First Brooks showed that endurance training reduces the amount of lactate that enters the bloodstream without affecting the amount of lactate that the muscle cells produce—a strong piece of circumstantial evidence that lactate is somehow used within the cell. In fact, as much as 75 percent of the lactate produced by any given muscle cell never leaves it. Then, in 2006, Brooks was able to peer through a confocal microscope and all but see aerobic lactate metabolism in the mitochondria, the intracellular site of aerobic metabolism. Gathered together there he saw the transporter proteins that deliver lactate to the mitochondria, the enzymes that catalyze the first step of lactate breakdown, and the protein complex where oxygen is used to complete the process of energy release. A smoking gun if there ever was one!
It would be difficult to overstate the magnitude of this discovery. George Brooks showed that there is a direct link between aerobic and anaerobic metabolism. In fact, what was previously thought to be anaerobic metabolism is actually just incomplete aerobic metabolism. During moderate-intensity exercise, most of the carbohydrate that is broken down for energy is processed aerobically and produces no lactate. But at high intensities, a second pathway—the lactate pathway—ramps up, giving the muscle two parallel pathways to release energy aerobically at very high rates to keep up with the muscle’s energy demands. In this second pathway, glycogen or glucose is broken down to lactate without oxygen, and then lactate is broken down to carbon dioxide and water with oxygen.
Brooks is not done yet. His most recent research has looked at the role of lactate in cell signaling. It suggests that the high levels of intracellular lactate that arise during intense exercise stimulate some of the beneficial fitness adaptations that occur in response to such training. Specifically, high lactate concentrations trigger the production of free radicals that “upregulate” a variety of genes. Some of these genes govern mitochondrial biogenesis. So it appears that intracellular lactate accumulation during intense exercise stimulates the muscle cell to produce more mitochondria, thus enhancing its ability to burn lactate (and other fuels) in future workouts.
If I had to package all of the forgoing science into a single upshot, it would be this: According to the classical theory of lactate, one of the highest priorities of training was to reduce the amount of lactate the body produces at higher exercise intensities so that the athlete can race faster without fatiguing due to high lactate levels. According to the new theory of lactate, one of the highest priorities of training is to increase the body’s capacity to use lactate during high-intensity exercise so that the athlete can race faster.
So what practical difference does this shift make in terms of how we train? In truth, not much, because the advanced training methods that today’s best-informed triathletes use were developed through blind trial and error, and were not fashioned consciously to conform to now-discredited ideas about lactate.
That said, for many years lactate-conscious coaches have counseled athletes to strictly limit the amount of training they do above the lactate threshold because the large amounts of lactate produced in such workouts are very stressful to the body. The rationale for this widely heeded caution has disappeared. It certainly remains true that the physiological stressfulness of exercise increases exponentially as its intensity does, such that the amount of training the body can handle is inversely related to its intensity. But lactate is not the reason. And lactate threshold intensity is not all that high. In the typical trained triathlete it corresponds to the fastest swimming, cycling, or running speed that can be sustained for one hour. There’s plenty of room to go faster in your training without wearing yourself down.
Furthermore, as we have seen, far from stressing the body, high lactate levels trigger some of the most important performance-boosting muscle adaptations. You might not be able to handle a high volume of training above the lactate threshold (again, for reasons that have nothing to do with lactate), but the new science of lactate suggests that you should go there frequently nonetheless. Many triathletes wait until the race phase of training to introduce supra-threshold training into their bike and run regimens (swimming, as always, is another matter. Training in this discipline is entirely based on high-intensity interval work). It would be better to do a small amount of supra-threshold training throughout the training cycle, with the greatest volume of such training immediately preceding races, for those who compete in short-course events (because lactate threshold pace is close to race pace at theses distances) and falling somewhat earlier for those who compete in long-course races.
How much supra-threshold training is enough? A Spanish study involving cross-country runners found that a mix of 81-percent moderate-intensity training, 10.5-percent lactate threshold training, and 8.5 percent supra-threshold training produced optimal results. That 8.5 percentage is a sensible median target. All triathletes should do 5 percent of their bike and run training at supra-threshold intensities as a baseline. Short-course specialists can peak at roughly 12 percent and long-course triathletes at 8-10 percent.
Research has shown that the greatest lactate exposures occur in workouts consisting of 3- to 5-minute intervals at VO2max velocity separated by 2- to 3-minute active recoveries and in 30- or 60-second intervals at the same intensity separated by active recoveries of equal duration. VO2max velocity is approximately the fastest speed you can sustain for 10 minutes in swimming, cycling or running. Lactate interval workouts featuring shorter intervals are a bit more manageable and should therefore come earlier in the training process. Never try to do more than 20 total minutes of VO2max-intensity swimming, cycling or running into a single session. If you do, you will boil alive in toxic lactic acid.