Edited by Kate Findley, The Great Courses Daily
When your legs start burning as you complete your first lap around the track, you may believe you have lactic acid buildup. In fact, as Michael Ormsbee, Ph.D., explains, lactate is actually a good thing. He also discusses the role of carbohydrates in fueling your body.
The Glycolytic System
The glycolytic system, or anaerobic glycolysis, is the second of three energy-producing systems in our bodies, following the creatine phosphate system. It relies on carbohydrates as a fuel source to make energy.
The glycolytic system lasts longer than the creatine phosphate system (which powers you through 30-second bursts of high-intensity activities like sprinting and weightlifting), but usually not more than two minutes while you are working hard.
It is also called anaerobic glycolysis because, as with the creatine system, you produce adenosine triphosphate (ATP) without the need for oxygen in your cells. Think of running a 400-meter dash or doing short swimming intervals or treadmill exercises that you could only maintain for one to two minutes before needing a break.
Glycolysis, as the name implies, is the breakdown of glucose to make ATP. Glucose is in any type of carbohydrate that you would eat or drink—a sports drink, a piece of bread, or even an apple.
Once the glucose is in your blood, you need to get it into your cells and break it down to either make ATP or store it as glycogen for later use. An elaborate system of proteins called GLUT transporters assists with this process.
The glycolytic energy system is one you rely on during any sort of physical activity. The three energy systems work on a continuum, but kick in to a much greater extent when required by the intensity for what you are doing, whether typing on your computer, lifting weights, or running a marathon.
Rethinking Lactate’s Reputation
During glycolysis, your body breaks down glucose to eventually produce molecules called pyruvate, as well as ATP. Glycolysis both uses up and produces ATP; so along this pathway the net amount of ATP produced is considered moderate at best.
Under conditions where oxygen is limited, like during hard exercise, pyruvate will go on to form lactate, and lactate or lactic acid is often considered to be a bad thing. Exercisers often dislike lactate because of the burning sensation in their legs when exercising. However, there’s more to the story.
Lactate is actually a storage depot for the excess hydrogen ions produced as a byproduct of this glycolytic energy system working to make ATP. When you break down glucose to make ATP, you also produce hydrogen ions.
If you are working out intensely, then the hydrogen ions will accumulate and build up, eventually causing a drop in the pH of your muscles. This drop in pH from the hydrogen ions is known as lactate acidosis, and it’s painful.
It is this acidity that makes you slow down or even stop for a breather during exercise. The lactic acid formed helps keep glycolysis functioning to make just a little more ATP, or to give you energy to perform more work for just a little longer before you have to slow down or stop.
Additionally, lactate can be transformed back into glucose for later use in this glycolytic energy system. The process of forming new glucose from non-carbohydrate sources—in this case lactate—is called gluconeogenesis.
Lactate leaves the muscle, is converted back into pyruvate in the liver, and is then re-formed into glucose; this process is called the Cori cycle. The newly formed glucose then makes its way back to the blood and then to the muscles to produce ATP.
Why Pain Is Generative
Imagine you are doing hard exercise intervals, like running uphill. Eventually your legs will begin to burn, which indicates that the energy is being produced and the acidity is increasing in your muscles. This again is caused by the hydrogen ions that are a product of glycolysis, and they ultimately combine with pyruvate to form lactic acid.
Most likely the pain associated with the acidity increasing in your legs will make you slow down. Now, having slowed down, you are able to actually use that built-up lactate by transforming it back into glucose to provide more fuel to produce ATP.
Because you slowed down and allowed yourself to catch your breath—that is, allowed oxygen to be available to your metabolic processes—you begin again to transform pyruvate, the end product of glycolysis, into another molecule called acetyl coenzyme A, or acetyl-CoA, rather than forming lactate.
It is this acetyl-CoA that can then move into the third energy system to provide even more ATP. Acetyl-CoA is actually called the common intermediate for energy production because in order to make lots of ATP in the third energy system (the oxidative system), you must first form Acetyl-CoA, no matter if you are burning carbohydrates, fats, or proteins as your energy source. It is the common point for entry for all of these fuels to make energy in the oxidative system.
Dr. Michael Ormsbee is an Associate Professor in the Department of Nutrition, Food, and Exercise Sciences and Interim Director of the Institute of Sports Sciences and Medicine in the College of Human Sciences at Florida State University. He received his M.S. in Exercise Physiology from South Dakota State University and his Ph.D. in Bioenergetics from East Carolina University.