Lactate intolerance

Growing up and doing martial arts I was always told I got sore because of lactic acid.  Acid? That stuff burns!  Makes total sense.  Many years later while learning metabolism from the one and only Hans Kornberg (discoverer of anapleurosis and writer of this lovely memoir), I started having some of those oh, this is how things work moments.  Since then I've been reading articles, both scholarly and semi-journalistic, on blood lactate accumulation and exercise.  This is the first of what I'm sure will be many posts.  This one is titled "Lactate intolerance" because it seems to be the villain molecule in amateur physiologists, like many cyclists and cycling coaches are.    But it's not, and is quite useful for many reasons.  Let's get into it.

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For a brief period, I studied the enzyme that catalyzes this reaction:

http://www.aaltoscientific.com/purifiedhumanproteins/images/lactate-dehydrogenase.jpg

Pyruvate is the end of glycolysis.  After pyruvate, the next step is rate determining (read: slow-ish, for enzymatic catalysis).  Pyruvate->acetyl-CoA happens more slowly than the reactions before it, so pyruvate can add up.  Other things that add up are NADH and H+.  During highly glycolytic times, NAD+ needs to be regenerated to be used in more glycolytic reactions.  And because the TCA cycle doesn't seem to be particularly fast compared to glycolysis, it can be slow regenerating NAD+.

Like many metabolic reactions at rest, lactate and pyruvate are kept in near equilibrium concentrations.  During exercise when glycolytic muscle fibers become engaged, pyruvate starts to pile up, and lactate dehydrogenase (LDH) regenerates NAD+ and mops up a spare H+ as well (it catalyzes the above reaction both ways, depending on the concentration of lactate and pyruvate).

Think of it like a bucket that has a divider in it.  As pyruvate piles up, it'll spill over into the lactate half.  Now they're both filling up.  Glycolysis is still happening because you're still exercising.  Now if lactate piles up too fast, so will pyruvate, which might fill up the bucket (bad metaphor, overflowing cells would be a serious problem) and back up all of glycolysis!  So now what do we do?  Throw out lactate.  Into the blood stream you go!

What happens to lactate in the blood?  Does it pile up?  It can, but it also gets taken up by other cells and converted back to pyruvate for aerobic metabolism.  One of the big lactate users during exercise is heart muscle, which is very, very aerobic.  When you eat heart, it's very dark meat because there's so much mitochondrial density, and the iron makes it dark.  Incidentally, aerobic training increases the density of enzymes that import lactate and pyruvate from the blood (MCT, monocarboxylate transporter).  And contrary to what you might read in some older literature, lactate itself is not oxidized, but is converted back to pyruvate.  Not that they were bad researchers, they just didn't have the tools that we have now.

Lactate appearance in the blood has several names, and they serve different purposes.  What most people say is "lactate threshold", researchers have defined as: the highest VO2 attained during incremental exercise before an elevation of blood lactate is observed; a blood lactate concentration 1mmol above baseline; blood lactate concentration 2.5mmol above baseline; &c.  Also there's onset of blood lactate accumulation (OBLA), the VO2 attained during incremental exercise corresponding to a blood lactate concentration of 4 mmol/L (source).  So if that's where your current understanding of LT is at, that study just cited as a source is about to blow your mind.

Category 2 and 3 cyclists' average steady blood lactate in a 20k TT is in the upper left hand graph.  In each graph the dotted line is one of the previously defined lactate threshold definitions.  Nuts, right?

It's fortunate for us as cyclists that we don't do physiological testing to define our training intensities.  A standard 20 minute test may well have some of us doing a steady 12mmol/L blood lactate.  Although if we did do such blood tests to set training zones, we probably would have figured out sooner that lactate isn't guilty of anything except being the most obvious blood molecule during exercise.

As it stands, the 20 minute test is generally not excellent at setting training zones, and how long an athlete can hold a certain % below or above the reference value indicates what the athlete is naturally good at.  I'm a sprinter/pursuiter, some people I know are good in a long breakaway but not so good in a short TT.  Incidentally, figuring out this stuff is another thing WKO4 does well (it's a feature I always pined for using trainingpeaks) as long as the athlete has good maximal power values all around, although it still takes a good eye to interpret the data and curves.

For the record, here's mine from WKO4.  You can guess what power zone I'll be working on next spring.  And just to make it easy, we can keep calling it lactate threshold.  And before you ask, there are many reasons I'm not an elite.  Like beer and cookies.