If you’ve ever participated in an exercise class of any type, you’ve probably heard something about heart rate zones. But what are they, and do they really matter? While exercise has been linked to several health benefits — such as reduced blood pressure, improved insulin resistance and decreased risk for chronic diseases (such as type 2 diabetes), there isn’t much clinical research demonstrating whether one type of exercise is better than another. The benefits of zone 2 cardio, defined as cardio workouts performed at 60-67% of your maximum heart rate, appear to be linked to improved mitochondria function. In this report, we’ll discuss what we know (and don’t know) about zone 2 cardio, how it impacts metabolic health, and why you might want to include some zone 2 training in your exercise regimen.

The relationship between workout zones and metabolism

Training zones were developed by exercise scientists as a sort of “cheat sheet” method for understanding which energy systems your body is using at any given exercise intensity. Training zones, which are defined by a percentage of your maximum (“max”) heart rate, cause your body to get — and use — energy in different ways. There are six training zones, defined by heart rate, but most guides only include zones 1-5 (all aerobic zones), as zone 6 is technically an anaerobic training zone and can only be sustained for very short periods of time.

• Zone 1: 50-60% max heart rate; very easy intensity
• Zone 2: 60-70% max heart rate; easy intensity
• Zone 3: 70-80% max heart rate; moderate intensity
• Zone 4: 80-90% max heart rate; hard intensity
• Zone 5: 90-100% max heart rate; maximum intensity
• Zone 6: 100% max heart rate; maximum intensity for very short periods (e.g., seconds)

To fully understand the significance of each training zone, we first need to understand how our bodies get energy. The fuel for our cells is a molecule called adenosine triphosphate (ATP). ATP can be produced in three main ways:

• Oxidation turns fatty acids (i.e., fat) into ATP within parts of your cells called mitochondria. This process uses oxygen and produces huge amounts of ATP.
  
  
• Glycolysis turns glycogen (i.e., carbs) into ATP. It also produces a lot of ATP, but not as much as oxidation. Unlike oxidation, glycolysis does not use oxygen.
  
  
• Energy recycling. ATP can also be recycled. When it’s used, it loses a phosphate and becomes adenosine diphosphate (ADP). Creatine can provide a phosphate to ADP, turning it back into ATP. This is why a lot of athletes use creatine supplementation.

For the purpose of understanding training zones, only the first two mechanisms for energy production matter. (Only the most intense training zone — zone 6 — uses energy recycling/creatine for energy production.) 

As a general rule of thumb, the more intense the exercise, the less oxygen is used for energy production; the less intense, the more oxygen is used. There are six training zones, divided based on exercise intensity, from low to high. Therefore, lower intensity zones (zones 1 and 2) use more oxygen and therefore burn fat for energy, while higher intensity zones (zones 4 and 5) use less oxygen and burn carbs for energy. Zone 3, in the middle, burns both fat and carbs.

What is zone 2 cardio training?

Zone 2 is like the “sweet spot” zone for burning fat: you’re stimulating your mitochondria to burn fat as much as possible without tipping your body over into glycolysis and carb burning. Because fat stores are essentially bottomless, unlike carb stores, you might feel like you can go forever when you’re “in the zone” (2, that is). Typical zone 2 training workouts are longer (30 minutes or more), low-intensity, cardio-style workouts: running, cycling or swimming are examples. 

There are several important health benefits associated with zone 2 training:

• Zone 2 cardio increases mitochondrial number, function and flexibility (the ability of mitochondria to use fat or glucose), which has a direct impact on your health. Mitochondria are like muscles — you need to use them or they become weak and inefficient. Dysfunctional mitochondria have been associated with a range of diseases, including metabolic syndrome, type 2 diabetes and cardiovascular disease.
  
  
• Improved athletic performance, which is the reason many elite athletes spend most of their time in zone 2 training
  
  
• Decreased resting heart rate and lower blood pressure
  
  
• Improved insulin resistance
  
  
• Improved longevity

It is important to note that these health benefits are related to the improvements in mitochondrial function associated with zone 2 training. However, there have been only a few randomized controlled trials directly tying zone 2 training specifically with improved health outcomes. And there are as many studies suggesting that high-intensity training is better for health and longevity as there are ones that support the benefits of low-intensity training (such as zone 2 cardio). 

Many scientists and doctors suggest that a combination of the two is best for maximum health benefits. As significantly, it’s clear that at the end of the day, exercise of any type, intensity, and duration has beneficial effects on health and well-being, and any exercise is better than none. Because zone 2 training is low-intensity and easy to do, it can be a great option for people who simply want to stay fit without becoming sore and exhausted at the end of every workout.

How do I know if I’m in zone 2?

Training zones, as mentioned, are defined on a percentage of your maximum heart rate. During zone 2 exercise, your heart rate is 60-70% of your max heart rate (see Note below). There are a couple of different ways to identify your zone 2 heart rate, but take note: most of these are estimates, at best. The only way to really know if you’re in Zone 2 is to measure the levels of lactate (i.e., lactic acid, which is produced during exercise and the culprit behind sore muscles) in your blood in a way similar to pricking your finger to measure blood sugar levels. Estimation methods, for the most part, are good enough for identifying a target heart rate range and experiencing the benefits of zone 2 cardio.

• The heart rate method. Tracking your heart rate is the most common method of estimating which zone you’re in; that’s why you see all of those people wearing heart rate monitors to their Orange Theory or spin classes. In zone 2, your heart rate will be at 60-70% of your max heart rate. So, if your max heart rate is 185, you should keep your heart rate between 111 and 130 beats per minute to ensure you’re in zone 2.
  
  
• The talking test. You may have heard about this one from a fitness or team sports coach. The general rule of thumb here is that during zone 2 exercise, you can still have a conversation, but you’ll be slightly out of breath.

Note: You can estimate your maximum heart rate by subtracting your age from 220. So, if you’re 35 years old, your max heart rate is 185. Note that several factors, including chronic disease, can impact your max heart rate; so the best way to get an exact number is to ask your doctor to do a max heart rate test.

Key takeaways

Training zones were developed as a quick and easy way to determine whether we’re burning fat or carbs during exercise. While low-intensity zone 2 exercise has been linked to improvements in both mitochondrial function and metabolic health, clinical research comparing the health benefits of low-intensity and high-intensity exercise is sparse. Instead, most doctors recommend a combination of low- and high-intensity exercise to reap the benefits of both while preventing injury, overtraining and waning motivation. An exercise plan works only if you can stick to it —  and variety is the spice of life (and exercise!).