It's true that meditation and concentration exercises will be the main thing for our spiritual striving, but when the elaboration of the astral body begins, the food that an esoteric eats will be of some importance.
It's especially important to avoid alcohol in every form. The bad effect of alcohol on the brain function has been scientifically shown, and knowledge of spiritual things is made completely impossible through its use. It's inadvisable to eat meat and fish.
Mushrooms are very harmful; they contain hindering lunar forces, and everything that arose on the old Moon signifies rigidification. Likewise legumes aren't very advisable because their nitrogen pollutes the etheric body. Proteins make mastery of sexual passions difficult. Sugar promotes independence, and should be avoided by egotistical people.
If someone wants to undergo training in thinking, he mainly needs a well-constructed, healthy brain apparatus. Since present-day parents seldom give their children such well-built brains, one needs help to strengthen one's brain apparatus. And here it's mainly filberts that supply the brain-building substance. All other nuts are of less value and peanuts should be avoided altogether. Milk butter is the best fat. Coffee supports logical thinking, but doesn't make one a logical thinker by itself. Drinking too much coffee leads to hysteria in people who don't think much. One can get good ideas by drinking tea or by doing special exercises.
It's especially important for an esoteric to lead a life of moderation. An ancient sage said: Moderation purifies feelings, awakens ability, cheers one up and strengthens memory; the soul loses most of its earthy weight and thereby enjoys greater freedom. 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 (Schwarzwurzel) 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.
A master doesn't need solid food, and temperaments no longer influence or control him. He uses the choleric temperament to do his magic deeds, he lets the things of the physical world pass by him like a sanguine, he'll behave like a phlegmatic in his enjoyment of life and he'll brood about his spiritual findings and experiences like a melancholic. But it'll take us awhile to get that far, so we should try to bring our whole life into harmony with our spiritual striving. You only get as much out of life as you put into it.
- Rudolf Steiner, no date or place given