Midwife Maven
VO2 Max? What is it? How it can help? Why you should know.
Zone 2 Training: The Most Underrated Thing You Can Do.
It’s not glamorous. It’s not intense. And it may be one of the most powerful tools available for women navigating the menopause transition.
There is a number that predicts how long you will live better than almost any other single measurement. Better than your cholesterol. Better than your blood pressure. Better, in most studies, than your smoking status.
That number is your VO2 max.
VO2 max is a measure of how much oxygen your body can use per minute during maximum effort. It reflects the combined efficiency of your lungs, your heart, your blood vessels, and your muscles. And the research on what it predicts is not subtle.
A 2018 study published in JAMA Network Open followed 122,007 adults undergoing exercise testing. The group with the lowest cardiorespiratory fitness had a mortality rate nearly four times higher than the group with high fitness. Going from the bottom fitness category to merely below-average was associated with roughly a 50 percent reduction in mortality over the follow-up period. That improvement, in cardiovascular fitness alone, outperformed the mortality risk reductions associated with being a nonsmoker or not having a diagnosis of heart disease.
This is not a marginal association. And it is particularly relevant to you right now, because perimenopause is one of the most significant windows for cardiovascular fitness decline in a woman’s life, and most of the standard exercise guidance does not tell you that.
What is Zone 2, exactly
Your body can exercise across a wide range of intensities, and different intensities produce different physiological effects. Exercise physiologists typically describe five zones based on heart rate and metabolic activity. Zone 2 is the second of those zones, the upper end of low intensity, where you are working hard enough to elevate your heart rate and breathing meaningfully but not so hard that conversation becomes difficult.
The clearest way to identify Zone 2 without a laboratory test is what’s sometimes called the talk test. You should be able to speak in full sentences while exercising, but you would not want to sing. There is real effort involved. You are not strolling. You are working. But you could sustain it for a very long time if you had to.
In terms of heart rate, Zone 2 corresponds roughly to 60 to 75 percent of your maximum heart rate for most people, though this varies enough between individuals that feel is a more reliable guide than formula. You can use the Maffetone formula as a starting estimate: 180 minus your age gives you approximately the upper boundary of your Zone 2. A 50-year-old would target somewhere around 130 beats per minute. But these are estimates. Your actual Zone 2 depends on your individual physiology, your current fitness level, and your fitness history.
Physiologically, Zone 2 is the intensity at which your body primarily burns fat for fuel, where lactate is being produced and cleared at roughly equal rates, and where your mitochondria are being asked to work steadily over an extended duration. It is this sustained aerobic demand, not brief intensity spikes, that trains specific adaptations in the heart, the blood vessels, and the cells.
What Zone 2 builds
The adaptations Zone 2 training produces are foundational in the deepest sense of the word. This is the biological infrastructure that underlies nearly every other health outcome.
Mitochondrial density and function.
Mitochondria are the organelles inside your cells that convert oxygen and fuel into ATP, the energy currency of cellular life. Mitochondrial function is so central to healthy aging that it is now classified as one of the hallmarks of aging itself. When mitochondrial capacity declines, virtually every energy-dependent process in the body becomes less efficient. Sustained aerobic exercise at Zone 2 intensity is one of the most potent known stimuli for mitochondrial biogenesis, the creation of new mitochondria, as well as for mitochondrial quality control, the process by which damaged mitochondria are cleared and replaced. Older adults who have maintained lifelong aerobic exercise show dramatically better mitochondrial function and density than age-matched sedentary peers, to a degree that in some cases resembles people decades younger.
Capillary density.
Mitochondria cannot function without oxygen delivery, and oxygen delivery depends on the network of capillaries serving each muscle fiber. Zone 2 training stimulates the growth of new capillaries within muscle tissue, a process called capillary angiogenesis. More capillaries per muscle fiber means more oxygen available when you need it, better nutrient delivery, better waste removal, and improved insulin sensitivity at the tissue level. Lifelong endurance exercisers have been shown to have over 35 percent more capillaries per muscle fiber than sedentary peers.
Cardiac efficiency.
Sustained low-intensity aerobic exercise, sustained long enough to keep the heart working continuously, gradually increases stroke volume, the amount of blood the heart pumps with each beat. A more efficient heart pumps more blood per beat and therefore does not have to beat as fast at rest or during moderate effort. Resting heart rate, which is closely related to stroke volume, is one of the clearest markers of cardiovascular health and a significant predictor of longevity. Zone 2 training over time produces a stronger, more efficient heart, and the adaptations are cumulative across years and decades of practice.
Metabolic flexibility.
Zone 2 is the intensity at which your body is most actively burning fat for fuel. Training consistently in this zone improves what exercise physiologists call metabolic flexibility, the capacity to efficiently shift between fat and glucose as fuel sources depending on demand. Reduced metabolic flexibility is one of the early hallmarks of metabolic dysfunction and insulin resistance. Improving it through consistent Zone 2 training has cascading effects on blood sugar regulation, body composition, and cardiometabolic risk. This is directly relevant during perimenopause, when insulin sensitivity tends to decline and abdominal fat accumulation accelerates.
A note on Zone 2 versus HIIT
There is ongoing scientific debate about whether Zone 2 specifically is uniquely optimal for mitochondrial adaptation, and the honest answer is that higher-intensity training also produces meaningful mitochondrial and cardiovascular improvements, sometimes greater ones in shorter timeframes. For women who have an existing aerobic base and can tolerate higher intensities, incorporating some high-intensity intervals does add benefit beyond Zone 2 alone. The reason Zone 2 gets emphasized is not that it is the only useful intensity but that most people chronically underdo it, often replacing it entirely with moderate-to-hard exercise that is too hard to sustain for long enough to build aerobic base and too easy to maximally stress the cardiovascular system. For women new to structured exercise or rebuilding after a period of inactivity, Zone 2 is the right place to start.
Why perimenopause makes this urgent
VO2 max naturally declines with age at a rate of roughly 10 percent per decade after age 40, even with consistent activity. This decline is partly driven by reduced cardiac output, loss of muscle mass, and reduced mitochondrial function, all of which accelerate during the menopause transition due to estrogen’s role in each of those systems.
Estrogen promotes endothelial function, the health and responsiveness of the blood vessel lining. It supports nitric oxide production, which keeps blood vessels dilated and responsive. As estrogen declines, endothelial function worsens, vascular stiffness increases, and the cardiovascular system becomes less efficient at oxygen delivery. Research comparing perimenopausal and postmenopausal women of similar age finds that VO2 max is meaningfully lower in the postmenopausal group, and that this difference is associated with increased cardiometabolic risk.
At the same time, physical activity levels tend to drop during perimenopause. This is partly driven by symptoms: disrupted sleep creates fatigue, hot flashes make sustained exercise less comfortable, mood changes reduce motivation. The combination of accelerated physiological decline and reduced activity creates a compounding problem. Fitness falls faster, which raises mortality risk, which also happens to be the period when women are least likely to be thinking about exercise as a longevity intervention.
This is one of the most significant gaps in how we care for women in midlife. The evidence that aerobic fitness is protective during this transition is clear. The conversation about how to preserve and build it is not happening often enough.
The scale of the mortality data
The Mandsager 2018 JAMA Network Open study of 122,007 adults found that people with low cardiorespiratory fitness had a mortality rate nearly five times higher than those in the elite fitness category over the study period. Moving from low fitness to merely below-average fitness reduced that risk by roughly half. The researchers specifically noted that the mortality risk associated with low fitness exceeded the risk associated with smoking, hypertension, or having a diagnosis of heart disease. And each 1 MET improvement in fitness, about 3.5 ml of oxygen per kilogram per minute, has been associated across multiple studies with a 10 to 15 percent reduction in all-cause mortality.
What Zone 2 training looks like in practice
This is where I want to be direct, because the conversation about Zone 2 sometimes gets abstracted into concepts rather than practical reality.
The most accessible forms of Zone 2 training for most women are brisk walking with some incline, cycling at a steady moderate pace, swimming at an easy continuous tempo, elliptical or rowing machine at sustained moderate effort, and jogging if your fitness allows it. The key is continuity at a steady effort level. You want to maintain the effort, not surge and recover.
Duration matters more than many people expect. To produce meaningful aerobic adaptation, you generally need 30 to 60 minutes of continuous effort in Zone 2 per session. Short bursts of easy exercise don’t build the aerobic base the way sustained effort does. Many people who believe they do Zone 2 regularly are actually completing 20-minute walks that are slightly too easy to be Zone 2 and too short to produce the desired adaptations even if they were. The goal is sustained effort in the right zone for long enough to matter.
In terms of weekly volume, the current evidence base around longevity benefits generally points to 150 minutes or more of moderate-intensity cardiovascular exercise per week at minimum, with greater benefits as volume increases up to a point. For context, that is roughly three to five Zone 2 sessions of 30 to 50 minutes per week. That is a real commitment, but it is a realistic one, and the evidence for its impact on longevity is stronger than for almost any other single lifestyle intervention.
You do not need specialized equipment or a laboratory test to start. You need a way to sustain continuous movement, a way to check your heart rate, and an honest calibration of effort. If you can carry on a conversation without gasping but you would choose not to, you are probably in the right zone.
What to add as your base builds
Zone 2 is foundational, but it is not the complete picture. As your aerobic base develops, adding some higher-intensity cardiovascular effort produces additional benefit, particularly for improving VO2 max more rapidly and strengthening cardiovascular adaptation at maximum effort levels.
The research on polarized training, a model where roughly 80 percent of training volume is at low intensity and 20 percent at genuinely high intensity with very little time spent in the moderate middle zone, consistently shows strong outcomes across fitness levels. For women who have established a Zone 2 foundation and want to add intensity, a practical version of this might look like one session per week of brief, genuinely hard intervals: four to six efforts of 3 to 4 minutes at a pace that challenges you to sustain, with recovery periods between. This kind of high-intensity session once a week, layered onto a consistent Zone 2 practice, produces measurable VO2 max improvements over time.
What the research is less supportive of is the training pattern most people actually follow: almost everything at moderate intensity, hard enough to feel like exercise but not sustained enough to build base and not hard enough to maximally stress the cardiovascular system. That middle zone produces some benefit, particularly compared to no exercise at all, but it is not where the most significant longevity adaptations are building.
The honest truth about starting where you are
I want to name something here that doesn’t get said often enough in exercise guidance. For many women in perimenopause, the conversation about Zone 2 training and VO2 max happens while they are already exhausted. Poor sleep, hot flashes, brain fog, a schedule that has not gotten any lighter, and a body that is responding to exercise differently than it did five years ago.
This matters because the research is also clear that any movement is better than none, and that the largest mortality risk reduction happens at the lowest end of the fitness spectrum. Moving from completely sedentary to occasionally active produces more mortality benefit than moving from moderately fit to very fit. If Zone 2 sessions feel out of reach right now, a daily 30-minute walk, even a relatively easy one, is a real intervention with real longevity data behind it.
The goal I am working toward with patients is not Olympic fitness. It is functional cardiovascular capacity that sustains independence, cognitive function, and energy into the decades ahead. Every woman’s starting point is different, and the right plan is the one you can actually sustain.
What I want you to understand is why it matters, because that understanding tends to change the calculus around prioritization. This is not exercise as punishment or obligation. It is one of the most direct levers you have on how long you live and how well you live while you’re doing it.
Sources
Mandsager K, et al. Association of cardiorespiratory fitness with long-term mortality among adults undergoing exercise treadmill testing. JAMA Network Open. 2018;1(6):e183605.
JACC. Midlife cardiorespiratory fitness and the long-term risk of mortality: 46 years of follow-up. Journal of the American College of Cardiology. 2018.
Kokkinos P, et al. Cardiorespiratory fitness and cardiovascular disease outcomes. Journal of the American College of Cardiology. 2022;80(6):598-609.
San-Millan I. The key role of mitochondrial function in health and disease. Antioxidants (Basel). 2023;12(4):782.
Hood DA, et al. Maintenance of skeletal muscle mitochondria in health, exercise, and aging. Annual Review of Physiology. 2019;81:19-41.
Storoschuk KL, et al. Much ado about Zone 2: a narrative review assessing the efficacy of Zone 2 training for improving mitochondrial capacity and cardiorespiratory fitness in the general population. Sports Medicine (2025).
Gries KJ, et al. Cardiovascular and skeletal muscle health with lifelong exercise. Journal of Applied Physiology. 2018;125:1636-1645.
Lavin KM, et al. Effects of aging and lifelong aerobic exercise on basal and exercise-induced inflammation in women. Journal of Applied Physiology. 2020;129:1493-1504.
Hulteen AJ, et al. Detrimental changes in health during menopause: the role of physical activity. International Journal of Sports Medicine. 2023.
Seiler KS, Kjerland G. Quantifying training intensity distribution in elite endurance athletes. Scandinavian Journal of Medicine and Science in Sports. 2006;16(1):49-56.
McKendry J, et al. Lifelong exercise maintains skeletal muscle capillarization in older adults. Journal of Applied Physiology. 2021.
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Chris Goldby is a writer and multimedia storyteller based in Hickory, North Carolina. He owns The Hickory Algorithm a hyper focused lens on Hickory, NC and is a contributing writer and Head of Documentary/Docuseries with Akula Literary Partners.
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