The prevailing dietary wisdom of the 16th century, as presented in these medical guidebooks, relied on two assumptions: first, that the process of digesting foods was actually a form of cooking. Indeed, cooking stood as the basic metaphor for the systems that sustained all life. Seeds were cooked into plants; when the plants appeared above the ground, the heat of the sun cooked them into ripe fruits and grains. If humans gathered these foodstuffs, they could cook them further to create edible dishes. Finally, the internal heat of the body turned the food into blood. The body then expelled as feces what was not digestible. Excrement joined putrefying dead animals and plants to begin the life cycle again.
The second assumption about food and health in this scenario involved maintaining a proper equilibrium of bodily fluids by eating a suitably balanced diet. Doctors and chefs of the time believed that four fluids, or humors, circulated in the body: blood, phlegm, yellow bile and black bile. These humors corresponded to the four Aristotelian elements–air, water, fire and earth. Because blood was hot and moist, it corresponded to air; phlegm was cold and moist and thus resembled water; yellow bile was hot and dry, similar to fire; black bile was cold and dry, connected to earth.
Ideally, the human body was slightly warm and slightly moist, although in practice the exact balance varied from individual to individual, depending on variables such as age, sex and geographic location. Older people were believed to be colder and drier than younger ones; menstruating women colder and wetter than men; southern Europeans more hot-blooded than their neighbors to the north. The perfect meal, like the perfect human temperament, was slightly warm and slightly moist, but combinations away from this center could be used as mild dietary correctives to warm and moisten the elderly, dry out the moister sex, and calm down the southerner or perk up the northerner.
https://www.scientificamerican.com/article/birth-of-the-modern-diet-2006-12/