Why Women Don't Need As Much Zone 2 Training As Men, From An Exercise Physiologist

When we talk about "zone training," we mean breaking down our training intensities into heart rate or power ranges in order to achieve specific physiological and metabolic adaptations.

Within this structure, zone 2 is relatively easy and long (60 to 70% of maximum effort for 45+ minutes), and you should feel like you can do it for hours. The current recommendation is that everyone should spend the bulk of their exercise sessions—three to four training sessions a week—in 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 muscles' ability to use fat as a fuel, spare carbohydrates, improve metabolic flexibility (the ability to switch between fat and carbohydrate oxidation rapidly), and better clear lactate during higher intensity exercise. 

Before we dig into why zone 2 isn't as beneficial for active females, let's dig into how it affects our muscle fibers. We all 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 cells) 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 that Type 1 fibers can generate using fat, so Type IIa and then Type IIb fibers are recruited. Type 2 fibers have lower mitochondrial density and a higher capacity to use glucose for energy. 

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

Research shows that females have more oxidative (Type 1) fibers, greater fatigue-resistant muscles, greater autophagy activity, and a higher reliance on lipid (fat) metabolism2 compared to males.

When we look at equivalently trained women and men, we see differences in mitochondrial oxidative functional capacity, specific to skeletal muscle3. 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 (p50_mito) than men.

What about increasing fatty acid utilization, then? Should females 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 (fat particles stored in skeletal muscle cells); a greater amount of the plasma membrane fatty acid transporter protein CD364, which increases 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.

Finally, women might need to be less concerned about improving their lactate clearance than men. Since men have a greater ratio of Type 2 to Type 1 fiber (remember, Type 2 is glycolytic, which produces lactate), they rely more on carbohydrate metabolism than fat metabolism during exercise, and they have higher circulating plasma lactate levels per unit of workload compared to women of similar fitness. We see that men exhibit greater MonoCarboxylate Transporter (MCTs): MCT4 and MCT1, which are specific to skeletal and cardiac muscle.

MCTs are proton-shuttling proteins that are found in many tissues of the body. During heavy exercise, the high energy demand of your contracting skeletal muscles triggers an increase in glycolysis6: the breakdown of a glucose molecule into pyruvate.

In anaerobic conditions, pyruvate becomes lactate. Because we now know lactate is not a "waste" metabolite7 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 goal of zone 2 training: to increase the number of MCT1 transport proteins and improve lactate clearance and mitochondria oxidation, which, in theory, will improve performance.

Here's the kicker: MCT1 expression increases in males regardless of exercise intensity. However, it appears that sprint interval training for 30 seconds 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.

Still with me after all that heavy science? Great, let's look at what this means for the best kinds of training for females.

From a health and longevity standpoint, the goal is not only to increase MCT1 and mitochondria respiration but also to increase glycolytic capacity for brain resilience8. Research shows that a minimum of three days a week of HIIT (high-intensity interval training) and SIT (sprint interval training) intervals dramatically increase MCT1 expression over six weeks and increase the formation of brain-derived lactate, improving glycolytic capacity in the brain.

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 suggesting that doing this kind of training is pointless—not at all! You need it to develop the strength and capacity to go long. But spending a significant amount of time in low-intensity/zone 2 training does not enhance your mitochondrial respiration or oxidative capacity the same way it does for men.

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 more 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.