This post is all about testosterone (T), we’ll look at what it is, what it does and where it comes from. We’ll cover the reasons that it is an important hormone for women as well as men and how low testosterone levels negatively impacts both sexes. We will detail some of the symptoms of low testosterone and what you can do about them. Then we will look at what the latest research reveals about the impact of exercise, both resistance training and aerobic, on acute and long-term levels of testosterone.
By the end of the article, you’ll know a lot more about testosterone and how to optimize your own T-levels!
Let’s start out by answering the most basic question: What is testosterone?
What is testosterone?
Testosterone (T) is the number one male sex hormone and an anabolic steroid. It has two main roles:
- as an androgen regulating male characteristics, and
- as an anabolic steroid, involved in synthesizing tissue, tissue maintenance and growth.
Where does natural human testosterone come from?
Testosterone like all steroid hormones starts out as cholesterol, which then goes through the following preferential conversion path:
What are normal circulating levels of testosterone?
Examples of factors that can influence testosterone levels acutely
Circulating T levels fluctuate throughout the day, here are some lifestyle factors that have an impact:
- Time of day/circadian rhythm– testosterone levels typically peak between 4-8am and are lowest between 4-8pm
- Food intake– eating a meal can cause a decrease in testosterone levels for up to three hours
- Exercise– testosterone levels usually increase after exercise
- Sleep– a good night’s sleep can increase T levels, a lack of sleep or a disturbed night will result in lower levels of T.
- Emotional stress– can reduce levels of T whilst elevating levels of the stress hormone cortisol.
What happens if T levels are chronically below normal?
“Hypogonadism” occurs when an individual’s sex glands produce a clinically lower than normal level of sex hormones. In men, between 10-50% of the population suffer from hypogonadism, the percentage grows from lower to the higher end with increased age.
Hypogonadism is associated with:
- Low lean body mass
- Low strength levels
- Reduced bone mass and osteoporosis
- Reduced sex drive
Testosterone Replacement Therapy
For an individual with low testosterone levels, testosterone replacement therapy (TRT) can bring levels back into the normal ranges and reverse the symptoms of hypogonadism. TRT is a medically prescribed treatment that effectively treats a deficit in T production.
Since the 1950’s many athletes and bodybuilders have self-experimented with taking exogenous testosterone, elevating their T levels significantly beyond the normal range. Supra-normal levels of T can increase lean mass and strength even without exercise.
Taking exogenous T, whether to normalize low levels or to maximize muscle and strength, will effectively shut off an individual’s own endogenous production, a side effect of which is gonad atrophy.
Athletes and bodybuilders
It is anecdotally reported that those who use testosterone specifically for muscle and strength gain can typically expect to see body mass increases of between 10-20kg and strength increases of up to 30%.
It is thought that anabolic steroids impact athletes in three beneficial ways:
- Turning on androgen receptors in muscle cells
- Stimulating an anti-catabolic environment in muscle by blocking cortisol from binding to cortisol receptors. This effectively halts muscle protein breakdown and enables free amino acids to be used for tissue repair and growth.
- May enhance perceived physical ability (the actual psychological/mental impact of steroids is however highly debated)
The practical outcome is clear: steroid using athletes can TRAIN HARD, RECOVER FAST. There are however downsides to be considered including increase in total cholesterol with a concurrent reduction in HDL-C, hypertension and the aforementioned shutdown of natural testosterone production.
Impact of resistance training on acute and chronic levels of testosterone
A normal anaerobic workout will temporarily elevate levels of circulating testosterone. Androgen receptors in muscle cells are upregulated for 3 hours or more, drawing testosterone into muscle cells and increasing protein synthesis. Circulating testosterone levels typically return to baseline within 30 minutes.
In young men circulating T levels may increase by as much as 60% following a resistance training workout. Senior men produce less T after a workout than young men and women do not tend to show any significant increase.
For comparison sake, it is worth noting that during a cycle, athletes using anabolic steroids will typically have circulating levels of T that are double that attainable by a young man after a workout. And remember that these levels are elevated for weeks or months, not just a 30-minute post workout window!
If you do want to produce an acute increase in T from your workout research so far suggests the workout should present a metabolic demand that elevates blood lactate and the use of a controlled tempo (particularly slower eccentrics), HIT certainly fits the bill.
Can chronic resistance training increase baseline T levels over the long-term? Some research suggests that it can even in experienced lifters, however current evidence is far from conclusive.
Testosterone and endurance exercise
Moderate intensity aerobic sessions of around an hour have also been shown to acutely elevate T levels. However, moderate or higher intensity exercise performed for longer than 90 minutes typically causes a temporary reduction in levels of circulating T.
Can endurance exercise decrease circulating T chronically?
Researchers have recently coined the term “exercise-hypogonadal male condition” to describe men who appear to have significantly reduced their baseline T levels as a consequence of engaging in long-term, prolonged endurance exercise.
Men considered to have this condition have low resting levels of T, their baseline is often only 50-80% of normal and it isn’t a transient occurrence caused by temporary overtraining. If this were the case when training is reduced or halted a bounce-back in T to normal-levels would occur, which does not happen for these men, who appear to have recalibrated their circulating testosterone set-point permanently downward.
This condition is most typically observed in long-distance runners (10k and up), cyclists, race walkers and triathletes, with 1-15 years of near daily training.
Take home message
Testosterone plays an important role in human physiology and low testosterone levels are problematic for men and women.
Individuals with lower than normal levels of testosterone usually have less muscle mass, less strength, less bone mass, fertility issues and a reduced sex drive.
Testosterone levels can be temporarily boosted by a resistance training workout, a moderate intensity-moderate duration aerobic workout and a good night’s sleep.
Conversely testosterone levels are temporarily decreased by a lack of sleep, excessive stress, long duration aerobic exercise and eating a meal.
Optimizing your own natural best testosterone levels over the long-term will likely be achieved by maintaining a healthful bodyfat level, performing regular resistance training, good sleep hygiene, spending time in the company of people you are attracted to, balancing stressors effectively and avoiding regular, long-duration aerobic exercise.
Endocrinological Roles for Testosterone in Resistance Exercise Responses and Adaptations. Hooper DR, Kraemer WJ, Focht BC, Volek JS, DuPont WH, Caldwell LK, Maresh CM. Sports Med. 2017 Sep;47(9):1709-1720. doi: 10.1007/s40279-017-0698-y. Available here.
Low testosterone in male endurance-trained distance runners: impact of years in training. Hackney AC, Lane AR. Hormones (Athens). 2018 Mar;17(1):137-139. doi: 10.1007/s42000-018-0010-z. Epub 2018 Apr 27. Available here.
Male Hypogonadism: Basic, Clinical and Therapeutic Principles. Stephen J. Winters, Ilpo Huhtaniemi. DOI 10.1007/978-3-319-53298-1 ISBN 978-3-319-53296-7. Available here.
Testosterone Levels by Age. Available here.
TRT UK. A Guide To Testosterone Replacement Therapy In The UK: How to increase testosterone naturally. Available here.