This section is from the book "Practical Dietetics With Special Reference To Diet In Disease", by William Gilman Thompson. Also available from Amazon: Practical Dietetics with Special Reference to Diet in Disease.
Mushrooms, on account of their nitrogenous matter, are of some slight use as food; but if eaten in sufficient quantity to yield much nutriment, they always disagree. Bauer says: "Judging from their chemical composition, they ought to have no small value as foods, but it is doubtful how far they are really utilised in the alimentary canal".
Mushrooms have a tempting flavour, which is developed on cooking, and while they agree with most persons in health and form an acceptable article of diet, there are some persons who can never eat even the simplest varieties without suffering more or less from acute gastro-intestinal irritation. They should never be eaten raw. They are usually served alone, broiled upon toast or as a dressing for beef, fillet, steak, etc., or they may be preserved in olive oil or by drying. Their digestion requires fully three hours. The common mushroom consists of 91 per cent water, besides 3½ per cent of carbohydrates, and 4 per cent of proteid, with salts and other substances.