… about medical knowledge in the world: if you have studied enough or lived long enough, that is, a fairly good number of years, you will find that with the same authority, the same certitude, the same conviction, at one time certain things are not only considered bad, but on the basis of an absolute knowledge, an unquestionable observation, they are reputed to have a certain effect, and at another time these very unquestionable observations lead to diametrically opposite results. Very often I give an example which I happened to observe, especially as regards the value of certain foods and their effects on the body, like certain fruits or vegetables: at a particular time in medical history—not so long ago, about fifty or sixty years ago—when you had a certain illness, the doctor gave you a list of things recommending to you with absolute seriousness not to touch any of these lest you become even more ill—I could give you the list, but it is not interesting. Well, about these very same things, fifty or sixty years later, not the same doctor perhaps but another one will tell you with the same seriousness, the same unquestionable certitude and authority that these are the very things you must eat if you want to be cured! So if you have observed things pretty well and have a slightly critical mind, you can tell yourself, “Oh! it must depend on people or perhaps on the period.” And I shall tell you, as the doctor-friend I knew in France forty or fifty years ago used to tell all his patients, “Take a remedy while it is in fashion, for then it will cure you.
Ref : Questions and Answers 1956