David Haye at 35 is fighting better than ever since adopting a vegan diet in 2014. He recently enjoyed a first round knockout victory against Australian Mark de Mori. The victory was made all the sweeter after de Mori taunted him about his veganism at the pre-match interview (see below)
Haye chose to become vegan for ethical reasons (after watching the documentary Earthlings) and because he realised it increased his recovery times and helped him to keep physically strong. He had previously used a plant based diet to lose surplus weight before fights.
His main reason for choosing veganism however was for the animals.