The most pressing problem for humans throughout their history has been the basic one of securing food to satisfy hunger and food-energy needs. But the fact that the human population in different parts of the world (despite subsisting on different diets), seemed before about 1900 to experience approximately the same level of health had led some physiologists to believe that all foods were rather similar (McCollum 1957). In fact, this view was rooted in the ancient “many foods-single aliment” concept of Hippocrates and Galen. By the turn of the twentieth century, however, views were rapidly changing, and although knowledge of the specialized roles of amino acids, vitamins, and minerals was hazy and often contradictory, proteins, fats, and carbohydrates (starches and sugars) could be distinguished and analyzed in foods and diets. Thus, with the advances in food analysis and refinements of nutritional needs the “many foods-single aliment” concept was no longer tenable.
Heat and Life
Because the human body became cold after death, it was not surprising that some mechanism of heat production came to be considered synonymous with life at an early stage in human history. Hippocrates (born 460 B. C.) noted in one of his aphorisms:
Growing bodies have the most innate heat; they therefore require the most food for otherwise their bodies are wasted. In old people the heat is feeble and they require little fuel, as it were, to the flame, for it would be extinguished by much. (Lusk 1933:8)
The Greeks explained the mechanisms of health and illness in terms of the four “humors” in the body. When the body was in health, these humors were in balance, whereas excess or deficiency caused illness. Special foods or drugs could restore the proper balance and, hence, health. It was also observed from the earliest times that when an adult partook of a great deal of food, he or she did not necessarily gain in weight. Hippocrates believed this was due to a constant loss of insensible perspiration and the elimination of heat, which he conceived to be a fine form of matter.
Half a millennium later, Galen (130-200 A. D.) was more specific than Hippocrates in drawing a direct analogy between flame and innate heat, writing that “[t]he blood is like the oil [of a lamp], the heart is like the wick and the breathing lungs an instrument which conveys external motion” (Lusk 1933:12).
The Middle Ages saw few developments in the area of nutrition, but during the Renaissance, Leonardo da Vinci (1452-1519) again took up the analogy of a flame to refer to life by stating that “where there is life there is heat” and “where flame cannot live no animal can sustain its existence” (Lusk 1933:18).This remark was later echoed by Robert Boyle in 1660, who wrote that “neither a flame nor an animal can live in a vacuum” (Lusk 1933: 25). A contemporary of Boyle, John Mayow (born 1643), who died at the age of 36, appeared to have an understanding of respiration and the role of blood (that was only rediscovered a century later) when he wrote:
Respiration consists furthermore in the separation of the air by the lungs, and the intermixture with the blood mass of certain particles absolutely necessary to animal life, and the loss by the inspired air of some of its elasticity. The particles of the air absorbed during respiration are designed to convert the black or venous blood into the red or arterial. (Lusk 1933: 35)
Development of Scientific Concepts
Fuel was long understood as burning to produce heat or power, but an understanding of the importance of air in supporting combustion and respiration had to await identification of the gases. Later, when burning was recognized as the combining of fuel with oxygen and the giving off of carbon dioxide and heat, the respiration of animals and humans was accepted as a special case of combustion. Although many still believed that a “vital” or God-given force controlled all life processes, quantitative relationships began to be developed between individual foods and their use in the body to produce predictable amounts of heat.
The concept of heat remained central to the explanatory framework of traditional physiology. Every part of the body had its own innate heat, and the heat in the extremities was continuously being dissipated into the surrounding environment. This heat had to be replenished from some source in the body, which was identified as the heart, and the heat, together with the beating of the heart, generated the “vital spirit” out of air and blood. The vital spirit was, in turn, carried by activated arterial blood.
This view was replaced following the discovery of circulation by William Harvey in 1628. In the process of searching for the origin of body heat, later investigators tried to decide whether the heartbeat was involved with producing it or whether the motion of the blood created friction, and therefore heat, as it passed through the vessels of the body. The lungs were considered by Harvey to represent an important heat-dissipating mechanism that prevented the cardiac generation of heat from exceeding the body’s tolerance.
The Discovery of the Gases
Physiology prospered in the early years of the eighteenth century but during its latter part lost prominence to chemistry. By then, chemists had developed analytical methods to at least begin to determine the ultimate constituents of body fluids and tissues (Kinney 1988).
With the work of Joseph Priestley (1733-1804) oxygen was discovered (1774).
Antoine Laurent Lavoisier, in turn, recognized that carbon dioxide was a compound of carbon and oxygen and further demonstrated that the respiration of animals involved CO2.A table from the 1790 English translation of his text Traite elementaire de chimie indicates that the names oxygen, hydrogen and azote (nitrogen) were replacing several older terms for the gases. By use of an ice calorimeter (a device whereby heat produced from a living animal could be estimated from the amount of ice that was melted), Lavoisier was able to demonstrate that respiration was very slow combustion and that predicted heat agreed well with that which was measured.
World History
