Key Takeaways

  • Dr. Jared Rutter explains that pyruvate inside cells faces two main paths: oxidation inside mitochondria or conversion into lactate.
  • Burning pyruvate converts carbon into carbon dioxide that leaves your body through your lungs, while turning it into lactate preserves carbon to build cellular biomass and proteins.
  • Metabolic tracing experiments by researchers like Joshua Rabinowitz at Princeton prove that circulating lactate serves as a primary energy fuel for both the brain and the heart.
  • Andrew Huberman notes that exercise-induced lactate functions as a signaling molecule, triggering brain-derived neurotrophic factor (BDNF) to support neuroplasticity.

The Trade-Off Between Energy and Biomass

Every biology textbook teaches that lactate is a toxic metabolic byproduct of oxygen-deprived muscles. That idea is wrong. Rutter explains that cellular metabolism faces a constant resource decision: produce immediate energy or preserve building blocks.

When a cell metabolizes glucose into pyruvate, it chooses between two physical fates. As Rutter states: “When pyrovate is made simplistically has two fates. It can go into the mitochondria we talked about. What we didn't talk about is the other major fate is to be converted to lactate and exported.”

Sending pyruvate into the mitochondria burns it for rapid energy. But that energy comes at a physical cost. “If you burn the pyrovate, that turns into carbon dioxide. We breathe it out. That the stuff is gone. We breathe it out. There's no stuff. There's just the energy. If you don't burn it, that stuff doesn't get eliminated as carbon dioxide and can turn into a protein.”

Lactate production is a carbon-saving strategy. Cells keep raw material inside the body instead of blowing it out into the atmosphere. As Rutter summarizes: “There's something about that production of lactate that enables ongoing production of biomass.”

A Preferred Fuel for the Brain and Heart

When muscle tissue works hard and oxygen drops, pyruvate conversion to lactate accelerates. Rutter explains: “When oxygen isn't available, that pyrovate cannot be burned and then it essentially has to be converted to lactate. That's why when we exercise and our muscle becomes hypoxic or doesn't have adequate oxygen, we make lactate and that lactate is what causes the burn that we feel.”

Far from being metabolic debris that your body struggles to eliminate, that exported lactate enters circulation as high-grade fuel. Rutter points to recent laboratory research: “There's been beautiful experiments done in the last 5 or 10 years. Joshua Benowitz, a friend of mine, a professor at Princeton, has done some of these that have demonstrated that lactate is a very important fuel on its own.”

The heart and brain actively absorb circulating lactate from the bloodstream and burn it for work. Huberman adds that lactate acts as an active messenger. High circulating lactate levels created during intense exercise cross physiological barriers and stimulate BDNF production in the brain, directly driving neuroplastic adaptations.

What to Do With This

Stop viewing the muscle burn during high-intensity training intervals as damage or waste accumulation. Program two weekly high-intensity interval sessions where you deliberately cross your lactate threshold, producing the circulating fuel and BDNF signals needed to support cardiac endurance and cognitive plasticity.