Dean Karnazes: the man who can run for ever

Some of you may have heard of Dean Karnazes, an ultra- marathon runner from California. He has performed such astonishing feats as running 50 marathons in 50 consecutive days. And while most runners have to stop when they reach their lactate threshold, Dean Karnazes can run for three days and nights without stopping. David Cox writing in The Guardian tries to discover his secret.