The Incredible Metabolic Benefits of Zone 2 Training

When it comes to endurance training, fat loss, and longevity, one application of low-intensity steady-state cardio comes out on top, and that's Zone 2 training. It's the deadlift of the endurance world and accessible to almost everyone. Today we will briefly discuss what Zone 2 training is and talk about its incredible metabolic benefits!

There will be many more posts about Zone 2 training in the future, but today we're going to focus on metabolic flexibility, fat vs carbohydrate oxidation, and lactate levels with exercise intensity. Before diving in, let's talk about what it means to be in "Zone 2".

Although heart rate is generally used to determine if you're in Zone 2, the technical definition is the highest metabolic output you can sustain while keeping your lactate level below 2 millimoles per liter (mmol/L). So, to really know if you're in Zone 2 while on a run, swim, or bike ride, you'd have to stick yourself and draw some blood to measure your lactate level. That's both inconvenient and expensive, so most people rely on heart rate and perceived effort instead.

To be in Zone 2, you have to maintain a heart rate that is 70-80% of your maximum. The best way to figure out your max is to wear an accurate heart rate monitor (such as a chest strap) and push yourself to your max while exercising. Easy, right? Alternatively, you can estimate your max heart rate by subtracting your age from 220. So, as a 26-year-old, my estimated max would be 194 beats per minute (220-26=194). Taking 70-80% of that puts me in a range of 136-155 bpm for Zone 2.

There's also the Maffetone formula which is calculated by subtracting your age from 180 and represents your max aerobic heart rate. Again, mine would be 154 beats per minute as a 26-year-old, so you can see how the use of either one of these formulas yields similar results. However, the accuracy of these formulas is limited! Elite athletes like Lance Armstrong (regardless of PED use) can expect to have max heart rates 30% higher than these formulas predict, and they're capable of maintaining 90-95% of that heart rate for about an hour. For those that are unfamiliar with monitoring their heart rates during exercise, that is an insane physiological accomplishment.

Lastly, you can subjectively determine if you're in Zone 2 through perceived exertion. If you can talk in full sentences, perhaps with occasional pauses to catch your breath, then you're likely in Zone 2. For example, you should be able to talk over the phone with a friend, but they would definitely be able to tell that you were exercising. This is what separates Zone 2 from simply walking around.

Interestingly, your ability to exercise in Zone 2 is highly dependent on your metabolic flexibility. In patients with insulin resistance such as with type 2 diabetes, any amount of exercise can immediately put them into Zone 3 or 4, and their baseline lactate levels may actually be around 2 mmol/L. But more on that later...

There are several different fuels that your muscles' mitochondria utilize during exercise including glucose (carbohydrates), fatty acids (fat), and lactate (byproduct of carbohydrate utilization). The fuel that you actually burn depends on a variety of factors including fitness level, mitochondrial flexibility, and effort. Remember that glucose and fatty acid derivatives are both oxidized in the mitochondria, so mitochondrial abundance and function are key to a healthy metabolism. To be "metabolically inflexible" means you have a decreased capacity to oxidize lipids and experience an early transition from fat to carbohydrate oxidation when exercising. Additionally, the metabolically inflexible experience a rapid elevation in blood lactate concentrations as exercise power output increases.

One of the special characteristics of Zone 2 training is that it optimizes fat oxidation as a fuel source! We can use indirect calorimetry to determine in any individual what percent of their fuel is coming from fat vs carbohydrate oxidation. This is calculated by comparing the oxygen and CO₂ concentrations of your exhalations during exercise. As your effort increases, such as in the case of Zones 3-5, there is a transition from fat oxidation to carbohydrate oxidation. This shift is due to a number of factors, most notably an increase in lactate. Therefore, if you can limit lactate buildup and increase clearance, you can rely more on fat oxidation.

In the metabolically impaired, the ability to prioritize fat oxidation is limited since their capacity to stay in Zone 2 is severely hindered by a lack of metabolic flexibility. Especially in an environment of hyperlactatemia, or lactic acidosis, patients with mitochondrial dysfunction have impaired fatty acid clearance and elevated plasma-free fatty acids, a common symptom of insulin resistance. These patients also lack the ability to quickly clear lactate (kicking them out of Zone 2), which causes a decrease in fat oxidation and an increase in carbohydrate oxidation.

In the case of elite athletes, they are able to spend longer periods of time in Zone 2 and are able to do so at much higher energy outputs! For instance, an elite marathon runner might be in Zone 2 while running a 7-minute mile and maintain that for hours. On the other hand, someone who is moderately healthy may only be able to run a 10-minute mile while staying in Zone 2. Part of an athlete's ability to remain in Zone 2 is dependent on their capacity to clear lactate, which is optimized in elite endurance athletes!

To reiterate, patients with type 2 diabetes or other metabolic syndromes may be wholly incapable of exercising in Zone 2 for an appreciable amount of time due to four main factors:
1. Increased baseline lactate levels
2. Decreased lactate clearance
3. Decreased fat oxidation
4. Increased carbohydrate oxidation

TANGENT: The criteria for diagnosing septic shock often include a lactate level of > 2 mmol/L in the absence of hypovolemia. Research from the likes of Iñigo San-Millán et al makes me wonder if baseline lactate levels will be considered in the future as lactate (in my opinion) becomes a more common biomarker to measure the metabolic health of patients. Should these criteria instead include a % increase from baseline, rather than a broad recommendation of > 2 mmol/L? Sounds like an interesting study! I find this especially intriguing since lactate clearance is often assessed during treatment for septic shock, so it would be great to know if it was truly elevated from baseline in the first place...

Let's make sure you understand the relationships between lactate, carbohydrates, fat, and exercise effort!

1. Lactate and fat oxidation are inversely related.
2. Lactate and carbohydrate oxidation are directly related.
3. Lactate and exercise effort are directly related.
4. Fat oxidation and carbohydrate oxidation are inversely related.

Hopefully this helps you visualize the metabolic benefit of Zone 2 training. In this Zone, lactate levels are low, therefore increasing your ability to oxidize fat over carbohydrates! The fitter you are, the higher "intensity related to peak power" you can maintain while still being in Zone 2. With that being said, I want to make one point clear for those using this data to determine what kind of exercise to do in an effort to lose weight or burn fat. Caloric consumption as a total will be higher in Zones 3-5 than in Zone 2 if done for the same amount of time, and this is due to an increase in relative intensity. While you'd be decreasing your relative fat oxidation in these zones, total calories burned would likely offset the difference. Therefore, spending longer amounts of time in Zone 2 would be required to compete with short amounts of time in higher Zones. Luckily, this is easy to do and is why you often hear the recommendation to stay in Zone 2 for at least an hour at a time (among other reasons we will talk about in upcoming articles). Makes sense! This does not mean you should avoid time in higher Zones as they are also very important to overall health, especially Zone 5.

As your fitness level increases, you'll be able to spend more time in Zone 2 at higher energy outputs, thus enhancing your metabolic flexibility and your ability to maximize fat oxidation. You may be wondering, who cares? Well... THIS IS HOW YOU AVOID METABOLIC DISEASE! If you want to avoid diabetes, feel strong and capable in your 60s and 70s, avoid needing medications in old age, and be able to go on hikes with your grandchildren, then this stuff matters. You simply must make it a priority. Metabolic disease is one of the main causes of death in the United States and it drastically increases your risk of other diseases like cancer, Alzheimer's, and cardiovascular disease. Therefore, metabolic flexibility is protective and nearly every study will support that conclusion. We know cardiovascular training, especially in Zone 2, enhances your metabolic and mitochondrial flexibility, so please don't ignore it! Your life may depend on it, and your quality of life definitely does.