Essays on Diet (1883); quoted in Prof. Francis William Newman http://www.ivu.org/history/europe19b/newman.html at the International Vegetarian Union website.
“They [the true instructors of the people] will accustom children to the vegetable régime. The peoples living on vegetable foods, are, of all men, the handsomest, the most vigorous, the least exposed to diseases and to passions, and they whose lives last longest. Such, in Europe, are a large proportion of the Swiss. The greater part of the peasantry who, in every country, form the most vigorous portion of the people, eat very little flesh-meat. The Russians have multiplied periods of fasting and days of abstinence, from which even the soldiers are not exempt; and yet they resist all kinds of fatigues. The negroes, who undergo so many hard blows in our colonies, live upon manioc, potatoes, and maize alone. The Brahmins of India, who frequently reach the age of one hundred years, eat only vegetable foods. It was from the Pythagorean sect that issued Epaminondas, so celebrated by for his virtues, Archytas, by his genius for mathematics and mechanics; Milo of Crotona, by his strength of body. Pythagoras himself was the finest man of his time, and, without dispute, the most enlightened, since he was the father of philosophy amongst the Greeks. Inasmuch as the non-flesh diet introduces with many virtues and excludes none, it will be well to bring up the young upon it, since it has so happy an influence upon the beauty of the body and upon the tranquillity of the mind. This regimen prolongs childhood, and, by consequence, human life.”
Vœux d'un solitaire, pour servir de suite aux "Études de la nature", as quoted in The Ethics of Diet by Howard Williams (University of Illinois Press, 2003, p. 175 https://books.google.it/books?id=o9ugCcZ13BMC&pg=PA175)
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Jacques-Henri Bernardin de Saint-Pierre 3
writer and botanist from France 1737–1814Related quotes
Philozoia; or Moral Reflections on the Actual Condition of the Animal Kingdom, and on the Means of Improving the same, Brussels: Deltombe and W. Todd, 1839, pp. 42 https://books.google.it/books?id=hdVq93Ypgu0C&pg=PA42-43.

Source: Mazdaznan Dietetics and Cookery Book (1913), p. 199

Interview in the documentary-film The Game Changers by Louie Psihoyos (2018).

Source: The New Dietetics, What to Eat and How: A Guide to Scientific Feeding in Health and Disease, Battle Creek, MI: The Modern Medicine Publishing Co., 1921, p. 366 https://books.google.it/books?id=TNsMAwAAQBAJ&pg=PA366.

“Most people don't bother about their friends in the vegetable kingdom.”
Source: Diet for a New America (1987), Ch. 10: An Ounce of Prevention

Source: Talks for the Times (1896), "The Importance of Correct Ideals" (1892), p. 281