One report states that a trainer named Pythagoras recommended a meat diet to the Olympic athletes he trained.
On the Sociology of Ancient Sport
The carnivore diet involves eating only animal products such as meat, fish, dairy, eggs, marrow, meat broths, organs. There are little to no plants in the diet.
A lack of animal-derived protein seems to have been rectified early in history, as there are two reports of the introduction of meat into the athletes’ diet. One report states that a trainer named Pythagoras recommended a meat diet to the athletes he trained. https://www.omicsonline.org/open-access/a-comparison-of-ancient-greek-and-roman-sports-diets-with-modern-day-practices-2473-6449-1000104.php?aid=69865
8 He [Pythagoras] is said to have been the first to train athletes on a meat diet. The first athlete he did this with was Eurymenes. Formerly they had trained on dried figs, moist cheese, and wheat. Some say that it was a trainer named Pythagoras and not the philosopher who was responsible for this innovative diet. For our Pythagoras prohibited killing, not to mention eating, life which possessed souls like our own.
Diogenes Laertius, Lives of the Philosophers 8.12
Sport and Recreation
To all appearances medical knowledge in general and nutrition in particular was widespread in Pythagorean circles in Southern Italy, especially in Kroton. Burkert has pointed out that the fmaous doctor Alcmaeon came from the Pythagorean center of Kroton. From the akousmata (secret doctrines), which doubtless contain old religious traditions, we learn that the Pythagoreans were not originally strict vegetarians; the consumption of meat was permitted, with the exception of lamb and the meat of draft oxen. Burkert considers there is some likelihood that Pythagoras introduced a meat diet for athletes. He believes this tradition arose when Pythagorean vegetarianism had not yet been completely developed. In later times, when vegetarianism had prevailed, a second tradition has possibly been invented, in which it was not Pythagoras, the philosopher from Samos and Kroton, but a homonymous person who wrot the great 'trainer's recipe book'.
57: There are similar issues regarding the transmission in later sources of Pythagoras advising the successful heavy athlete Eurymenes of Samos to use a special meat-based diet instead of the dried figs and cheese he had previously been eating.
On the Sociology of Ancient Sport - page 43.
Province of Crotone, Italy