A study in the American Physical Education Review studied meat eaters and vegetarians who ran marathons to see how their habits affected their performance.
January 1, 1911
Effects of Severe Exertion - Diet as to Meat Eating
This was very interesting. Six were classed as vegetarians, one of whom was not permitted to run owing to a marked arrhythmia of pulse and generally poor condition. The other 5 did not quite hold their own with the meat eaters, 60 per cent finishing. The greater number ate meat once a day, and of them 65 per cent finished; out of the 10 who ate meat twice a day 70 per cent completed the race. Beef was by far the most popular meat.