Zone 2 training: Burn Fat the Easy Way

Remember those brightly colored “heart rate zone” posters hanging ubiquitously on the walls of school gyms and fitness centers a few decades ago? Well, they haven’t gone away. In fact, with the proliferation of wearable heart rate tracking devices, heart rate (HR) zone training is more accessible than ever.

The heart rate training zones were first described in the 1970s when the first wearable HR tracking technology became available to consumers. Within a few years, the poster seen above was on the wall of every gym, where it still lives today. Fortunately, the science behind HR zone training has evolved, as have the wearable technologies. For less than $100, you can now get reasonably accurate HR tracking on your wrist. First lets take a closer look at the zones and how they’re calculated, then delve into the nitty-gritty science to back it up.

The Zones

The zones are simply based on training at a percentage of maximum heart rate (MaxHR).  So what is your maximum heart rate?  You could do an intense time trial on a stationary bike or rowing machine, but its probably easier to use this simple formula:  

Max Heart rate = [208 – (0.7 x age)]

With your MaxHR value in hand, simply refer to the chart below which outlines the 5 commonly accepted heart rate training zones, and multiply your maxHR by the percentage to find out what zone you’re in at each level of exertion as measure by your heart rate. 

Each zone has its own benefits, and elite athletes will adhere to training schedules with time prescribed in each zone according to their goals. But even for the average weekend warrior like you and I, its good to spend time training each zone as they each have unique benefits.

Zone 1 is great for recovery days or off days, and it squares perfectly with primal principle #3: move frequently at a slow pace.  While there’s nothing wrong with Zone 1, training here exclusively won’t help you achieve any real significant fitness goals.

For a real fitness transformation there’s nothing like the zone 4 work you’ll get from HIIT or CrossFit. For those wishing to push their conditioning to the max, its great to touch Zone 5 once a week with some all-out sprints. But depending on your goals and baseline fitness level, a higher intensity workout is not necessary and can be risky.

If you are like many people and your primary goal is metabolic conditioning and fat loss, you’ve probably focused on things like jogging, spin, Zumba, etc. These activities typically put you in zone 3 and most folks think this is the sweet spot for weight loss and fitness. But let’s take a closer look at the science and see why the nearly forgotten zone 2 is for you!

The science

I’ll try to explain this in in a technical enough way without getting too much jargon and detail. If you are a sucker for detail–and I mean DETAIL–check out Peter Attia’s interview with Inigo San Milan, one of the world’s leading experts on HR zone training. As I do my best to explain in much fewer words than Peter and Inigo use, follow along on the graph below to help visualize.

When exercising, our muscles need energy in order to contract. This cellular energy is called ATP (adenosine triphosphate). ATP can be generated through two main fuel sources: fats (fatty acids) and carbohydrates. At relatively low levels of exertion, such as those seen in zone 1, there is abundant oxygen and fatty acid availability for the muscles to use their preferential fuel source–fatty acids–which are readily available (with a nearly inexhaustible supply) and are efficiently converted into ATP energy through a process known as mitochondrial beta-oxidation. Simultaneously, the muscle can also produce ATP by metabolizing carbohydrate in the form of glycogen stored in the muscle via the Krebs Cycle (remember that from college biochemistry?). As seen on the far left of the graph above, fatty acid oxidation and carbohydrate metabolism are both occurring even at low levels of exertion, with fat metabolism being the dominant pathway. With me so far?

As exertion level rises and we go to the right on the graph, you enter zone 2. Here’s where the magic happens. Energy requirements are still low enough that most of the cellular ATP requirement can be met through fat oxidation. Eventually, there reaches a point of maximal fat oxidation (FatMax). This point is different for each individual based on their level of metabolic health and fitness, but is typically found somewhere in the Zone 2 HR spectrum.

As you continue to exert more energy and HR rises into zone 3, Different muscle types are recruited. In Zone 3 type IIA fibers (fast twitch) are recruited to help achieve the body’s movement goals. This allows for faster, shorter, and more powerful contractions but comes at the cost of easier fatigability and more lactic acid (lactate) production. Additionally, more carbohydrate is used for fuel because the demand for ATP is too high to achieve from burning fat alone.

As we progress with exertion into zone 4 and 5, we will see that fatty acid oxidation is almost completely tapped out and carbohydrate metabolism is the only fuel source capable of producing enough ATP to keep up with such high levels of exertion. (n.b. I left out lots of detail here for simplicity, including the contribution of the phosphagen system. If you want to learn more, get a PhD and read this)

Why zone 2?

With the detailed scientific explanation out of the way, lets turn our attention to why Zone 2 training is important. First and foremost, the point of maximal fat burning is in Zone 2. If you’re interested in burning fat (and who isn’t), Zone 2 training is the sweet spot.

Added bonus? Its enjoyable…and dare I say easy. For me, Zone 2 training means going for a brisk walk on a treadmill with a slight incline or walking about as fast as I can while swinging my arms. Here in flat Florida, there aren’t any hills to walk, but the thick soft sand of the beach does a good job as well. A decent pedal on a road bike will probably do the trick too. But even an “easy” row (is there such a thing?) or swim usually gets me beyond zone 2 into 3 or even 4 depending on intensity.

Zone 2 training isn’t just for the average weekend warrior or amateur athlete; Inigo San Milan argues that even world class athletes will benefit. The reason? Zone 2 training promotes slow twitch type I muscle fibers that serves as a “sink” for lactic acid. Type IIa and IIb muscle fibers used with more intense activity produce a lot of lactic acid, which notoriously builds up and causes muscle soreness. But as it turns out, having a huge reservoir of type I fibers will help as these fibers reabsorb and use the lactic acid. It’s a win-win situation.

The takeaway

So what’s the takeaway? HR zone training is actually based in some pretty cool science. When looking for fat burning and improved mitochondrial health, Zone 2 training is the sweet spot. The added bonus is that its relatively easy, less intense, and safer than most exercise programs. Whether you are looking for elite fitness or just trying to get off the couch, you need a good base of zone 2 training, preferably 2-3 hours per week. So get out there and MOVE…a little slower and less intensely sometimes!

~Brendan