Say a person has recently become vegetarian. Then the following specific process will take place in the lower organisation. Certain material forces will change into spiritual forces. When these spiritual forces are not used, they will work in a harmful way and may even affect the brain activity negatively.
If this person does not become active in a different way than, for example, a banker or an ordinary scholar, it can be very damaging. He must learn to utilise the forces that become available through his vegetarian lifestyle, by assimilating spiritual ideas.
Therefore, the vegetarian must adopt a spiritual lifestyle at the same time that he or she becomes vegetarian, otherwise, it would be better to eat meat; this person’s memory may suffer, certain parts of the brain can be damaged and so on. It is not enough to eat only fruit in order to reach the highest spiritual development.
Source (German): Rudolf Steiner – GA 96 – Ursprungsimpulse der Geisteswissenschaft – Berlin, 22 October 1906 (page 174-175)
Translated by Nesta Carsten-Krüger