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The Best Exercise For Metabolic Health: Zone 2

1/11/20


When it comes to holistic health, many of the answers lie in the mitochondria. Efficient mitochondria means efficient fuel usage. It means more insulin sensitivity, lowering the likelihood of disease and potentially fostering a longer, healthier life. Of course, most people understand that there are benefits to working out and eating right, but often the improved health comes at a high cost of time spent in the gym and sacrificing favorite foods. Because of this, it's always helpful to know which practices pack the biggest punch in keeping you as healthy as possible.


In dieting, fasting is one of the most potent tools for training your metabolism, decreasing insulin resistance, and burning fat. It's another option in your toolkit. In exercise, the most influential training you can do is Zone 2. Before getting into the details of Zone 2's impact on the body, let's first understand the function of the "powerhouse of the cell" that you heard about in high school biology.


The mitochondria is the organelle at the cellular level that is responsible for energy production. It uses fatty acids and glucose to generate ATP, the energy currency. If you're more interested in the biochemistry of how that's actually done, just look up glycolysis or beta oxidation.. pretty intense stuff. For the purposes of this article, you just need to know that there are aerobic (with oxygen) and anaerobic (without oxygen) means of energy production. Most of what we do on an average day is aerobic and uses mostly fatty acids. Glucose is a more quickly delivered fuel and it's used greatly in anaerobic production. That's when your body is not getting oxygen quickly enough to produce the large amount of energy that's needed. Additionally, we have three main muscle fibers that are used. Type 1 are the "slow twitch," because their contractions are less forceful, they fatigue very slowly, and they rely mostly on fatty acids for fuel (we are almost always using some glucose). Type 2A is the middle ground between 1 and 2B, and 2B is the other extreme, fatiguing quickly and using almost all glucose. Those are the "fast twitch" muscles that our bodies recruit for intense activity like sprints.


So how does this all tie together, and how do we apply it to improving the efficiency of this system? To start to understand this, it helps to relate the whole process to a car. In a car, as output required/intensity increases the engine becomes more stressed. Past a certain point the RPMs are too much to handle, and we have to shift to prevent breaking down. It's the same in our bodies. Most of the time, we are using Type 1 muscle fibers and oxidizing fat to produce energy. As we ramp up the intensity, we "shift gears." These "gears" are known as Training Zones. The lower zones recruit the Type 1 fibers, which have the highest density of mitochondria. They oxidize fat well, then, because fat can only be broken down in the mitochondria. Zone 1 is for walking down the street or climbing stairs. In the middle zones, the body starts to produce more and more of its energy through glycolysis rather than fat oxidation. In Zone 6, the body taps into the small amount of ATP that is stored in the muscles themselves - this is reserved for the "all out" pushes, where energy is needed at the highest rate.


But let's take a closer look at the transition between fat oxidation and glycolysis. What really happens when your engine "revs" and the "shifts" occur? As we move through Zone 1 and start to touch Zone 2, more oxygen is used, more mitochondria are required, and more fat is burned to keep up with the rising intensity and energy requirements of muscle contraction. Simply put, as we get to Zone 2, we are producing maximum ATP possible under completely aerobic conditions. That means it also entails peak fat oxidation. You read that right. It's a natural thought to say that we must burn the most fat when we're going the hardest, but that's not true. Touching Zone 3, Type 2A muscles fibers start to engage and more glycolysis means less fat is being burned.


The kicker is that the maximal usage of Type 1 mitochondria (Zone 2) actually changes them. It increases their size and density, as visible in muscle biopsies of world class aerobic performers. It improves mitochondrial function and therefore makes your body more efficient in using fat as fuel. It trains your body, just like fasting trains your body, to use fat instead of storing it. As we've explored previously, allowing the body to get comfortable storing fat increases insulin resistance, an omen of a really unhealthy domino effect.


This kind of training also improves lactate clearance. Lactate is a byproduct of glycolysis, and the hydrogen ions in it causes that burning muscle fatigue. So by better clearing lactate, you also increase the Zone 2 threshold, so you can go harder for longer (something clearly important for competing endurance athletes).


So Zone 2 is the best of both worlds! You're optimally training your mitochondria and metabolism to be more efficient without trying annoyingly hard. You don't have to be overly uncomfortable, and there is less opportunity to hurt yourself. It'll keep you satisfied with your workout and returning to the gym fired up next time, which is important.


OK, so we know Zone 2 is good for us. But how do you know you're in Zone 2, and how much do you actually have to do to see a benefit from it? Well, your level of output (measures in watts) will differ from other people, because everyone has different mitochondrial function. Competitors in the Tour de France will cruise non-stop in Zone 2 at a ridiculous output. They'll operate with no issue around 5 watts per kilogram of body weight, while someone with Type 1 diabetes will reach Zone 2 at a lower output, like 1.5 watts/kilo.


Most accurately, you know you're in Zone 2 when your blood lactate levels are around 1.5 - 2 millimole. If you've ever done a lactate test for a sport, you know that higher levels (>10 millimole) come with the highest zones. If you don't have access to a blood lactate test, like most of us, it's still easy to know you're operating around Zone 2. It's about when you are breathing as hard as you can, while still being able to maintain a normal, coherent conversation. Don't come back and blame me when people are giving you weird looks at the gym.


In terms of dosage, a study done by Dr. San Millan (see below) showed that 2 hours of Zone 2 training a week is enough to maintain mitochondrial function, while 3 to 4 hours and above will improve it. I assume the maintaining part refers to after I actually up my baseline and get up off the couch...


It's nice to know that Zone 2 is the most metabolically influential exercise you can do, but at the end of the day it comes down to personal balance. For example, I like to enjoy a few beers on the weekend. Although not an ideally healthy habit, I'm not necessarily unhealthy because of it. I make up for it and more in other areas. If a habit contributes to your happiness or de-stresses you, it might always be worth it. After all, cortisol may be one of the most damaging hormones, so if a glass of wine a night keeps you levelheaded, by all means continue. If a slice of pie in non-negotiable for you, then you can double down in other areas and pull other levers to keep you at a health level you're happy with. No one is perfect in their habits, and it comes down to what you want. Having options is great, though, and for me, Zone 2 and fasting are the most intriguing to implement and balance.


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If you're interested in going deeper into this topic, I recommend checking out Peter Attia's podcast with Dr. San Millan, a former pro cyclist and expert in mitochondrial function. He lost about 60 pounds despite keeping his diet of pasta and wine by using only Zone 2. In his studies, he has performed some really interesting experiments with both world class athletes and diabetics, assessing mitochondrial function through some really cool tests, such as the respiratory exchange rate (the ratio of oxygen on an in breath to CO2 on an out breath can actually tell you how efficient your mitochondria are).

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