Orphicism and Pythagoreanism, two common ancient Greek religions, suggested a different way of life, based on a concept of purity and thus purification (κάθαρσις katharsis) — a form of asceticism in the original sense: ἄσκησις askēsis initially signifies a ritual, then a specific way of life. Vegetarianism was a central element of Orphicism and of several variants of Pythagoreanism.
Empedocles (5th century BC) justified vegetarianism by a belief in the transmigration of souls: who could guarantee that an animal about to be slaughtered did not house the soul of a human being? However, it can be observed that Empedocles also included plants in this transmigration, thus the same logic should have applied to eating them. Vegetarianism was also a consequence of a dislike for killing: "For Orpheus taught us rights and to refrain from killing".
The information from Pythagoras (6th century BC) is more difficult to define. The Comedic authors such as Aristophanes and Alexis described Pythagoreans as strictly vegetarian, with some of them living on bread and water alone. Other traditions contented themselves with prohibiting the consumption of certain vegetables, such as the broad bean, or of sacred animals such as the white cock or selected animal parts.
It follows that vegetarianism and the idea of ascetic purity were closely associated, and often accompanied by sexual abstinence. In On the eating of flesh, Plutarch (1st–2nd century) elaborated on the barbarism of blood-spilling; inverting the usual terms of debate, he asked the meat-eater to justify his choice.
The Neoplatonic Porphyrius (3rd century) associates in On Abstinence vegetarianism with the Cretan mystery cults, and gives a census of past vegetarians, starting with the semi-mythical Epimenides. For him, the origin of vegetarianism was Demeter's gift of wheat to Triptolemus so that he could teach agriculture to humanity. His three commandments were: "Honour your parents", "Honour the gods with fruit", and "Spare the animals".
Aelian claims that the first athlete to submit to a formal diet was Ikkos of Tarentum, a victor in the Olympic pentathlon (perhaps in 444 BC). However, Olympic wrestling champion (62nd through 66th Olympiads) Milo of Croton was already said to eat twenty pounds of meat and twenty pounds of bread and to drink eight quarts of wine each day. Before his time, athletes were said to practice ξηροφαγία xērophagía (from ξηρός xēros, "dry"), a diet based on dry foods such as dried figs, fresh cheese and bread. Pythagoras (either the philosopher or a gymnastics master of the same name) was the first to direct athletes to eat meat.
Trainers later enforced some standard diet rules: to be an Olympic victor, "you have to eat according to regulations, keep away from desserts (…); you must not drink cold water nor can you have a drink of wine whenever you want". It seems this diet was primarily based on meat, for Galen (ca. 180 AD) accused athletes of his day of "always gorging themselved on flesh and blood". Pausanias also refers to a "meat diet".