The lactate loop between muscle and liver
During intense effort, muscle breaks down glucose faster than its mitochondria can oxidize the resulting pyruvate. The excess pyruvate is converted to lactate by lactate dehydrogenase, which regenerates the NAD+ that glycolysis needs to keep running. Lactate then diffuses into the blood and travels to the liver.
In the liver, lactate is converted back to pyruvate and rebuilt into glucose through gluconeogenesis. That glucose returns to the bloodstream and can be taken up again by muscle, closing the loop named after Carl and Gerty Cori.
Why the cycle costs net energy
The cycle does not create energy. Making glucose from lactate in the liver consumes more ATP than muscle gained from breaking that glucose down. The net effect is to shift the energy burden to the liver, letting active muscle keep producing ATP quickly under low-oxygen conditions and delaying fatigue rather than removing the cost.