Dr. Michael Ruscio, DC is a clinician, Naturopathic Practitioner, clinical researcher, author, and adjunct professor at the University of Bridgeport. His work has been published in peer-reviewed medical journals and he speaks at conferences around the globe.
Zone 2 training is simply exercise that gets your heart rate in a low-to-moderate zone of effort that promotes fat burning, mitochondrial function, and cardiovascular health.
Examples of zone 2 training include walking, jogging, biking, swimming, and most activities we would consider “cardio,” as long as they aren’t getting your heart rate up higher than a low level of effort.
It’s important to balance zone 2 training with resistance training for the combined health benefits of cardiovascular health, strength, mobility, and maintaining muscle with aging.
In general, you want to be doing zone 2 training in a 1:1 or 1:2 ratio with strength training. Start with equal days of cardio and strength training, and build up from there as you become more comfortable with regular exercise.
A surge in the popularity of fitness for overall health (as opposed to only aesthetics) has caused some new terminology to be tossed around in both online and offline conversation. One term that I keep coming across in my discussions with other practitioners and health enthusiasts is this concept of zone 2 training.
Despite the ambiguous name, zone 2 training is relatively simple to understand. Zone 2 refers to a low-intensity heart rate zone, or alternatively a lower level on the scale of breathing intensity. At this level of intensity, the idea is that you can do whatever exercise you’re doing with a lower amount of effort, such that you’re able to hold a conversation while you do it (i.e. not breathing so hard you can’t get a word out) but at the same time your heart rate is still somewhat elevated. Zone 2 training also refers to cardio, aka aerobic exercise, as opposed to resistance training or anaerobic exercise.
Why is zone 2 training important? Zone 2 training corresponds with increased fat loss, cardiovascular health, and healthy mitochondrial function. Basically, talking about zone 2 training is just a fancy way of saying something most of us already know: some amount of cardio-type exercise — that gets your heart rate up and stimulates circulation, metabolism, and lymphatic flow in the body, as well as detoxification through sweat — is good for you.
However, there’s a little bit more nuance to explore in the details and the research behind zone 2 training, so let’s dig into the theory, benefits, risks, and recommendations around this type of exercise.
What is Zone 2: The Zone 2 Training Theory
Though zone training is still making its way from pop culture into the scientific literature, there is a well-agreed-upon understanding of how heart rates that fall into the zone 2 range affect us physiologically. And it all comes down to the health of our mitochondria, our energy systems, and our ability to break down fat.
Mitochondria are the organelles known as the “powerhouse” of our cells and are the primary producers of our main energy source, adenosine triphosphate, or ATP. They also help with fatty acid oxidation, aka fat breakdown. Glycolysis, the breakdown and metabolism of glucose, is the second top contributor to our ATP reserves .
Overt mitochondrial disease is caused by genetic (and typically more severe) or non-genetic dysfunction that causes serious conditions like hearing loss, blindness, cardiomyopathy, dementia, poor cognition, and brain swelling . However, more subtle mitochondrial dysfunction typically doesn’t lead to clear clinical outcomes, but is believed to contribute to conditions like diabetes, autism, dementia, and early aging [3, 4].
When mitochondrial dysfunction starts to occur, the ratio of ATP production begins to favor glycolysis, and our energy factories begin to lose their ability to break down fat (lipolysis) leading to these disease states. Proponents of zone 2 training state that maintaining your heart rate in the “zone 2 range” can help combat this dysfunction by increasing mitochondria, supporting the breakdown of fat, and increasing mitochondrial efficiency .
We still have a lot of research to do to further validate this statement on zone 2 training. However, preliminary scientific evidence does support the hypothesis that light-intensity exercise may increase fatty acid oxidation and lipolysis and limit inflammation, which is a known culprit of many diseases .
The Different Heart Rate Zones of Exercise
According to the National Academy of Sports Medicine, heart rate zones are ranges of heart rates (number of heart beats per minute, BPM) that can be used to track the intensity of your workouts or other activities . Proponents of utilizing heart rate zones state that they can be helpful for designing your optimal cardio program and progressing toward your fitness goals at a reasonable and sustainable pace .
There are a number of different versions of heart rate zones floating around, but we are focusing on the five-zone standard as described by the American College of Sports Medicine (ACSM) [7, 8].
Here is a breakdown of the different zones based on the percentage of an individual’s maximum heart rate, the highest their heart rate should go safely when exercising.
Once you know your estimated maximum heart rate, you can calculate your personal heart rate zones. You can estimate your max heart rate by using this formula: 208 – (0.7 x age) = HR max. For example, a 43-year-old’s estimated HR max (rounded to the nearest ten) is 178 beats per minute.
If the 43-year-old wants to make a chart of her own estimated HR zone ranges, it would look like this:
Heart rate zone
ACSM Heart rate range in beats per minute
Other common Heart rate range in beats per minute
Intensity of exercise
Near max to maximal
Are Heart Rate Zones a Valuable Metric?
Although the common calculations of maximum heart rate and heart rate zones are generic and don’t take into account aspects of your life that may affect your resting heart rate (like stress level, hormonal imbalances, or cardiac conditions), heart rate zones are a useful way to estimate your training intensity .
The point is to be able to identify which types of exercise push you to your maximum heart rate and max effort, and which ones keep you in a low to moderate range of effort. Knowing how much effort you’re putting into your workouts tells you what fuel source your body will use to sustain that effort: either fat, glucose, or both.
An Alternative to Heart Rate Zones: The Talk Test
A potentially more accurate way to determine your unique training zones is to note not your heart rate, but the effort it takes for you to speak while exercising. This breathing-based system includes only 4 zones, but zones 1 and 2 should be roughly equivalent to zone 2 on the 5-zone heart rate training scale . This system might seem a little ambiguous at first, but it has some notable advantages.
Using the breathing-based scale, also called the talk test, your age and heart rate don’t matter [10, 11]. (No calculations or heart rate monitor required!) Instead, your ability to string words together or not tells you how hard you’re working, your intensity zone, and what kind of fuel your body is using [7, 12].
Equal fat and glucose toward increasingly less fat, more glucose (carbs)
Breathing increases and continuous conversation becomes difficult
3 (aerobic to anaerobic)
Increasingly less fat, more glucose toward mainly glucose
Breathing is vigorous and talking is limited to short phrases
Talking is impossible or limited to grunts of single words
Roughly speaking, the sweet spot that corresponds to heart rate zone 2 goes from being able to speak easily to speaking more choppily, with several words strung together interrupted by breaths . At the lower end of this range, your main fuel is fat, and at the top of the range you’re using more glucose than fat .
Examples of Zone 2 Training
Generally, zone 2 training is working out at light intensity in an aerobic capacity, aka doing steady-state cardio. The intensity of exercise in zone 2 will vary for each person, depending on their fitness level and the kinds of exercise they regularly do .
For example, an elite 30-year-old triathlon runner may be in her zone 2 range while jogging a 9-minute mile, whereas the same exercise and pace could place a 30-year-old who only lifts weights in zone 3 or zone 4. Our weightlifter may find that fast-walking is a great way to work out in zone 2.
Likewise, someone with a desk job who rarely exercises or who has been unable to exercise for a long time as a result of a serious injury may find that walking at a moderate pace is enough to get to zone 2 .
Your zone 2 may also change over time as your body becomes better trained at a particular exercise . For instance, if you’ve never used a rowing machine before, you may hit zone 2 pretty quickly the first time you try it. But over time, as your body develops cardiovascular and muscular adaptations to rowing, you may find you have to work harder and may take a longer amount of time than before to get to zone 2. That’s a sign you’re getting stronger, improving endurance, and increasing mitochondrial function.
Other possible examples of zone 2 training include:
Benefits of Zone 2 Training
Benefits of light-intensity activity are primarily cardiometabolic and include better cardiovascular health, lower risk of mortality, improved insulin levels and blood sugar regulation, and better body weight [13, 14, 15, 16].
For example, a literature review assessed 27 clinical trials and 45 observational studies to understand the relationship between time spent doing light exercise and cardiometabolic health and mortality in adults.
Compared to sitting, short but frequent bouts of light-intensity exercise each day reduced blood sugar by 17.5% and insulin by 25.1%.
More than 150 minutes per week of light physical activity reduced body fat and improved blood pressure and cholesterol levels.
More time spent doing daily light-intensity exercise was associated with a 29% lower risk of death from all causes.
Overall, light-intensity physical activity may help improve cardiometabolic health and reduce the risk of death in adults .
However, lower-intensity exercise may not have the same brain health benefits as other forms of exercise, such as high-intensity interval training (HIIT) [17, 18].
Balancing Cardio and Resistance Training
This brings us to the importance of balancing your aerobic and anaerobic exercise, cardio, and resistance training. Light aerobic training is best balanced with anaerobic training to experience the full spectrum of exercise benefits, including increased muscle strength and endurance [19, 20].
Additionally, adding in higher level activity (moderate and high-intensity exercise) can promote benefits that light intensity does not, like its superior effects on brain and immune health [18, 21].
In general, cardio and resistance exercises should be implemented in a 1:1 or 1:2 ratio. To start, try to have equal days of cardio (at whichever intensity is feasible for you) and resistance training. Start zone 2 training in small increments but frequent sessions, and build up your tolerance over 2–3 months until you are active for about 150 minutes per week. Your first week might only have two 10-minute sessions of cardio and two 10-minute resistance workouts. These could even be 5-minute training sessions. Starting out with a training time commitment that seems so easy you have no reason not to do it is key to begin building your exercise habit (or any habit for that matter).
Risks of Not Doing Zone 2 Training
Sedentary behavior is consistently linked in research to increased risk of mortality, including cancer and cardiovascular-related deaths, insomnia, and poor mental health [22, 23, 24, 25, 26, 27], though sedentary behavior has a relatively low-risk association with these outcomes compared to other unhealthy behaviors.
Therefore, the biggest risk of not exercising may not be the increased risk of inactivity on your health, but that you will miss out on the many health benefits of exercise as a whole. The real power of exercise lies in its ability to optimize our health, as opposed to preventing negative health outcomes. And better health is what we all want, isn’t it?
Additionally, the research shows that physical activity at any level can decrease the risk of mortality, with some researchers suggesting that light-intensity activity is equally effective as other intensities . Finally, proponents of zone 2 training state that it can break up more intense physical activity and build endurance, preventing injuries and overtraining .
Can You Do Too Much Zone 2 Training?
For the general population, focusing exclusively on zone 2 training (i.e. light-intensity exercise) is unlikely to pose serious risks, as it typically doesn’t involve any overly strenuous movement. The main “risk” is that over-focusing on aerobic activity can detract from anaerobic exercise, like weight-bearing exercise, that can offer other health benefits.
Additionally, too light of aerobic activity may not provide the same health effects as moderate to higher exercise intensity, as seen in the case of combating viral illness and boosting brain health [18, 21].
There is some evidence that prolonged exercise and overtraining can lead to intestinal permeability and the increase of toxic metabolites in the body, but this is more likely to occur with high-intensity exercise, exercise longer than two hours, and exercise in high-heat environments .
There are a few populations that may want to be more cautious when performing any type of aerobic exercise, primarily those with known cardiac issues like uncontrolled hypertension, recent heart attack, or advanced congestive heart failure . If you have a history of any of these, you’ll want to check in with your doctor before you start a zone 2 training program.
However, for the majority of people, the benefits of exercise outweigh the risks .
The Final Verdict on Zone 2 Training
Zone 2 training remains an important part of a regular exercise routine to support cardiovascular health, increase the output and number of mitochondria in our cells, and more efficiently break down fat. However, zone 2 training should ideally be balanced with resistance training for additional health benefits like strength, coordination, and mobility, especially with aging.
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