You must have probably heard of heart rate training zones and how elite endurance athletes, and more recently, a growing number of recreational athletes, use them to guide their training. Training zones can significantly enhance the results one gets from cardio and interval training regardless of whether they are looking to break and triathlon record, get healthier or lose weight. As a result, they should be top of mind for everyone with a fitness routine.

This blog explains what training zones are, how they are determined, the effects each one has on human physiology, and how they can be used to inform training and nutrition better.

Exercise intensity zones and metabolic response

The human body has a different metabolic response depending on the exercise intensity it’s exposed to. Its metabolic response is defined based on the amount of energy (i.e. calories) and type of fuel (i.e. proportion of fats and carbohydrates) it uses at each particular intensity. For example in zone 2 one may be burning 4.5 kcal/min with a fuel mixture consisting of 75% fats and 25% carbohydrates. These attributes will gradually change as one moves to a higher level of exercise intensity zones because the energy demand increases, and the fuel mixture required to sustain the growing levels of energy release needs to change. To better understand this phenomenon we should first examine the process of energy generation in the human body.

During exercise, your body typically burns a mixture of fats and carbohydrates to release the energy required (i.e., calories) to move. This process typically uses oxygen and is thus also referred to as the oxidation of fats and carbohydrates. Fat releases more energy than carbs when burnt (i.e., 9 kcal per gram of fat vs. 4 kcal per gram of carbs) but has a slower burning process making it suitable for low exercise intensities where the rate of energy demand is low. Carbohydrates, on the contrary, require less time to burn and can therefore support higher exercise intensities where the rate of energy demand is larger.

As a result, the fuel mixture changes from predominantly fats when exercise intensities and energy requirements are low to carbohydrates dominant as exercise intensity increases and the rate with which the body needs to release energy increases. In short, as the demand for energy increases, cells need to rely more on carbohydrates since they release energy faster. On the other hand, cells rely on fat as a fuel source despite its slow energy release process for as long as energy demand remains low.

Figure 1 shows the difference between fat and carbohydrate burn as exercise intensity increases during a treadmill test.

 

Figure 1 Dark green: fats, Turquoise: carbohydrates, Light green: heart rate

Going from intensity zones to heart rate zones

As described above, exercise intensity zones are defined based on the body’s metabolic response when exercising at a specific intensity. The metabolic response is broadly defined by the calories expended and the contribution of fats and carbohydrates in the calorie-burning process at that particular intensity. To measure calorie expenditure along with fat and carbohydrate consumption, one needs to analyze an individual’s expired oxygen and carbon dioxide using a metabolic analyzer. Since metabolic testing can only be done in a test or evaluation setup and not on every training session or athletic event, one needs to use a proxy metric that can be easily tracked daily to map a person’s metabolic response against it. The most commonly used proxy metric to achieve this is heart rate.

By measuring heart rate during a metabolic test, we can establish a correlation between the different metabolic states the test subject undergoes and heart rate. Since this correlation remains fairly constant, once established, it can be used to infer the subject’s metabolic response just by tracking heart rate during training or athletic events. We refer to this as “getting your personalized training zones.”

How different are my heart rate zones from the person sitting next to me?

The answer is Very! There are many ways to estimate heart rate zones, including predictive equations and wearable devices. However, the metabolic particularities of an individual render these methods highly inaccurate, with some cases reaching up to 50% deviation from one’s training zones. Simply put, unless one uses metabolic analysis to evaluate metabolic response in conjunction with heart rate, it’s impossible to identify their true heart rate zones accurately.

Do training zones change over time?

Yes! Training and nutrition will impact the way your cells work and consequently will affect your metabolism. In broad terms, this means that you will burn a different number of calories and use a different mixture of fat and carbohydrates at specific exercise intensities. For example, after three months of endurance training, you have likely become more economical in your running. You now burn fewer calories at a given pace and perhaps more fat adapted, meaning that the proportion of fat metabolism in your energy generation process has grown. Since heart rate zones correlate between heart rate and metabolic response, if your metabolism changes, so will its correlation with your heart rate profile. As a result, depending on how rapidly you progress through your fitness journey or training, you should re-assess your training zones. The recommended testing frequencies are the following:

  • Every six months for individuals in a static or maintenance routine
  • Every three months, individuals progress through their fitness program at an average pace
  • Every 4-8 weeks, athletes undergo rigorous training

Moreover, it’s also very important to highlight that training zones also change based on the type of exercise. This is due to the fact that the amount of calories, fats, and carbohydrates you burn are heavily affected by the movement your body conducts. This inevitably means that the correlation between your metabolic response and heart rate profile will change depending on the exercise. For example, you may be burning 30% fats and 70% carbs at 140 beats per minute when running but only 15% fats and 85% carbs at 140 beats per minute when cycling. This may also explain why you feel a burning sensation in your legs when biking at a relatively low heart rate range while feeling much more relaxed when running at the same heart rate.

Training zones and physiological adaptations

Each training zone elicits different physiological adaptations on the human body. These physiological adaptations include improved cellular fitness, heart fitness, lung capacity, VO2max, and more. Each zone has specific effects meaning that spending time in it will improve only a specific set of these systems and not everyone. As a result, when one is looking to get the most out of their cardio training by targeting specific weaknesses their body faces, following a training program with accurate training zones is essential. For example, spending a specific period in zone 2 is the essential part of a training program if one is looking to improve their fat burning efficiency. If this person hasn’t measured their personalized training zones and instead of training in Zone 2 they’re actually training in Zone 3; they may be getting as much as 40% less positive adaptation in enhancing their fat burning capacity.

The five zones system is the most frequently used one, accurately capturing the difference in metabolic states while remaining practical enough for everyday usage.

Each zone is used for a different purpose as it inflicts different metabolic adaptations on your body. Here are the adaptations each zone will affect:

Zone 1

Training intensity is typically used for warmup or active recovery (i.e. recovering from intense exercise while moving).

Zone 2

  • Zone Training will develop your mitochondrial function and improve your fat-burning efficiency.
  • It’s highly recommended for long-range endurance sports and individuals suffering from metabolic syndrome (e.g. Type II Diabetes).
  • The improved mitochondrial function will also significantly support recovery capacity helping you to recover faster after intense bouts of exercise.

Zone 3

  • Zone 3 training can help strengthen your pulmonary muscles and improve cardiovascular function.
  • It’s an ideal intensity when suffering from a lung or heart problem since its moderate- intensity offers a solid stimulus to the heart and lungs without being exhausting or overly strenuous.

Zone 4

  • Zone 4 training will help improve your VO2max and ability to sustain high-intensity exercise for prolonged durations by improving lactate shuttling.
  • Lactate is a byproduct of anaerobic metabolism, which can also be used as fuel by your muscles.
  • For as long as your body can clear fatigue byproducts faster than their being produced, the exercise intensity remains sustainable.
  • As a result, the greater your lactate shuttling capability the greater your ability to sustain high exercise intensities for long periods.

Zone 5  

  • Zone 5 training will improve your VO2max and peak power output capability (e.g. maximum speed or cycling wattage).
  • This exercise intensity is sustainable for 60 to 120 seconds and requires one to train at full potential.

Dr. Stacy Sims shares about Zone Two for Aging Female Athletes

Zone 2 training is a hot topic, but like many things, it’s different for females.

If you even remotely follow fitness and training media, you will have seen the buzz around “Zone 2 training”–generally known as steady, conversationally-paced exercise–and how it is the golden child of endurance (and health) training programs. And while there’s no doubt that easy days have an important place in health, fitness, and training, for women specifically, the benefits are being oversold.

When we talk about “zone training”, we mean breaking down our training intensities into heart rate or power ranges that are used to form a structured training plan or workout. The purpose of stratifying intensities in this way is to achieve specific physiological and metabolic adaptations through our training. In this structure, Zone 2 is relatively easy and long (60 to 70% of max for 45+ min) and you should feel like you can go for hours. The current recommendation is to have the bulk of your exercise sessions–three to four training sessions a week–be in Zone 2.

Why the emphasis on Zone 2? The thought is that Zone 2 is a low enough intensity to stimulate mitochondrial and other adaptations within the muscle cell that improve the skeletal muscle’s ability to use fat as a fuel, spare carbohydrate; improve metabolic flexibility (the ability to rapidly switch between fat and carbohydrate oxidation), as well as to better clear lactate during higher intensity exercise.

Female Muscle Makeup & Zone 2 Training

  • To understand what this all means for female athletes, let’s dig into how this works within our specific muscle fibers.
  • We have two primary types of muscle fibers:
    • Type 1 fibers, called “slow twitch” fibers, and Type 2 fibers, called “fast twitch” fibers (which are broken down into subtypes Type IIa and Type IIb).
    • Type 1 fibers have the greatest mitochondrial density (mitochondria are the “powerhouses”of the cell) and are highly oxidative, meaning they are very efficient at using fat as a fuel.
    • As intensity heats up and muscle contractile speed increases, we need more energy than Type 1 fibers can generate using fat, so Type IIa and then Type IIb fibers are recruited.
    • Type II fibers have lower mitochondrial density and high capacity to use glucose for energy.

Because Type I fibers are so efficient at using fat as a fuel to keep going, the concept around Zone 2 training is that by spending more time tapping into Type I fibers, we can increase their mitochondria density and respiratory rates (the metabolic reactions that require oxygen to convert fatty acids into the usable ATP), as well as increasing the transport proteins (MCT-1) needed to clear lactate quickly and efficiently (during exercise, lactate is produced by the Type II fibers, but primarily cleared by Type 1).

Sounds great, doesn’t it? Go long and easy to boost your metabolic health, endurance capacity, and improve your overall performance! 

But, hold on a minute.

If the main goal of Zone 2 training is to increase the number and functionality of mitochondria within the skeletal muscle, and increase fatty acid utilization, we need to question the validity of this concept for women.

Yes, you’ve got it.

  • This conversation around Zone 2 benefits does not take into account sex differences.
  • Research shows that females (e.g. XX chromosomes), have more oxidative (Type 1) fibers, have greater fatigue-resistant muscles, have greater autophagy activity and a higher reliance on lipid (fat) metabolism as compared to males(e.g. XY chromosomes).
  • We know that training status does have an impact on muscle mitochondria adaptations (basically increasing muscle oxidative capacity), but when we look at equivalently trained women and men, we see that there are differences, specific to skeletal muscle,
  • in mitochondrial oxidative functional capacity.
  • Women have approximately one-third greater mitochondrial intrinsic respiratory rates (the amount of mitochondrial respiration occurring for a given amount of mitochondrial protein) and greater mitochondrial oxygen affinity (p50mito) than men.

What about increasing fatty acid utilization then? Should women spend time in Zone 2 to increase their ability to use fat? Again, no.

Research shows, as compared to similarly trained men, women have a greater amount of intramyocellular lipid droplets (aka, fat particles stored in skeletal muscle cells), a greater amount of the plasma membrane fatty acid transporter protein CD36 which increase fatty acid uptake into the cell, and also a greater sensitivity to malonyl-CoA (M-CoA) (a metabolite that can inhibit fatty acids getting into the mitochondria),

  • What about metabolic flexibility? Women are already metabolically flexible!
  • Not only do women oxidize more fat and less carbohydrate during prolonged exercise; women also have greater metabolic flexibility because there is a greater ability to switch between fatty acid and glucose use,depending on what nutrients are available.

Finally, when it comes to improved lactate clearance, it may be more important for men to improve their clearance capacity.

  • We see that men exhibit greater MCT-4 and MCT-1.
  • For one, because they have a greater ratio of Type II to Type 1 fibers (remember type II is glycolytic, which produces lactate); men rely more on carbohydrate metabolism than fat metabolism during exercise, and they have higher circulating plasma lactate levels per unit of workload (fitness matched to women). When it comes to endurance training, men upregulate MCT-1 more so than women, which may look like a bad thing, but in the big picture, it makes sense with respect to sex differences in glycolytic fibers and circulating lactate during exercise.

Should women be concerned about a reduced expression of MCT-1?

The short answer is no. Because women’s bodies fuel exercise differently than men, although we do produce lactate, because we have less overall glycolytic activity, we will have less overall expression of MCT-1.

When women do a block of specific high intensity work, and up regulate MCT-4 (the transporters that pull lactate out of cells) there is also a response to upregulate MCT-1 (to clear the lactate).

Where does all this leave women in the Zone 2 conversation?

  • For women, Zone 2 training is great for active recovery, a certain amount of base building for endurance athletes, and social exercise.
  • Yes, there is merit in Zone 2 BUT if you are planning the bulk of your exercise time for Zone 2 training to enhance mitochondria function and fatty acid utilization, you may want to revisit that concept (more on that in a future post).
  • One final and very important point is that when you do train in Zone 2, really make it Zone 2.
  • This is really one of the biggest problems I see: women spending too much time going hard or “kind of hard” and not enough time going truly easy.
  • When Zone 2 training enters the conversation, they think they need to spend hours doing this, but it very often becomes hours doing moderate intensity that is harder than Zone 2 and is eventually counterproductive in that it just makes them worn out without the training gains.
  • Stay tuned for part 2 where I’ll talk about what training scheme works best for women.

https://www.drstacysims.com/blog/what-women-need-to-know-about-zone-2-training


Key takeaways from PNOE 

Regardless of age, gender, and fitness level, every person has one or more systems that limit fitness or health. Targeting these limitations effectively requires the precision that stems from focusing your cardio and interval training in the zone(s) that will bring about the adaptations needed to overcome them.

The metabolic analysis provides gold-standard accuracy in determining your training zones and the plan that puts them to effective use. Understanding how your body responds metabolically and building your program around your metabolism is a foundational step toward maximizing your workout’s efficiency and achieving your health or performance goals faster and with less effort.

Dr. Stacy Sims suggest doing this instead of Zone 2 if you are my age range…

The popular zone 2 protocol doesn’t work the same way for women. Here’s what to do instead.

In my last blog, I reviewed down to the cellular level, why zone 2 training, which is currently being promoted across the internet as the priority training for health, longevity, and performance, isn’t as effective for women as it is for men. Now let’s talk about what type of training scheme delivers those benefits best for females.

First a very quick review: Zone 2 is relatively easy and long (60 to 70% of max for 45+ min) and you should feel like you can go for hours. The current recommendation is to have the bulk of your exercise sessions–three to four training sessions a week–be in Zone 2. The theory is that Zone 2 is a low enough intensity to stimulate mitochondrial and other adaptations within the muscle cell that improve the skeletal muscle’s ability to use fat as a fuel, spare carbohydrate; improve metabolic flexibility (the ability to rapidly switch between fat and carbohydrate oxidation), as well as to better clear lactate during higher intensity exercise.

As reviewed in that last blog, putting a precedence on zone 2 training in women may not be the ideal way to improve skeletal muscle mitochondria function (to include increasing mitochondria expression, improving fatty acid uptake, and improving lactate clearance). If you want to follow me further into the rabbit hole for a moment, I have discovered additional compelling data regarding exercise intensity’s impact on MCT1 and MCT4 expression that should make women rethink their training priorities and why.

First, let’s review what MCTs are. MCT stands for “MonoCarboxylate Transporter”. Basically, they are proton-shuttling proteins that are found in many tissues of the body. For our purposes here, we’ll talk about MCT1 and MCT4, which are specific to muscle (skeletal and cardiac) and their ability to transfer lactate and pyruvate across the plasma membrane. During heavy exercise, the high energy demand of your contracting skeletal muscles trigger an increase in glycolysis; the breakdown of a glucose molecule into pyruvate and in anaerobic conditions, pyruvate becomes lactate. Because we now know lactate is not a “waste” metabolite but can be pulled into the mitochondria of skeletal and heart muscles and used as fuel, exercise researchers are interested in how lactate shuttles across the cell membranes. This is where MCT1 and MCT4 transport proteins come into play. We see that MCT4 pulls lactate out of the cells, whereas MCT1 pulls it into the cells (as pyruvate) where it can then be oxidized in the mitochondria to produce energy for muscular work.

Research shows exercise alters MCT expression, which is the basis of the zone 2 conversation: to increase the number of MCT1 transport proteins, to improve lactate clearance and mitochondria oxidation; which, in theory, will improve performance. However, we see there are sex differences in the expression of MCT1 and MCT4 based on intensity of exercise. Regardless of intensity, MCT1 expression increases in males, but it appears sprint interval training, (e.g. 30s or less), increases MCT4 expression only after a series of training sessions have occurred (meaning that the increase in MCT4 expression happens later in a training block). For women, low intensity training shows minimal increases in either MCT1 or MCT4 expression, but sprint interval training does increase MCT1 expression in women (regardless of the recovery between intervals). The thought is that by increasing lactate within the cells, an upregulation of MCT1 occurs; but the increased lactate does not have a signaling effect on MCT4. This may be due to the decreased glycolytic capacity of women as compared to men for similar workloads; and also the influence of estrogen on MCT1 expression.

Now, if you are still with me, after the heavy science, let’s look at the practical aspects of what kind of training a female should do.

From a health and longevity standpoint, the goal is not only to increase MCT1 and increase mitochondria respiration; but also increase glycolytic capacity for brain resilience (enhancing brain glycolysis can increase neuronal metabolic strength to sustain a better cognition and slow down or prevent the progression of Alzheimer’s disease ).  Research shows that a minimum of 3 days a week of HIIT and SIT intervals dramatically increases MCT1 expression over 6 weeks, and increases the formation of brain-derived lactate (improving glycolytic capacity in the brain).

From a performance perspective, HIIT and SIT are critical, not only to improve lactate production and clearance, but to also stimulate an increase in the fast twitch, Type II fiber cross-sectional area (size and function); of which female have significantly less as compared to the slow twitch Type I fibers. Hello power and speed!

What About Endurance Athletes?

Now, I hear all of the endurance athletes out there saying “what about the long slow training I need to do for my long-distance race?!”.  I am not saying it is pointless to do this kind of training, not at all! You need it to have the strength and capacity to go long. What I am saying is that to spend a significant amount of time in the low intensity/Zone 2 training does not enhance mitochondrial respiration or oxidative capacity like it does for men. In fact, I love this quote from an article from the Journal of Physiology , “physical activity is essential for males to maintain mitochondrial integrity in conjunction with more coupled respiration like females, even though their bioenergetic capacities may remain lower than females…”. Translation:because females have better mitochondria respiration and mitochondria density than men, men need to do the long slow aerobic work to be more like women (go figure…!). By peppering your long slow work with specific high intensity work, you will improve your mitochondria capacity and anaerobic capacity by the nature of the high intensity work.

If you’re not an endurance athlete, I hope this helps clear up some of the confusion around the zone 2 messaging. Remember, just because it is making the rounds in popular media, does not mean a full, deep dive into the literature to look at both male and female physiology has been done. This may be a perfect example of why the tagline Women Are Not Small Men comes into play- male data has been generalized to everyone without really looking at the viability for positive outcomes.