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“The world’s best athletes stay competitive by interval training. The intense exercise generates big lactate loads, and the body adapts by building up mitochondria to clear lactic acid quickly.
 If you use it up
[as an energy source], it doesn’t accumulate.”

                                          ~ George A. Brooks, UC Berkley Professor of integrative biology (News Release)

New Thinking on Lactic Acid

* * *

Practical Applications

You’ve probably heard that lactic acid is a waste product that burns and shuts down muscles, and makes you sore a day or so later. It’s something athletes and fitness exercisers are urged to avoid. You’re told to work out just below your lactate threshold, where lactic acid begins to accumulate. That, it seems, is mostly wrong. Scientists have discovered that lactic acid is an important fuel; it’s central to carbohydrate metabolism. Unfortunately, if you don’t train properly, it can also impair performance. Your goal should be to burn lactic acid more effectively, not produce less of it.

First, the idea that lactic acid causes soreness makes no sense, as Auburn University Professor L. Bruce Gladden explained to NYTimes health columnist Gina Kolata (May 16, 2006): “Lactic acid will be gone from your muscles within an hour of exercise. You get sore one to three days later. The time frame is not consistent, and the mechanisms have not been found.”

After that, however, it gets a little more complicated. Let’s start with a simplified explanation of the process by which energy is supplied to the body.

There are two primary energy systems: aerobic and anaerobic. Both systems burn carbohydrate in the form of glucose (blood sugar) and glycogen (muscle sugar). The main difference between the two systems is the presence of oxygen--and the end product. Moderate exercise is mostly aerobic, with oxygen; it’s clean burning, so there is no end product. When exercise becomes more intense and the aerobic system can’t provide enough oxygen, the anaerobic system kicks in to provide energy, without oxygen. The end product of anaerobic exercise is lactic acid.

The two energy systems have been thought to operate as separate and distinct systems. Lactic acid was considered the enemy of aerobic metabolism, with the power in sufficient accumulations to bring it to a halt.

Enter UC Berkley integrative biology professor George A. Brooks, who has been studying lactic acid since doing his doctoral dissertation on the subject in the ‘70s. Told by coaches when he was in college that running too hard would produce lactic acid and slow his performance, Brooks decided to investigate when he got to graduate school.

The old lactic acid theory is based on an experiment performed on dissected frog legs. I remember hearing about it in biology class. After being subjected to electric shock, the frog legs contracted a few times and then stopped working. The motionless frog legs were found to be saturated with lactic acid.

Voila! Lactic acid is bad; it brings muscles to a halt.

“It was one of the classic mistakes in the history of science,” Brooks told Kolata. “I gave rats radioactive lactic acid, and I found that they burned it faster than anything else I could give them.”

Dr. Brooks concluded that lactic acid is an important source of energy—and later research has proved him to be correct.

“The understanding now is that muscle cells convert glucose to lactic acid,” Kolata explains. “The lactic acid is taken up and used as a fuel by mitochondria, the energy factories in muscle cells. Mitochondria even have a special transporter protein to move the substance into them.” As we will see below, aerobic metabolism and anaerobic metabolism operate side by side in the mitochondria.

Thomas D. Fahey, Ed.D, Professor of Exercise Physiology at California State University at Chico, fills in more of the details on the new findings. “Dr. George Brooks describes the dynamic production and use of lactic acid in metabolism in his Lactate Shuttle Theory. This theory describes the central role of lactic acid in carbohydrate metabolism and its importance as a fuel,” Fahey explains. “The heart, slow-twitch muscle fibers, and breathing muscles [actually] prefer lactate as a fuel during exercise.”

“The body converts glucose, a substance removed from the blood only sluggishly, to lactate, a substance removed and used rapidly,” Fahey continues. “[Moreover] using lactic acid as a carbohydrate middleman helps you [metabolize] carbohydrates from your diet, without increasing insulin or stimulating fat synthesis.”

We’ll come back to Fahey on training. First, however, let’s look at the latest discovery concerning Dr. Brooks’ long postulated “intracellular lactate shuttle,” as summarized in a UC Berkley press release dated April 20, 2006.

The Clincher

“This experiment is the clincher, proving that lactate is the link between glycolytic (anaerobic) metabolism, which breaks down carbohydrates, and oxidative metabolism, which uses oxygen to break down various fuels,” Brooks said.

 Post-doctoral researcher Takeshi Hashimoto and staff research associate Rajaa Hussien made the actual discovery. (Forgive the technical jargon; you’ll get the idea.) The researchers located and labeled “three critical pieces of the lactate pathway: the lactate transporter protein; the enzyme which catalyzes the first step in the conversion of lactate into energy; and the protein complex where oxygen is used. Peering at skeletal muscle cells through a microscope, the two scientists saw these proteins sitting together inside the mitochondria, attached to the mitochondrial membrane, proving that the intracellular lactate shuttle is directly connected to the enzymes in the mitochondria that burn lactate with oxygen.”

Brooks says that these finding “can help athletes and trainers design training regimens. Athletes may instinctively train in a way that builds up mitochondria, but if you never know the mechanism, you never know whether what you do is the right thing.”


So lactic acid is a significant energy source, a good thing. How do we train to use more of it? Brooks gives us a powerful hint in the press release. Dr. Fahey provides more details. Gina Kolata also weighs in on the subject. It’s up to each person, however, to figure out what’s best based on their individual needs and goals.

The hint from Brooks is obviously aimed at those of us interested in maximizing our potential: The world’s best athletes stay competitive by interval training. The intense exercise generates big lactate loads, and the body adapts by building up mitochondria to clear lactic acid quickly. If you use it up [as an energy source], it doesn’t accumulate.”

I’ve included hard intervals in my training for years. Looks like instincts were right on. (See The Aerobics Problem/Solution in Ripped 3.) What’s different is that I now have a better understanding how my body uses lactic acid. My job is to challenge my body to handle more of it. Gradually increasing the intensity of my intervals seems to be the best way to do that. (See our Aerobic Exercise category for articles on interval training, including 10, 11, 12, 30, 112 and 152.)

Gina Kolata suggests more endurance training: “Running longer and longer distances, for example, increases the mass of [an athlete’s] muscle mitochondria, letting them burn more lactic acid and allowing the muscle to work harder and longer.” Almost as an afterthought she adds, “Just before a race, coaches often tell athletes to train very hard in brief spurts.”

Fahey says, “Lactic acid formation and removal rates increase as you run, bike or swim faster. To improve your capacity to use lactate as a fuel, you must increase the lactic acid load very high during training.”

Fahey warns, however, that too much lactic acid is still bad. “Lactic acid is a powerful organic acid, and its accumulation can cause distress and fatigue during exercise.” Professor Brooks concurs. “Overtraining,” he cautions in the press release, “can kill muscle cells.”  

Fahey recommends a combination of high intensity and endurance “to improve the capacity to use lactic acid as a fuel during exercise and recovery.” As indicated above, both forms of training increase and improve the mitochondria.

Fahey says that high intensity training also develops cardiovascular capacity, and that endurance training also increases the use of fat as fuel. Increased cardiovascular capacity, of course, translates into improved blood flow and oxygen delivery to the muscles and other tissues. That means “you have less need to breakdown carbohydrate to lactic acid,” Fahey explains. Burning more fat as fuel has a similar function; “[it] decreases lactate formation and speeds its removal.”

That leaves it to you to decide on the best combination of high intensity and endurance training to meets your needs. Marathoners and other endurance athletes will probably want to rely more on endurance training, while bodybuilders and strength athletes will likely favor interval training. Most of our readers are probably somewhere in between the two extremes.

Wherever you fall on the endurance-strength continuum, remember that lactic acid is a fuel, not a poison.   

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