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Endurance Training and Low Testosterone – What You Need to Know

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As a regular gym goer or athlete you’ll probably understand the importance of testosterone for maintaining a good physique.

And whilst strength training has been found to elevate male hormones, there’s also a fair bit of research out there that shows excess cardio training can make them plummet.

But what if endurance training is your sport, passion or hobby and you don’t want to give it up?

Is there a way to put the brakes on your ever-decreasing T levels? And what are the consequences if you don’t?

Read on to find out more…

Testosterone – The Male Hormone

Your masculinity, strength and muscle mass are all controlled by testosterone. As is your physical performance, libido and virility.

Produced in the Leydig cells of the testes, testosterone is a natural steroid hormone. It is classed as an androgen as it develops, controls and maintains your male characteristics.

It has two categories of effects:

  • Anabolic – actions primarily include development of male characteristics – increased strength, voice deepening and hair growth.
  • Androgenic – actions include increased protein metabolism and inhibition of protein breakdown.

How Does Testosterone Work?

The release of T from the Leydig cells is all controlled by a very elaborate system called the hypothalamic-pituitary-gonodal (HPG). This is made up of the hypothalamus and pituitary glands which both live in the brain. The third part – referred to as the gonads – is just another word for your testes.

Within this three part system are a number of hormones that have their own individual jobs. Here are the ones you need to know about:

  • Gonadotropin-releasing hormone (GnRH) is released by the hypothalamus gland and sends signals a very short distance to the pituitary gland. It is like the command centre for the whole operation.
  • The pituitary gland then releases its own hormones which are luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These are pushed into the main circulation of the bloodstream and then bind to receptors in the testes.
  • Once the Leydig cells receive LH they begin to produce and then release testosterone into the bloodstream ready for you cells to pick it up.

Any upset to to any part of the HPG axis will result in lower testosterone being secreted into the bloodstream. Think of the whole system being like a chain. If at any point the chain breaks, the whole system falters.

The Free Testosterone Connection

Lastly, it is also worth knowing that free T levels are different to total T.

A large percentage of the testosterone in your blood is bound to two proteins – albumin and sex hormone biding globulin (SHBG). In fact, only around 3% of testosterone is actually bioavailable or ‘free’ to be taken up by your tissues.

If levels of SHBG go up then you have less testosterone available to do it’s job. And this is an important feature in the link between testosterone and endurance training.

The Testosterone and Endurance Training Relationship

A number of research studies have shown that prolonged endurance training can disturb hypothalamic-pituitary gonadal axis hormones.

In fact, at rest testosterone concentrations appear to be lower in endurance athletes than untrained males. And much lower than that of strength athletes.

For example, a study published in the journal of Endocrinology found that intensive, long-term running reduced the efficiency of a number of HPG hormones [1].

In the study, 286 runners were split into one of two groups:

  • Moderate-intensity exercise at 60% of maximal oxygen uptake
  • High-intensity exercise at 80% of maximal oxygen uptake

Each group exercised 5 times per week for 60 weeks – a very long period of training. The sessions lasted 2 hours too!

Every few weeks the volunteers had their blood taken and tested. And in as little as 12 weeks, both groups showed a decrease in total and free testosterone with an increase in SHBG. All other HPG axis hormones including GnRH, LH and FSH were lower than at the start of the study.

Similar results have been found in as little as 2 weeks too. A study by Hug et al [2] reported that when elite level cyclists were asked to ramp up their endurance training, T levels fell. As did other anabolic hormones such as IGF-1 and IGF-BP3.

Review studies have actually proposed a 100 km per week threshold for avid distance athletes [3]. Anything above this is said to make testosterone levels plummet.

And lastly it’s not just chronic endurance training that lowers T levels.

A Brazilian study from the university of Sao Paulo study found that after running the 26.2 miles of a marathon, male athletes had testosterone levels that were 50% lower than before the race [4].

The Cortisol Connection

Chronic endurance training can elevate levels of the stress hormone cortisol. This is another type of steroid hormone, but rather than boosting testosterone, it has the opposite effect.

In normal amounts, cortisol is necessary for metabolic functions and energy regulation, but when levels get too high from intense or regular endurance training it can have a negative effect.

This includes a loss of muscle mass, blunted immune system, increased inflammation and decreased T levels.

Studies have shown that cortisol increases in both moderate and high training intensities – specifically any intensity above 60% maximal oxygen uptake [5].

And this of course is the exact intensity that an endurance training athlete trains at.

Endurance Training and Overtraining

One feature of excessive endurance training is that of overtraining. This is a disorder characterized by fatigue, lethargy and loss of performance.

When you are in an overtrained state you find it hard to maintain workload, you recover slower and you’re much more likely to pick up colds and other illnesses.

Studies have shown that when endurance athletes are overtrained their testosterone levels begin to slump. Cortisol rises too.

High level endurance athletes are likely to train in excess of 20 hours per week. This high volume is much more likely to shift the athlete into overtraining than lower training volumes. In fact, one study showed that at any one time, up to 20% of endurance training athletes will be overtraining [6].

The prevalence of OT is thought to be highest in endurance-based sports requiring high volume, intense training, such as swimming, cycling, triathlon and marathon running. [6]

Why is Low T Bad for Endurance Athletes?

Now that we’ve established that endurance training blunts your T levels it’s time to look at the health consequences.

Testosterone is responsible for regulating muscle mass, optimizing body composition and regulating energy levels. It is the number one hormone that ensures male health and performance.

But when it isn’t working optimally, things can go wrong.

Increased Belly Fat

Regardless of how many miles you cover on your runs or cycles, the combination of high cortisol levels and low T is notorious for increasing the risk of belly fat [7]. Throw excessive amounts of cardio into the mix and straight away that speeds up the process.

Low T and obesity go hand in hand. And they trap each other into a vicious cycle.

The loss of muscle mass associated with hours and hours of endurance training will hammer at your metabolism too. And the only fix? Getting those T levels back up!

Higher Metabolic Disease Risk

The increase in body fat combined with a loss of muscle can result in a number of metabolic disorders such as insulin resistance.

More than half of men with low T develop diabetes type 2 [8], a disease where you are unable to produce enough insulin or your body doesn’t react to it properly.

Research also suggests that marathon runners may suffer from higher levels of artery plaque and that endurance athletes in general are at a higher risk of heart wall fibrosis [9].

And as diabetes and disorders relating to arteries are primary risk factors for heart disease, it’s important that you avoid them at all costs.

Loss of Strength and Power

A loss in muscle mass not only contributes towards belly fat but can also decrease your strength too.

Bigger muscles can generate more force, and testosterone helps muscle tissue DNA repair and recover quicker. Endurance athletes are typically weak in comparison to other athletes. Low testosterone is part of the reason why.

Low Bone Density

Testosterone protects your bones in the same way that estrogen does in women. And when T levels drop, so does your skeletal density.

A study published in the British Journal of Sports Medicine [10] suggested that bone loss could result from chronic suppression of testosterone levels with distance running, and that high mileage endurance training athletes are at the highest risk.

Summary Endurance Training and Testosterone

Regular distance athletes and endurance enthusiasts are at a higher risk of low testosterone. In fact, it’s pretty conclusive that excessive cardio blunts your male hormone levels.

Endurance training can decrease production and regulation of signalling hormones in the brain, or directly decrease Leydig cell production too.

The consequences are that you’ll lose muscle mass and strength. That in turn can contribute towards belly fat, insulin sensitivity and higher risk of heart disease.


  1. Safarinejad, MR et al. The effects of intensive, long-term treadmill running on reproductive hormones, hypothalamus–pituitary–testis axis, and semen quality: a randomized controlled study. J Endocrinol. 2009; 200: 259-271
  2. Hug, HM et al. Training modalities: over-reaching and over-training in athletes, including a study of the role of hormones. Best Pract Res Clin Endocrinol Metab. 2003; 17(2): 191-209
  3. De Souza, MJ et al. The effect of endurance training on reproductive function in male runners. A ‘volume threshold’ hypothesis. Sports Med. 1997; 23(6): 357-74
  4. França, SC et al. Divergent responses of serum testosterone and cortisol in athlete men after a marathon race. Braz Arch Endoc Metab. 2006; 50(6)
  5. Hill, EE et al. Exercise and circulating cortisol levels: the intensity threshold effect. J Endocrinol Invest. 2008; 31(7): 587-91
  6. Mackinnon, LT et al. Overtraining and overreaching: Causes, effects and prevention. In: Garrett W.E., Kirkendall D.T., ed. Exercise and Sport Science.  Philadelphia: Lippincott Williams & Wilkins, 2000: 487-98
  7. Rebuffé-Scrive, M et al. Effect of testosterone on abdominal adipose tissue in men. Int J Obes. 1991; 15(11): 791-5
  8. Rao, PM et al. Testosterone and insulin resistance in the metabolic syndrome and T2DM in men. Nature Reviews Endocrinology; 9: 479-493
  9. O’Keefe, JH et al. Potential Adverse Cardiovascular Effects From Excessive Endurance Exercise. Mayo Clin Proc. 2012; 87(6): 587–595
  10. MacKelvie, KJ et al. Bone mineral density and serum testosterone in chronically trained, high mileage 40–55 year old male runners. Br J Sports Med. 2000; 34(4): 273-8