Berkeley physiologist George Brooks is the man we have to thank for our current understanding of lactate -- for the view that it's a valuable fuel for hard-working muscles rather than just a corrosive waste product. And his group continues to churn out interesting studies trying nail down exactly what the role of lactate is. His latest, currently in press in the Journal of Applied Physiology, takes a closer look at how the creation and disappearance of lactate changes near the lactate threshold.

First, since people mean lots of different things by "lactate threshold," here's a graph showing what he means:

That's power output on a stationary bike along the horizontal axis, and lactate concentration on the vertical axis. The lactate threshold is the point at which lactate levels start to increase more quickly than the steady straight line they've been on up to that point. You can see that the trained subject is able to put out lots more power before he hits that point, but the shape of the graph is pretty much the same.

The experiment involved six trained competitive cyclists and six untrained but healthy controls. Both groups did 60 minutes of cycling at lactate threshold, after being infused with isotope tracers to track their use of lactate (and other markers); the trained group also did a similar session at 10% below threshold. The main point of the experiment was to figure out what causes the threshold. One view is that it's the signal that you're no longer supplying enough oxygen to your muscles, so they starting to go anaerobic. Brooks's view, on the other hand, is that it's more a question of balance between production and removal (to be reused as fuel) of lactate. And this new experiment backs that up.

First (and not surprisingly), he finds that the trained subjects produce AND reuse far more lactate at threshold than the untrained group. If you look at the graphs above, you see that both groups have lactate concentrations of about 4 mmol/L at threshold. But don't be fooled: the trained group is actually producing about 60% more lactate at that point, and clearing/reusing it 60% faster.

Interestingly, the threshold isn't the point at which lactate clearance is maximized. The slightly lower intenstity, 10% below maximum, produced the highest values for lactate clearance, about 30% higher than at threshold. In other words, the sharp increase of lactate levels above threshold isn't because you've suddenly started to produce more lactate; it's because your ability to clear lactate can no longer keep pace. There are a number of reasons for this, including the rate at which lactate can be transported across cell membranes, the decline in blood flow to the liver (which is where lactate is converted to glucose), increased activation of the sympathetic nervous system, and so on. The basic point is that it's not a simple question of oxygen shortage.

So what about training? Does the fact that maximum lactate clearance occurs below lactate threshold have any significance? Brooks writes:

In that sense, it is interesting to note that in most endurance activities (e.g., cross-country skiing and rowing) athletes predominantly train at exercise intensities below the LT.

I don't know... If you're trying to improve lactate clearance, should you train at the intensity where it's already optimal, or at a higher intensity to improve it where it's not optimal? I don't really see any compelling reason to think that either approach must be "right." Assuming you're aiming for the same overall training load, there are always tradeoffs between intensity, the length of time you can sustain it, the time necessary to recover, and so on. To me, it seems likely that subtle adjustments in workout intensity produce subtle adjustments in adaptation, rather than that there's an on-off switch that clicks on when you're hitting the "right" intensity.

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