Point/Counterpoint: Lactic acid, debunked
Earlier this week, we published a training article that originally ran in the Dec. 2013 issue of Velo magazine. That article, which focused on dispelling misconceptions over lactic acid and lactate, drew a great deal of positive feedback, but also brought on two reader letters that were as detailed as they were heated. To address those letters, we’ll run them here, followed by the response from Trevor Connor, author of the original article, which you can find below. If you have more feedback, please visit our Facebook page to comment.
Correction on article about lactic acid
I was reading your article about lactic acid and noticed a couple of inaccuracies, specifically the part about lactic acid existing in humans.
First off, I need to be clear on the definition of an acid. An acid is a compound that releases a proton (a hydrogen atom without an electron –H+) into solution, be it blood or water or alcohol. Acidity is measured in pH, which is the -log (concentration of H+).
Another thing that needs to be made clear is that not all acids are equal. Some acids are strong and some are weak. A strong acid releases all of its H+s in water, while weak acids don’t necessarily release an H+ into solution. What that means is that if you put one million molecules of a weak acid into water, even if only one of those molecules loses its H+, it is still an acid. On the other hand, if you put one million molecules of a strong acid into water, every single molecule would loose an H+. Therefore an equal concentration of a strong acid will have a lower pH than that of a weak acid.
Now that that’s out of the way, here are the errors I found.
1) In the middle of the section, the article says that “If you had lactic acid in your blood, you’d have to have a pH under six…” That isn’t true. At low concentrations, the pH would remain in the sixes.
2) Near the end of the same section, there was a statement that “an acid is simply a positively charged hydrogen.” This is incorrect as well. The H+ is what we measure to determine the acidity of a solution, but it isn’t an acid by its self. To have an acid, you need a hydrogen bound to at least one more atom.
I strongly urge you to take down the article by Trevor Connor. Far from debunking myths about lactic acid, I believe it contains basic errors in chemistry and physiology that need to be properly addressed. It could simply be read by any professor of chemistry or biochemistry for an authoritative vetting.
What struck me immediately is the misunderstanding of the functional difference between the ionized form of lactic acid (“lactate”) and lactic acid (protonated form). They are functionally the same thing. For example, in my work I typically describe vitamin C in an organism as “ascorbate” because ascorbic acid exists in the ionized form at typical physiological pH. The vitamin C is still there, and is still fully functional, whether it is referred to as “ascorbate” for “ascorbic acid”. Same thing with lactate/lactic acid. Its net production (buildup) shows that an athlete’s muscles are no longer able to produce sufficient energy aerobically, such as in a long sprint. It doesn’t help your readers understand sports physiology by reading an argument that “lactic acid” is not being produced.
There is a great need for educational articles, and I just suggest having them “reviewed” more stringently.
Assoc. Research Scientist
Dept. MCDB & EEB
University of Michigan
Trevor Connor’s rebuttal
I want to thank our readers for their thoughtful feedback on the recent lactic acid piece. I will admit that it proved to be more difficult than I expected to explain lactic acid chemistry in a short article targeted toward many readers with little chemistry background. Certainly some things needed to be simplified. For any of you interested in a complete explanation, I would definitely refer you to two great reviews that were critical to my article. The first is titled “Lactic acid and exercise performance,” written by Simeon Cairns. The second is “Biochemistry of exercise-induced metabolic acidosis,” by Robergs, et. al. The former, which provides a great history of lactate physiology states early on, “it should be noted that virtually no lactic acid exists in the body in this neutral form; instead it is represented by two ionic species: lactate ions and hydrogen (H+) ions.”
One reader pointed out that lactate/lactic acid are functionally the same thing. And certainly in the literature acids and their base are often used interchangeably. However, in this case I do not feel that lactate and lactic acid can be viewed interchangeably for several reasons. First, lactic acid is an acid with a dissociable hydrogen ion. Lactate is a base without the ion. That difference is important when looking at its impact on pH balance. Second, the pKa value of lactic acid is 2.67. This is the pH at which the substance exists 50/50 as lactic acid and lactate; 2.67 is not physiological, so it is simply not possible for the bound form (lactic acid) to exist in the body. This is what I was getting at when I said, “if you had lactic acid in your blood, you’d have to have a pH under six.” Meaning our blood pH would have to be well below six before it would exist as lactic acid.
Third and most importantly, at no point in its physiological pathway does lactate function as an acid. Lactate is formed by binding a hydrogen ion to pyruvate inside cells, so as recent research is showing, lactate formation not only doesn’t contribute to acidity but in fact acts as a buffer by binding intracellular protons. Further, the lactate that is pumped into the blood never has a physiologically dissociable hydrogen ion that can contribute to acidosis. A hydrogen ion does leave the cell with lactate, but it is believed that there again, the lactate is helping to maintain intracellular pH by acting as a co-transporter. Robergs et al. concluded that the hydrogen ion was not produced by lactic acidosis but by ATP hydrolysis. They go on to say, “there is no biochemical support for lactate production causing acidosis. Lactate production retards, not causes, acidosis.”
What all this means is that lactate never contributes to acidity but in fact acts as a buffer, so I do not feel it is appropriate to say lactic acid and lactate are functionally identical in this case.
The reader states that what’s important is that lactate is produced when we can no longer produce sufficient energy aerobically, but this again is not the case. Lactate is constantly being produced in the body and actually acts as an important fuel. Its rise in the blood has as much to do with our ability to clear it as its formation. I have in fact worked with athletes who have very high lactate levels while working at low aerobic levels due to poor clearance mechanisms.
Another reader does point out that I was inaccurate when I referred to hydrogen ions as acid. This is true. However, physiologically speaking, the body doesn’t sense particular acids, it senses pH levels and it is the concentration of hydrogen ions that determine pH. It would have been more accurate to say the hydrogen ions pumped out with lactate cause the drop in blood pH. I hope you can forgive the simplification to make the article digestible.
Ultimately the key point I was trying to get across was that at no time does lactate physiologically function as an acid or contribute to metabolic acidosis. So I think it’s important for our readers to stop thinking of “lactic acid” — a substance that only appears when we work aerobically, that increases acidity, and leads to fatigue. Instead, it is important they instead think of “lactate” — a substance that is constantly produced (even at rest), that can be formed as an acid buffer, and serves as an important fuel shuttle within our bodies.
Dept. of Health and Exercise Science
Colorado State University