Lactate conversion to pyruvate is thermodynamically favorable and does not require energy input.

“Yeah, it is. It's more reduced, so it actually has more energy than pyruvate. And, as you said, energy comes free.”

“I always think about lactate, as a source of energy, being thermodynamically favorable because you don't need energy to convert lactate into pyruvate, but you need energy to convert glucose into pyruvate.”