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Low testosterone levels are never good. Clinically low testosterone leads to poor muscle mass development, increased body fat, and compromised athletic performance. Worst of all, reproductive function drops and libido is compromised. 

 

It’s safe to say that optimizing testosterone is a top priority, which is why you should know that endurance exercise leads to depressed testosterone production in men. 

 

For women, endurance exercise increases testosterone slightly, however, other reproductive hormones such as FSH and LH are typically reduced in female endurance athletes, which can lead to reproductive problems. 

 

Simply, very large amounts of endurance exercise are no good for either gender, though the exact mechanism differs. 

 

This article will review the negative effect of endurance exercise on testosterone and other androgen hormones and give you tips for what you can do about it.

 

Studies consistently show that high-volume endurance exercise reduces baseline androgen hormones by 20 to 40 percent. The androgen hormones include testosterone, estrogen, and DHEA. Follicular Stimulating Hormone (FSH) and luteinizing hormone (LH) are related hormones that affect reproductive function and are released from the pituitary gland. 

 

In men, testosterone, LH, and FSH are all reduced, whereas in women, typically only LH and FSH are low. For men, this means low libido and reduced semen quality. For women it manifests primarily in terms of menstrual irregularities and poor fertility. 

 

Endurance exercise leads to larger acute elevations of the key stress hormone cortisol than strength and power training. Over time, repeated bouts of intense, long-duration training lead to adrenal gland enlargement due to high cortisol output.

 

In normal amounts, cortisol is necessary for metabolic function, but chronic elevations from high levels of training stress leads to an impaired stress response that is almost always associated with low testosterone in endurance athletes. 

 

This is problematic because excessive cortisol has a catabolic effect on muscle tissue, breaking it down and leading to higher levels of persistent inflammation. High cortisol also suppresses immune function, which is why endurance athletes have high rates of colds and illness during high-volume training phases. Fat burning is also reduced, which can lead to fat storage in the right metabolic environment. 

 

Testosterone does just about everything cortisol doesn’t, which is one reason the ratio between these two hormones is used to assess training status and performance potential. 

 

Scientists have hypothesized that high increases in cortisol on a daily basis may reduce testosterone via inhibitory actions. 

 

Two additional symptoms typically coincide with low T and high cortisol in endurance trained men: Luteinizing hormone, which normally follows a pulse-like pattern, is inhibited, and prolactin is elevated.  When LH is blunted, testosterone release is reduced from the testes. Elevations in prolactin correspond with dysregulation of the hypothalamic-pituitary-adrenocortical axis. 

 

It’s well known that female endurance athletes with low reproductive hormones have greater risk of low bone density and bone fracture, but less attention has been given to the impact of low testosterone on male bone health. 

 

Lower bone mass has been reported in a study that compared male endurance runners with sedentary controls and case reports show similar bone strength problems. 

 

Endurance athletes experience less muscle mass development due to a phenomenon known as “interference.” This may be partly due to reduced testosterone and androgen hormones since these hormones play a role in muscle tissue repair. 

 

For example, testosterone increases DNA activity on muscle tissues and leads to greater release of growth hormone, which enhances amino acid uptake and tissue repair. 

 

Endurance athletes also have much lower power output than strength athletes or even sedentary controls, which may be due to their poor androgen hormone status. In addition, the ability of cells to bind with androgen hormones is reduced in response to highly stressful exercise, whether it’s endurance modes or a long-duration high-intensity model, such as CrossFit. 

 

Running a few miles a few days a week is unlikely to cause low testosterone and other related reproductive problem. 

 

Athletes who are most at risk are those training a high volume (intense workouts lasting longer than an hour) everyday. Marathoners, long-distance runners, triathletes, and CrossFitters are most susceptible. 

 

In addition, if you have a crazy stressful life, lower training volumes may take a toll on you testosterone-to-cortisol ratio. Athletes often ignore the “mental” stress factor, but it may be the reason that studies into the effect of training on hormone levels are highly variable.

 

We do know that high-volume, serious endurance training is “too much.” For example, a 2009 study had 286 men run for 2 hours on a treadmill, 5 times per week for 60 weeks. Half the men ran at moderate intensity (60 percent of VO2 Max) and the other half ran at high intensity (80 percent of VO2 Max). 

 

Testosterone, LH, and FSH were all significantly reduced in both exercise groups, but much more so in the high-intensity athletes. They also experienced significantly decreased sperm counts and lower reproductive health. 

 

The good news is the effects aren’t permanent: The study included a 36-week recovery phase in which participants radically reduced exercise intensity to 30 percent of VO2 Max and all reproductive markers including testosterone rebounded to normal levels. 

 

If you’re a hardcore athlete who is concerned about your testosterone levels, getting a blood test is a smart move. “Normal” testosterone levels for men encompass a wide range that is useless for all intents and purposes. According to the Manual of Laboratory Diagnostic Tests normal values for total testosterone are anything above 270 ng/dl. 

 

For peak performance and recovery, you want your levels to be significantly higher—in the 500 to 800 ng/dl range. For cortisol, morning values of 5 to 10 mcg/dL are ideal and will mean your stress levels are well under control. Up to 23 mcg/dl is considered “normal,” but if you are running around with that level for long, you’re in for a fall—either due to overtraining, persistent inflammation, or metabolic problems such as insulin resistance, poor muscle mass, or abdominal fat. 

 

Once you know your testosterone and cortisol readings, measuring your T-to-C ratio is a useful way to assess your training status and recovery ability. Assuming you have normal testosterone levels at baseline, research suggests that anything beyond a 30 percent drop in the T-to-C ratio will have a negative effect on recovery. 

 

Of course a slight drop is ideal because it indicates that you’re effectively overloading the body. Therefore, changes in the ratio of less than 10 percent represent inadequate stress if you want performance improvements. Another way to say this is that performance is optimal if your ratio is between 10 and 30 percent in the 24 to 48 hours after a killer workout. 

 

Although cutting back on training volume and taking planned recovery weeks are probably the most effective strategies for raising levels of testosterone and related hormones, there are a few key additional actions you can take:

 

Too much volume is the factor that limits testosterone production in endurance athletes. A well-tested solution is to dial back training volume in favor of a few high-intensity, SHORT, weight or interval workouts. Lifting heavy weights or doing interval repeats will target your neglected powerful type II muscle fibers to make you faster and more powerful, without compromising endurance. Do these workouts separately from endurance workouts.

 

Testosterone is produced in the body out of cholesterol, which you get from dietary fat. Studies show that men with a higher fat intake have significantly higher testosterone than those who restrict fat. Be sure to get a healthy intake of saturated fat from meat, dairy, and coconut oil. Round out fat intake with fish, avocado, olives, seeds, and nuts. 

 

Carb intake is important for testosterone production and it also helps lower cortisol. Therefore, vegetables, fruit, and other whole carbs from starch, beans, and select grains can be beneficial for healthy testosterone levels. 

 

Diets high in fast-digesting and refined carbs are contraindicated. This is important to high volume endurance athletes who are notorious for using simple carbs for quick refueling. 

 

Research shows that in response to a blood sugar spike from high-carb foods, testosterone is temporarily reduced by 25 percent. Therefore, unless you must replenish glycogen immediately post-workout in order to train again in the next 4 to 6 hours, opt for slower digesting complex carbs, or at least a high-quality carb powder. 

 

Just one night of short sleep will jack cortisol up and alter testosterone release, leading to a lower level in the morning. Combine the stress of lack of sleep with the pervasive training stress of endurance exercise and you’re in for major hormonal dysregulation.