Testosterone is a naturally secreted ‘androgenic’ hormone. It is responsible for the promotion of secondary male-sex characteristics, and plays a significant role in muscle growth and neuromuscular adaptation.  

Testosterone exerts an anabolic effect on muscle via two mechanisms: (1) The stimulation of amino acid uptake, and thus protein synthesis; and (2) inhibiting protein breakdown by countering cortisol signalling, a ‘catabolic’ hormone. The literature suggests that reductions in testosterone over time appear to correlate with an observed decline in muscle mass and strength. In men, muscle mass levels tend to peak in the third decade of life and subsequently decreases on average by approximately 1-2 % per year, lead by changes in muscle fibre type and size. Additionally, muscle strength decreases on average by approximately 1.5-3.0 % per year, with the rate of decline becoming steeper after age 50. 

Ageing 

Convincing data show that serum free and total testosterone levels fall with normal ageing at an average rate of 2-3 % per year, and 1.6 % per year respectively, due to changes at all levels of the hypothalamic-pituitary-testicular axis. For this reason, the prevalence of testosterone deficiency increases with age, with clinically low testosterone levels seen in ~10 % of men in their 50’s, 20 % of men aged over 60, rising to an expected 50 % prevalence in those over 80.   

The symptoms of mild deficiency include lack of energy, reduced strength, loss of libido, decreased sexual performance, depression, and mood change. Due to the similarities between normal ageing and the symptoms of mild testosterone deficiency, the clinical diagnosis in ageing men is challenging, and is often missed. 

In regards to body composition and exercise performance, there is evidence that testosterone treatment (TRT) can restore muscle mass levels and reduce visceral fat, in a variety of clinically deficient populations, including ageing men. However, reported improvements in maximal strength with TRT alone are inconsistent.  

Exercise 

The effect of exercise on testosterone in ageing men has been extensively investigated but remains unclear, with some research reporting significantly higher basal (resting) levels of testosterone in well-trained/athletic older men vs age-matched sedentary controls. However, other studies have failed to distinguish differences in lifelong trained vs sedentary populations. 

Resistance Training 

Numerous studies have demonstrated that resistance training can cause acute changes in serum testosterone levels, which appears to be responsive and proportional to the activation of muscle mass, i.e. training small muscles in isolation, even at high intensity does not elevate testosterone levels, whereas total body involvement elicits significant increases up to ~30 %. Middle-aged and older men appear to show similar relative testosterone responses to those of younger men to a single bout of high-intensity resistance exercise, however, this increase is transient and begins to return to baseline after just 30 mins. In isolation, the acute fluctuation is unlikely to play a significant role in the long-term adaptive response to an exercise session. Despite the strong associations between muscle strength/size and testosterone levels in middle aged and older men, data suggests that resistance training intervention does not significantly influence basal testosterone. 

Endurance Training 

Similar findings are reported in regards to the effects of endurance training on acute changes in testosterone levels. There appears to be a relative exercise intensity that must be reached in order to induce changes in testosterone concentrations, with significant increases (~20 %) in serum testosterone levels only reported in ‘high intensity’ exercise conditions (stated as ≥ 90 % VO2 max in one particular study), returning to baseline within 1 hour post-exercise. Despite increasing the frequency, duration, and intensity of endurance training over longer periods (≥14 weeks), no significant increases in basal plasma testosterone concentrations are reported.  

Interestingly, some studies have shown that chronic endurance training is inversely correlated with basal serum testosterone concentrations, e.g. professional cyclists tend to have lower basal testosterone levels after major competitions when compared to pre-competition baseline. Although there is no clear consensus on the mechanism at play, several researchers have hypothesised that exercise induced production of reactive oxygen species can suppress the hypothalamic pituitary axis and lead to diminished function of the testes and low testosterone production. 

Take home messages

• Basal testosterone levels appear to be largely genetically determined and typically decrease with ageing, with a more prominent decline occurring after the age of 50. 
• Despite the fact both endurance and resistance training seem to induce an acute rise in testosterone, this effect is not reliably sustained. 
• A rapid return to baseline levels of testosterone post-exercise is likely attributed to (a) sympathetic nervous system stimulation and the subsequent inhibition of testosterone production; and (b) changes in testosterone metabolism. 
• Chronic endurance exercise may lower basal testosterone levels. 
• Symptoms of testosterone deficiency should be clinically evaluated by an appropriately qualified medical professional.  

References

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