Zone 2 Cardio and Testosterone: The Endurance Training Sweet Spot

Cardio and testosterone have a complicated relationship. On one hand, cardiovascular fitness is good for heart and metabolic health. On the other, years of very high endurance training have been linked to lower resting testosterone in some men. Zone 2 training, easy steady aerobic work you can hold a conversation during, sits at the low-stress end of that spectrum. This article separates what research shows from the claims that have grown up around it, including the idea that Zone 2 raises testosterone, which we could not find evidence for.

What Is Zone 2 Training?

“Zone 2” comes from the common five-zone heart-rate model, where it is the easy-to-moderate aerobic band. In the three-zone framework used in endurance research, it falls in the upper part of the lowest intensity zone, below the first lactate or ventilatory threshold. In studies of nationally and internationally competitive endurance athletes, about 80 percent of training sessions were performed at low intensity (below about 2 mmol/L lactate), with about 20 percent at high intensity.^[1] There is no single universally agreed definition, and published zone systems differ.

In practice, Zone 2 is the effort at which you are working steadily but can still hold a conversation. Be careful with heart-rate percentage rules. In a laboratory study, the ventilatory threshold occurred at about 82 percent of peak heart rate on a cycle ergometer and 89 percent on a treadmill,^[6]which is above the “60 to 70 percent of max heart rate” band often quoted for Zone 2. Thresholds differ between people, so treat any percentage as a starting guess.

Very High Endurance Volume and Testosterone

The strongest evidence of a link between endurance training and lower testosterone comes from long-term endurance athletes. In the comparison above, men who had trained for years had lower resting total testosterone (4.99 versus 7.25 ng/mL) and free testosterone (17.2 versus 23.6 pg/mL) than untrained men, with normal cortisol and prolactin and normal LH pulse patterns. The authors suggested chronic endurance training may lower testosterone possibly by impairing testicular function, and that cortisol and prolactin were not the cause.^[3]A review describes this as the “exercise-hypogonadal male condition”: persistently reduced basal testosterone in men chronically exposed to endurance training, with an unclear mechanism, apparently limited to men who have trained for years and not highly prevalent.^[4]

A common claim is that testosterone falls because of rising cortisol. The evidence is more nuanced. In a study of 12 trained men, 30 minutes of exercise at 40 percent of VO2max did not raise cortisol, 60 percent raised it by about 40 percent, and 80 percent raised it by about 83 percent.^[7] That shows cortisol responds to intensity, but Zone 2 typically sits between the lowest and middle intensities tested, so this study does not tell us its exact effect, and the trained-versus-untrained comparison above found no cortisol difference.

Hackney also notes that exercise-related lowering of testosterone can occur in different forms: temporarily with low energy availability (RED-S) or excessive training load, which resolves with appropriate changes, and a more chronic form lasting years.^[8] Not eating enough for the training you do is a common and fixable contributor. For more on running specifically, see whether running lowers testosterone.

Does Zone 2 Preserve or Raise Testosterone?

We did not find trials that tested Zone 2 training specifically on testosterone. The closest evidence is the meta-analysis of 11 randomized trials in insufficiently active men aged 19 to 75, covering aerobic, resistance and combined training lasting a median of 12 weeks. It found a negligible effect on resting total testosterone (standardized mean difference 0.00, 95% CI -0.20 to 0.20), regardless of training mode, age or body mass status.^[2] In other words, moderate training did not lower testosterone in these men, and it did not raise it either. Cardio is worth doing for heart, metabolic and mental health, but it is not a testosterone booster. Weight management is a better-documented route; see testosterone and belly fat.

What About the Mitochondria Argument?

You may read that Zone 2 uniquely builds mitochondria, and that because testosterone synthesis begins in mitochondria, Zone 2 must support testosterone production. Two parts of that chain are weaker than they sound. First, the rate-limiting step of steroid hormone synthesis, conversion of cholesterol to pregnenolone by the enzyme P450scc, does take place in mitochondria,^[9] but we found no human evidence that exercise-induced mitochondrial changes in muscle translate into higher testosterone output from the testes. Second, in a meta-regression of 353 studies, mitochondrial content in muscle rose by 23 percent with low- or moderate-intensity endurance training, 27 percent with high-intensity training and 27 percent with sprint interval training, differences that were not statistically significant, although per hour of exercise the higher-intensity formats were more efficient.^[5] Zone 2 is a good way to build an aerobic base with low stress, but it is not the only way.

How to Find Your Zone 2

Pick the method that fits your equipment and how precise you want to be. Treat every estimate as a starting point.

The talk test is supported by validation work,^[6] and Tanaka and colleagues' equation is the basis for the example above.^[10] If you have a heart condition, are new to exercise or are over 40 and sedentary, check with a doctor before increasing training.

Weekly Programming

There is no evidence-based weekly Zone 2 prescription for testosterone, so base your plan on general guidelines. The WHO recommends 150 to 300 minutes of moderate-intensity (or 75 to 150 minutes of vigorous) aerobic activity per week plus regular muscle-strengthening activity.^[11] Easy sessions can make up a large share of that. An example week: resistance training on three or four days, one to three easy Zone 2 sessions of 30 to 60 minutes, and at least one rest day. Adjust to your recovery, and increase volume gradually.

Almost any sustained aerobic activity works for Zone 2. Incline treadmill walking and cycling make it easy to hold a steady effort, and rowing, swimming or the elliptical trainer also work. If you find traditional cardio boring, consider rucking, which adds a load to walking. For how cardio and lifting compare, see HIIT versus weight training.

Signs Your Total Training Load Is Too High

A joint consensus statement of the European College of Sport Science and the American College of Sports Medicine describes overreaching and overtraining syndrome as arising when the balance between training and recovery is not respected, with symptoms such as fatigue, performance decline and mood disturbances. It also notes that hormonal and other markers used to detect overtraining do not currently meet the criteria for general use, so symptoms and context matter more than any single blood value.^[12] Practical warning signs include persistent fatigue that does not improve with a rest day, declining performance, disrupted sleep, mood changes and falling motivation. The usual response is to reduce load, sleep more and make sure you are eating enough, which also applies if you are cutting calories while training hard.

Related Reading

• How to Increase Testosterone Naturally: where exercise fits in the full picture.
• The Complete T-Maxing Guide: the habit stack that cardio should fit into.
• Testosterone Levels by Age: age-specific context.

Putting It All Together

Zone 2 cardio is a low-stress way to build aerobic fitness and fits easily around resistance training. The evidence does not support the claim that it raises testosterone, and it does not suggest that moderate amounts lower it either. The testosterone concern is about years of very high endurance volume, often combined with not eating enough. Start with two easy 30-minute sessions a week, build gradually, watch your sleep, energy and mood, and see a doctor if symptoms persist.

Thrive is a wellness app that helps you track training, sleep and recovery habits so you can see how your routines add up. Thrive is not a medical device and does not diagnose or treat any condition. Try Thrive free on the App Store.