Some fish sprint better than others, but even the Carl Lewises of the fish world have to keep going for long periods at more sedate speeds. Fish power slow swimming with red muscle, which is packed with mitochondria that generate energy rich ATP. Mitochondria can oxidise both lipids and carbohydrates to drive ATP production. Out of the two fuels, lipids produce more energy than carbohydrates, but carbohydrates release their energy more quickly. Chris Wood wondered which fuel the red muscle consumes as the fish change pace. Working with a team of students, they tested how the fish metabolise lipid and carbohydrate at different stages of an endurance swim. To their surprise, the fish kicked off by consuming carbohydrate, but switched to lipid metabolism once they'd established a steady pace(p. 2067).
Jeff Richards, a graduate student in Wood's group, was already familiar with the way that white muscle metabolises...
