(Wasn’t sure where to stick this, so moderators, feel free to move if appropriate.)
One of the justifications offered for avoiding excursions to higher intensities during endurance workouts is to avoid suppression of lipolysis by lactate. The unstated assumption is that to get better at oxidizing fat you need to oxidize fat. However, as I have pointed out here and on the Inside Exercise podcast, neither of the above are true (something that has been known for a half century). First, the reduction in fatty acid concentration during high intensity exercise is not due to lactate-mediated suppression of lipolysis, but rather “entrapment” of fatty acid within adipose tissue due to diversion of blood flow to exercising muscle. This is why there is a massive “overshoot” in plasma fatty acid concentrations when exercise is over, despite the fact that lipolysis itself rapidly decreases. Second, the increased ability to oxidize fat after training is primarily the result of a training-induced increase in mitochondrial respiratory capacity, which is stimulated by contractile activity itself, regardless of the fuel source (i.e., fat or carbohydrate).
Nonetheless, there are apparently coaches and sports scientists who continue to promote these false ideas. That led to somebody emailing me, asking for references. I pointed them towards studies by Issekutz and by Bulow in the 1970s, but then got digging around in the literature to refresh my own memory. In doing so, I came across this study, which I don’t recall seeing before:
In addition to directly testing the hypothesis that lactate suppresses exercise-stimulated lipolysis in human adipose tissue in vivo, perhaps most noteworthy is the fact that they tested the effect of lactate at a higher concentration than Miller, Brooks, et al., who “clamped” lactate at a plasma (and therefore lower interstitial) concentration of “only” 4-5 mmol/L (and also found no effect).
Elevated lactate concentrations do not suppress lipolysis in exercising humans. This is a falsehood that needs to be buried once and for all.