Dr. Michael Greger recently completed a 17-part video series examining the effect various vegetables have on athletic performance. All of the videos are linked on that page if you would like to check them out.
The conclusion? Nitrate in vegetables can be turned into nitrates and then nitric oxide in our bodies. Studies have found that, on average, increased nitric oxide boosts stamina and allows people to engage in high-intensity exercise up to 20% longer.
If you only want to watch one of the videos, watch this one in which Dr. Greger talks about how athletes can use vegetables (especially dark, leafy greens) to nitrate load.