The evidence is convergent. Multiple independent sources reach the same conclusion, the underlying mechanism is well-characterized, and even the field's most cautious voices treat it as worth doing.
But yeah, you know, lots of debate around what's the best way to increase mitochondrial content because that in turn is going to set the upper limit for fat oxidation capacity. And athletes in particular want to have a very high rate of fatty oxidation, because even in very lean individuals, there's ample fat on board, and carbohydrate tends to be a very precious and limited fuel.
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So if you're doing a high-intensity interval training session, and you're going above 80% max heart rate, you're now going to this lactate threshold where you're basically producing more lactate quicker than you can consume it, potentially you're anaerobic maybe. I mean, so this idea is like that you're not going to be burning fat during that part of the exercise, but that doesn't necessarily mean that you're not going to be able to burn more fat after, because it does, as you just mentioned, increase mitochondrial biogenesis. So you're actually increasing the capacity to oxidize fat later on.
So I think that the most important adaptation or a critical adaptation in muscle is increasing mitochondrial content, which then will allow a greater fat oxidation capacity as well as a greater carbohydrate oxidation capacity.
So, you know, mitochondrial content largely limits or determines fat oxidation by an individual muscle or fat oxidation during exercise is largely determined by mitochondrial content.