Medieval concepts of a healthful diet were largely based on theories from antiquity, especially the idea that the body's four humors, or bodily fluids—blood, phlegm, yellow bile and black bile—must be balanced through nutrition. It was believed that the humors were connected to the elements and had different properties—for instance, blood was associated with air and considered hot and moist, and phlegm was associated with water and was cold and moist. All foods were classified according to the humor they were thought to affect, and the diet was adjusted according to what fluid was thought to be lacking (or overabundant).
According to Food in Medieval Times, a 2004 book by Melitta Weiss Adamson, Arab physicians of the middle ages expanded on these ideas by also ascribing to foodstuffs an intensity, from "weak" to "extreme" (Galen, the ancient Greek physician, first described this system of " gradus" but applied it only to medicines, not foods.)
Haly Abbas, a medieval Persian author of medical texts, distinguished between "remedial foods" and "pure foods," the difference being that remedial foods "change the body until the body gains power over them and transforms them into its own nature," while pure foods "are those which the body changes and transforms into itself." His list of remedial foods included lettuce, onions and garlic.
One of the most popular books of dietetics was written by a Christian physician in Baghdad, Ibn Butlan, and translated into Latin as Tacuinum sanitatis, or "table of contents of health." It includes descriptions of the natures and degrees of various foods: cherries are considered cold in the second degree and moist in the third, roasted meat is hot and dry, and veal is hot and moist (I wonder how these were determined—I haven't been able to find any information on the subject).
According to Ken Albala's Eating Right in the Renaissance, diet books proliferated in the two centuries after the invention of the printing press, in 1440. "Courtly dietaries," intended for courtiers who were frequently required to attend lavish banquets, included both advice and recipes but were not very restrictive (some even offered remedies for drunkenness). One 15th century author, Marsilio Ficino, actually advised drinking human blood, writing, "Why shouldn't our old people, namely those who have no recourse, likewise suck the blood of youth?" This idea did not catch on, it appears.
It wasn't until the 16th century, Albala writes, that diet books began forbidding certain foods, notably cakes and other sweets, that were considered gluttonous. Other foods, like onions, leeks and garlic, were proscribed for (presumably educated) readers because they were "appropriate for barbers and journeymen."
It would be another three centuries before the modern concept of dieting for weight loss took hold. Check back soon for the final chapter in the series.