The popular zone 2 protocol doesn’t work the same way for women. Here’s what to do instead.
I have previously 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.
Putting a precedence on zone 2 training in women may not be the ideal way to improve skeletal muscle mitochondrial function (to include increasing mitochondrial 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.
