Dr. Rhonda Patrick: What do you think about, so metabolic flexibility, being the capability to shift between using glucose as a substrate and using fatty acids? I mean, this is something that—

Dr. Peter Attia: This is the Zone 2 thing, right? This is exactly why we train that Zone 2 system. And that's why we have our patients spend 80% of their cardio training time in Zone 2. That's really pushing that metabolic flexibility. This is the training system for making sure you expand the capacity of your mitochondria to, under ever-increasing demands, have the ability to utilize fatty acids for oxidative phosphorylation, and glucose for that matter.

Dr. Rhonda Patrick: But if you were to do, let's say you're doing more high-intensity interval training, which I do a lot of, that increases the capacity because it's such a potent stimulator of mitochondrial biogenesis. So maybe, and I hesitate to say, I think a lot of times when I'm doing my HIIT, I'm still really using my mitochondria. Like, you know, I'm not like doing an all-out sprint, but like, you know, I do shift into using glucose, of course.

Dr. Peter Attia: We just think that only 20% of the cardio training volume should be there. And the reason for that is actually kind of an empirical observation. If you ask the question, who are the most metabolically flexible, healthiest specimens we have on this planet, they are high-level endurance athletes, namely cross-country skiers, distance runners, and cyclists. So what do we know about this group? We know that they have the highest VO2 maxes of any humans on the planet, and we know that they are the most metabolically flexible of any humans on this planet. Now, my experience is far more with cyclists, and so I usually just talk about this through the lens of a cyclist.

Dr. Peter Attia: And the other thing I like about cycling compared to skiing or running is we can use wattage, because we can put people on power meters and we can get the numbers. A world-class cyclist is able to put out four watts per kilogram of power while keeping lactate below 2 millimoles per liter. In fact, the best cyclists in the world are probably at about 4.2, 4.3 watts per kilo. So let's just do the math on that. For anyone listening who has ever been near a power meter: if you're 80 kilos, you're 175 pounds, that means you're able to put out 330 to 340 watts, which, by the way, most people who weigh 80 kilos can't do that for one minute. Literally, they can't do that for one minute. These people can do it for hours and keep their lactate below 2 millimoles per liter.

Dr. Peter Attia: It's the single greatest demonstration of metabolic flexibility that you will ever see. How do these people train? This is one of the questions my patients ask me: “Peter, where is this 80/20 coming from?” Where is the study that demonstrated this? And I said, well, the studies are all based on what do you have to do to achieve that level of performance? So these athletes and their coaches have all figured out that to produce the highest VO2 max and to produce the greatest degree of metabolic flexibility, you think of it as a pyramid, where the base of the pyramid is your Zone 2 efficiency and the peak of the pyramid is your VO2 max. And the area, total area of the pyramid is your cardiorespiratory engine.

Dr. Peter Attia: So you want not a narrow base with a high peak, not a wide base with a short peak. You want a big base, big peak. And the way to get that is about 80-20. If you try to do too much high intensity, you simply don't have the aerobic base on which to build it. So yeah, you might have more mitochondria, but they're not as efficient. If you only do the low intensity stuff, they're efficient, but you might not have enough. This is a bit of an oversimplification, but you want the best of both worlds, right? You want both the breadth and the peak effectively. So what we basically do with our patients is we start from a standpoint of time. How much time are you willing to exercise a week? I'm not gonna tell you what you need to do. Let's start with you telling me what you're willing to do.

Dr. Peter Attia: And then the simplest approach is we'll put half of that into strength and stability, half of that into cardio. Of the cardio, it's 80-20. 80% of that will be Zone 2, 20% of that will be VO2 max. And VO2 max training, by the way, is pretty hard because it's slightly longer intervals than what people think of as traditional HIIT. So traditional HIIT works. I'm just saying it's not the best way to get there.

Dr. Peter Attia: It's a good way to get there. We know, even just looking at the Tabata studies, that Tabata is neither one nor the other. Twenty seconds on, 10 seconds off, for eight rounds is not pure Zone 2. Nor is it VO2 max training, because the VO2 max sweet spot is 3 to 8 minutes with one-to-one work-to-recovery: three minutes on, three minutes off. That's a lower intensity than most people are doing in a HIIT class. Most people in a HIIT class are doing shorter intervals and pushing much harder.

Dr. Rhonda Patrick: I just had a talk with Martin Gibala, and I asked him that question. He was like, “Rhonda, you've got to do more three-minute intervals,” because I wanted—I was like, I want to do VO2 max training. I do a lot of, you know, 16 rounds of 20 seconds on, 10 seconds off. But my heart rate is still pretty high during the 10 seconds off. So he's like, “You've got to do, like, three minutes at least, you know?” And so I've shifted my training now, and it's absolutely true: I am not going as hard.

Dr. Peter Attia: You can't.

Dr. Rhonda Patrick: Yeah, you just can't go as hard.

Dr. Peter Attia: And it's an art form. You'll figure it out, because you'll realize: I went too hard and was dead at a minute and a half, and I was loafing the last minute and a half. Or I held back too much, and by the end of the three minutes, it was like, oh, I actually could have gone harder. That's okay. You'll sort of figure out what that sweet spot is. But 3 to 8 minutes is the optimal zone for generating VO2 max power.