many kilocalories of energy that would be released if they were metabolized to carbon dioxide and water in the body. (The amount of heat required to raise the temperature of 1 kilogram of water by 1°C is 1 kilocalorie. This is the unit used in nutritional literature, but commonly written as Kcal, the K indicating that the unit is the kilogram rather than the gram unit.)

It was possible to estimate the energy value of foods by burning them in an atmosphere of pure oxygen (to make complete combustion more rapid), using a submerged metal casket (usually called a "bomb"), and measuring the heat released from the increase in temperature of the surrounding water. This "gross" figure needed correction for the proportion of the food that was indigestible. With ordinary human foods, this was only a small correction, and for practical purposes values of 4 Kcal per gram for carbohydrates and 9 Kcal per gram for fats have been found adequate. Protein not needed for new tissue synthesis and broken down also yields approximately 4 Kcal per gram. Energy requirements were found to vary from as few as 2,000 Kcal per day in sedentary men to as many as 4,000 Kcal in someone doing long hours of physical work. Women, in general, had lower requirements in proportion to their smaller muscular frame. In practice, more than 90 percent of the food we eat is needed as fuel, that is, as a source of energy, rather than for the replacement of worn-out tissue.

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