The high ideals of the poet, artist and philosopher kept athletics comparatively pure for a short time, but when the patriotic wave that followed the Persian war had spent its force, the decline in amateurism was rapid, and we enter the third period where too much competition begat specialization; specialization begat professionalism, and that in itself was death to true sport. Even the good athlete could not hope for success unless he put himself under a rigorous and prolonged course of training. The trainers had to concentrate on the preparation for single events. "The runner," says Socrates, "has over-developed his legs, the boxer his arms and shoulders." 

Up to this time the athletes had lived a simple natural life in the open air, eating figs, cheese, porridge and meal cakes, with meat only occasionally. The introduction of a meat diet is ascribed to Pythagoras of Samos(c. 570 – c. 495 BC), a trainer of boxing and other sports. It was momentous in that it at once created an artificial distinction between the life of the athlete and the life of the ordinary man, who ate meat but sparingly, just as our training tables place the athletes in an artificial and unnatural class by themselves, being used for this purpose quite as much as for any special diet that may be prescribed. 

The object of a meat diet was to make weight, for there was no classification in Greece of boxers and wrestlers into light, middle, and heavy weights. Weight then was important, and to produce this bulk the trainer prescribed vast quantities of meat, so that eating, sleeping and exercise occupied the athlete's entire time. 

Euripides calls such an athlete "the slave of his jaw and his belly," and the generals and soldiers condemned this training because it left no time for the practice of military exercises, and failed to produce the all-round development necessary for the useful soldier and citizen. The sacrifice for supreme excellence in a specialty was too great to make success a sufficient reward. Athletics had now passed that point where they could serve their true purpose of providing exercise or recreation. The competition was too severe and the training too artificial and exacting. It became the monopoly of the few professionals who devoted their entire time to it, while the rest of the young men, despairing of success, took to the hill as spectators. The amateur could not compete with the professional. Before the close of the fifth century, the word athlete had come to denote a professional, and amateur athletics were no longer practiced by the fashionable youth of Athens. Socrates, taunting an ill-developed youth with his unprofessional condition of body, meets the answer, "Of course, for I am not a professional but an amateur." 

Whereupon Socrates reads him a lecture on the necessity of developing his body to the utmost, saying: "No citizen has a right to be an amateur in the matter of physical training. It is a part of his profession as a citizen to keep himself in good condition and ready to serve his state at amoment's notice." "What a disgrace it is for aman to grow old without ever seeing the beauty and strength of which his body is capable."

Fun Wikipedia Facts about Pythagoras:

Pythagoreanism also entailed a number of dietary prohibitions.[107][156][172] It is more or less agreed that Pythagoras issued a prohibition against the consumption of fava beans[173][156] and the meat of non-sacrificial animals such as fish and poultry.[166][156] Both of these assumptions, however, have been contradicted.[174][175] Pythagorean dietary restrictions may have been motivated by belief in the doctrine of metempsychosis.[146][176][177][178] Some ancient writers present Pythagoras as enforcing a strictly vegetarian diet.[e][146][177] Eudoxus of Cnidus, a student of Archytas, writes, "Pythagoras was distinguished by such purity and so avoided killing and killers that he not only abstained from animal foods, but even kept his distance from cooks and hunters."[179][180] Other authorities contradict this statement.[181] According to Aristoxenus,[182] Pythagoras allowed the use of all kinds of animal food except the flesh of oxen used for ploughing, and rams.[180][183] According to Heraclides Ponticus, Pythagoras ate the meat from sacrifices[180] and established a diet for athletes dependent on meat.[180]