Lactate: A Fuel, A Vehicle And A Highway Combined

The new study confirms that lactate plays the same role during normal non-exercise activity and resting as in during intense exercise.

It's evidence to show that lactate shouldn't be associated with anaerobic metabolism. It's just a normal response to consuming carbohydrates or to exercise. In exercise, lactate is utilized as the dominant fuel source. That's why our blood lactate increases as we exercise a little harder. It's not that we're making it as a waste product. It's getting into the blood because it needs to go to tissues that need it to continue their physiological performance.

The researchers found that the bodies of the volunteers began converting the dietary glucose into lactate before it even left the intestines. Levels of lactate began rising in arterial blood a mere 5 minutes after the meal, while glucose, often touted as the energy currency of the body, only showed up in the bloodstream 15 to 30 minutes after glucose ingestion.

The first carbohydrate after a glucose meal gets into the blood as lactate because a) that's what intestinal cells do and because b) most of the glucose is captured by the liver before it is released into the blood for the muscles, where glucose is going to be converted to lactate. This shows that lactate is just a major energy highway for distributing carbohydrate—carbon energy flux.

In his 1984 book, “Exercise Physiology: Human Bioenergetics and Its Applications” George Brooks coined the term "lactate shuttle" to describe the body's metabolic feedback loop in which lactate is the intermediary sustaining most if not all tissues and organs.

Dr Brooks has shown, for example, that in many tissues, lactate is preferred as a fuel over glucose. During intense activity, the muscle mitochondria burn it preferentially and even shut off glucose and fatty acid fuel use. Brooks used tracers to show that human skeletal muscle, heart muscle and the brain prefer lactate to glucose as fuel and run more strongly on lactate. Lactate also signals fat tissue to stop breaking down fat for fuel.

One gap in these studies was what happens during normal non-exercise activity and resting. The current study fills that gap and supports the idea that when lactate levels in the blood remain high, it is a signal that something is disrupting the lactate shuttle cycle, not that lactate itself is harming the body.

"It's really informative about various medical conditions," Dr Brooks says. "I think what's significant about the current result is that it's just not a muscle thing. It starts with dietary carbohydrate. This was a missing piece in the puzzle."