Lactate is a preferred fuel for the heart.

“it's now the favorite fuel for the heart”

“skeletal muscle can produce lactate under fully aerobic conditions. So there's always some lactate production happening. And certainly, during more intensive exercise where we produce lactate inside the muscles, it can be released from active skeletal muscle, it can circulate to other places like the heart, like the liver, like the brain, but certainly in heart. Heart can be a big consumer of lactate, and so it takes up that lactate, can convert it back to glucose, and then utilize it during exercise.”