Throughout the first half of the twentieth century, a series of experimental observations, many of them from Richter’s laboratory, raised questions about what is meant by the concepts of hunger, thirst, and palatability, and how they might reflect metabolic and physiological needs. For example, rats whose adrenal glands are removed cannot retain salt, and will die within two weeks on their usual diet, from the consequences of salt depletion. If given a supply of salt in their cages, however, or given the choice of drinking salt water or pure water, they will choose either to eat or to drink the salt and, by doing so, keep themselves alive indefinitely. These rats will develop a “taste” for salt that did not exist prior to the removal of their adrenal glands. Rats that have had their parathyroid glands*132 removed will die within days of tetany, a disorder of calcium deficiency. If given the opportunity, however, they will drink a solution of calcium lactate rather than water—not the case with healthy rats—and will stay alive because of that choice. They will appear to like the calcium lactate more than water. And rats rendered diabetic voluntarily choose diets devoid of carbohydrates, consuming only protein and fat. “As a result,” Richter said, “they lost their symptoms of diabetes, i.e., their blood sugar fell to its normal level, they gained weight, ate less food and drank only normal amounts of water.”

The question most relevant to weight regulation concerns the quantity of food consumed. Is it determined by some minimal caloric requirement, by how the food tastes, or by some other physical factor—like stomach capacity, as is still commonly believed? This was the question addressed in the 1940s by Richter and Edward Adolph of the University of Rochester, when they did the experiments we discussed earlier (see Chapter 18), feeding rats chow that had been diluted with water or clay, or infusing nutrients directly into their stomachs. Their conclusion was that eating behavior is fundamentally driven by calories and the energy requirements of the animal. “Rats will make every effort to maintain their daily caloric intake at a fixed level,” Richter wrote. Adolph’s statement of this conclusion still constitutes one of the single most important observations in a century of research on hunger and weight regulation: “Food acceptance and the urge to eat in rats are found to have relatively little to do with ‘a local condition of the gastro-intestinal canal,’ little to do with the ‘organs of taste,’ and very much to do with quantitative deficiencies of currently metabolized materials”—in other words, the relative presence of usable fuel in the bloodstream.†133