Warning! Long-winded answer agead, but I promise I will get there…
As I have been pointing out to people for a long time, the muscular adaptations to exercise training are both exceedingly complicated, yet rather simple. That is, while repeated bouts of exercise influence the expression of thousands of genes, at the end of the day these changes (and the accompanying post-translational modification of proteins, etc.) result in only one of two things:
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An increase in the ability of muscle to produce greater maximal force or power output, or
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An increase in the ability of to sustain an already submaximal force or power output for a longer period of time, i.e., greater resistance to fatigue (failure).
The latter set of adaptations could then be subdivided into adaptations that result in greater fatigue resistance during very high intensity, non-sustainable exercise (e.g., an increase in muscle buffer capacity) vs. those that contribute to fatigue resistance during lower intensity, more sustainable exercise (e.g., an increase in the capacity to transport fatty acid across the sarcolemma), with some overlap between these subcategories (e.g., an increase in capillarization would both enhance delivery of substrates during lower intensity exercise and removal of potentially fatiguing waste products during higher intensity exercise).
Viewed from this perspective, I think it far easier to understand why you don’t really need to train at very specific intensities to induce specific adaptations. The body really has only one of two “programs” by which it can respond, which belies the notion that you can directly target just one thing (e.g., an increase in MCT). It is also important to recognize that 1) the idea of exercise “intensity” is a bit nebulous at the level of the individual motor unit (since by and large they are either “on” or “off” and don’t really care much about what their neighbors are doing), and 2) although during exercise ATP can be supplied both anaerobically and aerobically, recovery from exercise is essentially a completely aerobic process. This further blurs the lines between training “zones”, and explains why something like HIIT leads to the same outcomes as continuous exercise at a much lower intensity.
TL, DR: It’s all good! In particular, training at a VERY wide range of intensities leads to an increase in mitochondrial respiratory capacity. There is absolutely nothing special about so-called “zone 2” in this regard, except perhaps the fact that you can do more of it, resulting in a greater overall “dose” of training (i.e., combination of volume and intensity).* Anybody touting the “magical” nature of zone 2 and mitochondria simply doesn’t know what they’re talking about (this includes folks like ISM and Attia).
*With that said, the studies in the literature demonstrating the largest increases in mitochondrial marker enzymes that have had individuals train the hardest, not the most, with initially untrained individuals achieving comparable to elite athletes after just a few months of training. So at the end of the day, it may not really matter, i.e., “all roads lead to Rome”. Instead, it may simply come down to the individual, how much time and motivation they have, how durable they are, etc.
Don’t say I didn’t warn you about a long answer! 