The term “balanced diet” had a very different meaning two hundred years ago. Well into the nineteenth century, nutrition relied on the ancient theory of humors. Humoral theory is based on the idea that the body is regulated by four fluids: blood, yellow bile, black bile, and phlegm. Each humor has its own qualities: blood was thought to be hot and moist; yellow bile, hot and dry; black bile, cold and dry; and phlegm, cold and wet. One’s predominant humor was thought to determine one’s personality or temperament.
According to humoral theory, diseases and disorders were caused by an imbalance in humors, and many doctors thought such imbalances could be corrected through diet. Foods were thought to have qualities that corresponded to the humors, and therefore could offset imbalance. For example, to cure depression, which was caused by an excess of black bile (the cold and dry humor), doctors may have prescribed beef, which was thought to be hot and moist.
By the end of the nineteenth century, the humoral theory had been abandoned in favor of food chemistry, but even that provided an incomplete picture of the nutrients people need to survive.