The non-naturals and humours are key to understanding medieval and early modern attitudes to diet and health so here’s a quick explanation:
The six non-naturals were categories in Ancient Greek medicine which you had to keep in balance in order to be healthy. They included: air; motion and rest (exercise); sleeping and waking; food and drink; excretion and passions/ emotions. An imbalance in any of these- too much/little exercise, too much/little sleep, too much/little etc. was thought to cause illness. Balance for health certainly isn’t a modern concept!
(If you want more information on this L H Curth has written a good article found here:http://bmj-mh.highwire.org/content/29/1/16.abstract).
The four humours were the basis of Medieval medicine and this continued into the Early modern period. They were: blood, yellow bile, black bile and phlegm. Each of these humours equated to qualities: blood= hot & wet, yellow bile= hot & dry, black bile = dry & cold, phlegm= cold& wet. These in turn related to the four temperaments: sanguine, choleric, melancholic and phlegmatic.
Foods were also thought to have hot/cold/dry/wet qualities and so diet had to be balanced to ensure that the body wasn’t taking in too much of one humour.
Imbalance led to illness, including melancholy…