Beetroot Juice.

We came across this article recently and as one of us was training for the rugby season, he was volunteered to test the stuff. Verdict: Has a strong after-taste. Is like being punished. Makes your pee go pink.

Science Daily (Aug. 7, 2009) — Drinking beetroot juice boosts your stamina and could help you exercise for up to 16% longer. A University of Exeter led-study shows for the first time how the nitrate contained in beetroot juice leads to a reduction in oxygen uptake, making exercise less tiring.

I also tried it and found it not too bad. I have not put my stamina to the test.