In an earlier post, I walked through a 2025 peer-reviewed review that challenged the popular claim that Zone 2 training is uniquely optimal for building mitochondria and improving general metabolic health. I stand by everything in that post. But it left an important question unanswered, one worth addressing directly: what about people actually training for endurance races? That's a genuinely different question, answered by a different body of evidence, and I want to walk through it honestly.
A familiar story
I've heard some version of this from more than one patient and runner over the years: someone spends months chasing faster mile splits, training hard on most runs, and their race times stall out or even get worse. Then a coach tells them to slow down — sometimes dramatically, capping heart rate well below what feels like real effort, for a long run once a week — and their race times improve meaningfully within a few months. It sounds almost paradoxical. I think it's actually one of the more well-supported findings in endurance sports science, once you look at the right literature.
A terminology mix-up worth clearing up first
The popular "Zone 2" framework I addressed in my last post — the one criticized for its general-health mitochondrial claims — describes essentially the same intensity range that competitive endurance coaching has called "Zone 1" for decades, in a three-zone model built specifically around lactate and ventilatory thresholds. So there isn't really a contradiction brewing here. It's largely two different research communities using different names for a similar physiological zone, while asking different questions about it.
The question my last post didn't answer
My previous post focused on a specific, narrow claim: that this intensity zone is uniquely optimal for building mitochondrial capacity and improving general metabolic health in the broader population. The evidence didn't support that specific claim being uniquely true of this zone. But "does this zone uniquely drive mitochondrial biogenesis better than other intensities" is a different question from "what training distribution produces the best endurance race performance in athletes who are already training seriously." That second question has its own substantial research literature, and it points in a genuinely different direction.
What the performance literature actually shows
Research specifically on training intensity distribution in competitive endurance athletes consistently points toward something called polarized training: a large majority of total training volume — often 75% to 85% — performed at low, genuinely easy intensity, a small portion at real high intensity, and comparatively little time spent in the moderate "gray zone" in between. Multiple controlled studies in well-trained runners have found this distribution outperforms programs weighted more heavily toward moderate-intensity effort, including measurable improvements in race performance itself, not just laboratory markers.
Lactate threshold specifically — the pace you can sustain before your body starts accumulating lactate faster than it can clear it — does appear to improve with this kind of training. Several studies have documented real gains in threshold running velocity using programs built around this same high-volume-easy, low-volume-hard structure. This is a genuinely different, more favorable finding than what I described in the general-health post, where a review of Zone 2-only training found no clear effect on lactate threshold in already-trained individuals — a meaningful distinction, since real coaching programs like the one described above aren't a single isolated intensity practiced in isolation. They're a full, periodized distribution across the whole training week.
I want to be honest that this isn't a perfectly settled question either. A study in elite rowers found polarized training wasn't clearly superior to an alternative "pyramidal" distribution, and a study in triathletes found no significant difference between polarized and more moderate distributions. The consistent theme across this literature isn't that one exact ratio is definitively the answer — it's that minimizing excessive moderate-intensity training, in favor of genuinely easy volume paired with genuinely hard effort, tends to outperform training that's mostly moderate.
What's probably actually happening when this works
I think there's a well-supported, honest explanation for why shifting toward more easy-paced volume so often produces real improvement in runners who were previously training the way described above, and I don't think it requires choosing between "the coach was right" and "the mechanism he described was oversimplified." A few things are likely happening at once. Many self-coached runners chasing faster splits end up living in exactly the moderate "gray zone" the performance literature warns against — hard enough to accumulate real fatigue, not hard enough to drive the strongest adaptations. Shifting a meaningful chunk of training to genuinely easy effort is one of the most consistently documented fixes for that specific pattern. Separately, adding a long run at a controlled, sustainable pace often simply adds real training volume that wasn't there before, and total volume is itself one of the better-supported drivers of endurance improvement in this literature, somewhat independent of the exact intensity used to accumulate it. And the lactate threshold explanation itself has genuine support, more so in this performance-specific context than in the general-health framing I covered previously.
What I'd want you to take from this
The practical coaching advice — build a large base of genuinely easy running, keep most of your hard efforts truly hard, and don't spend most of your training in the moderate middle — is well-supported by real endurance performance research, even when the specific mechanistic explanation attached to it (a single zone uniquely unlocking superior fat-burning) is a simplified version of a more layered reality. This is, I think, a good illustration of something worth holding onto generally: a training or health practice can be genuinely well-supported by evidence even when the popular explanation for why it works is imprecise. The two aren't the same question, and it's worth being able to tell them apart.
Curious how training intensity fits into your own performance or metabolic health goals?
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