A man wouldn’t be able to generate productive thoughts if he ate too much and too often, because his forces would be used in digestion, and there wouldn’t be any left for thinking. Schiller, Shakespeare and many other writers lived on very little food. The mind is never so clear as after long fasting. The greatest saints lived on fruit, bread and water, and no miracles were ever done on a full stomach.
When a man works on himself he harmonizes his temperaments, but until then a melancholic pupil should eat fruit, so that its sun forces permeate the solidifying and rigidifying element in melancholics. Phlegmatics shouldn’t eat black roots because they would only increase his inner love of ease. Whereas a sanguine would benefit by eating root vegetables. One could almost say: A sanguine must be fettered to his physical body by food, otherwise he might fly away. The ego is predominant in cholerics, so they should avoid hot spices and stimulating food.
Source: Rudolf Steiner – GA 266 – ESOTERIC LESSONS 1: Number 56 – Unknown place and date