The Talmud teaches that people should eat enough to fill a third of their stomachs, drink enough to fill another third, and leave a third empty...It may seem absurd, but it's less absurd than a lot of diet advice, and lofty metaphorical visualizations like this may be better physically and psychologically than fussing over calories and carbohydrates and latching onto the latest report of a scientific study somewhere. This is a realization that extends beyond diet advice. It's a more general idea about the role religion plays for people who are not able or willing to put the time into long, brooding studies of moral philosophy.
Rashi, a medieval French rabbi, interpreted the Talmud to mean that the final empty third is necessary so that the body can metabolize emotions. If one ate until one’s belly was completely full, there’d be no room left to manage one’s emotions and one would burst asunder.
However absurd this may seem to us today, it made physiological sense in the premodern world as the emotions were considered physical things that, like food and drink, were metabolized by the body. A body stuffed with food and drink is full only of biology; it leaves no room for biography, for what makes us human.
ADDED: Maybe Rashi's "burst asunder" referred to vomiting. Presumably, that drink that filled a third of the stomach was alcoholic (in the old days, before water was a reliably healthy drink). With a third food and a third wine in your stomach, piling on more risks losing it all — a waste. You don't need the scientific method to arrive through observation and experience at the idea that one third of the stomach should be left free.
Quite aside from the problem of vomiting — which would be much worse when food was not abundant — there is the sluggishness of mind that we all experience when we've eaten too much. You don't need to know any physiology about blood going to the stomach or whatever to come up with advice about eating less so you can manage your mental processes.