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The following is an excerpt from a book by Dion Fortune - Moon Magic, most likely.
Fortune was an influential UK occultist who codified the esoteric practice of her time for the mass public. Although she died, of leukaemia, in 1948 her books later became widely-read at a time of heightened interest due to the counter-culture and modernist disruption of traditional religious and scientific belief systems.
Her observations are drawn from her personal experience of the sector of British society in which vegetarian practice was most common - the lunatic fringe.
"The question of diet also requires to be considered in this connection. The widespread propaganda of the Theosophical Society has caused vegetarianism to be regarded as a sine qua non of occult training. This, however, is not the case. The Western Esoteric Tradition does not make vegetarianism any part of its system, but teaches that a man should partake sparingly and temperately of the food of the land in which he finds himself. Personally I am inclined to think that occultism and vegetarianism are apt to be an injudicious mixture for a European, the result being a hyper-sensitiveness that makes life very difficult in our hard-driving civilisation.
Vegetarianism has to be thoroughly understood and exceedingly well done if it is to be successful, and even so, there is a goodly proportion of people who are incapable of digesting vegetable proteins, which are not nearly so easily dealt with as animal substances. Nothing but experience and experimentation can show whether a vegetarian diet suits a given person. Indigestion is not the only indication that all is not well. Loss of appetite, loss of energy, loss of weight, or a flabby stoutness are all danger signals which if disregarded will cause chronic ill-health. Vegetarianism may agree with a person well enough at first, but after a considerable period, possibly years, they may find that they are becoming subject to neuritis, neuralgia, sciatica, or one or another of the nerve pains. This is a sure indication that a vegetarian diet is affording insufficient nourishment, not because it does not contain the necessary food units, but because the digestion is unable to assimilate them and they are passing out of the body unchanged. Wherever there is a history of neuralgic pains complicating a case of psychic disturbance, I should be inclined to suspect chronic malnutrition as the cause of a hypertrophied psychism. In such cases it will probably be found that a gradual return to a nourishing mixed diet will bring about a reduction of the hyper-sensitiveness, the undesirable contacts that have been formed will fade, and the condition return to normal. The change of diet, however, should always be made gradually lest the digestion be upset."