The Impact of Physical Exercise on Male Fertility Through Its Association with Various Processes and Aspects of Human Biology

2. Impact of Physical Activity on Fertility Through Endocrine Action

The endocrine system supports the body’s homeostasis and adapts it to external stimuli. Hormonal signaling plays various roles in both anabolism (tissue growth, repair, and regeneration) and catabolism (metabolic regulation and tissue breakdown). During training, hormonal mechanisms are part of a complex, integrated signaling system that indirectly modulates metabolic and cellular processes in skeletal muscles, the nervous system, and connective tissue. However, as hormones do not act in isolation, every hormonal or adaptive response must be considered in the context of the entire endocrine system and its interplay with other physiological systems [59].

From the physiological perspective, male reproduction results from the collaboration of several hormones responsible for spermatogenesis and sexual function. To produce these hormones, the hypothalamus, the pituitary gland, and the testes work together, which is why they are often treated as a single entity, i.e., the hypothalamic-pituitary-gonadal (HPG) axis. When a male reaches sexual maturity, gonadotropin-releasing hormone (GnRH) is released from hypothalamic neurons in pulses, stimulating the pituitary gland to produce two other hormones, namely luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which then stimulate spermatogenesis and testosterone production in the testes [25,60]. Approximately 44-60% of the testosterone transported in the bloodstream is bound to sex hormone-binding globulin (SHBG). Free testosterone, on the other hand, accounts for approx. 2% of the total cholesterol in circulation and is taken up by tissues to bind with androgen receptors (AR) and mediate regenerative processes [61].

The nature of the impact of physical activity on the male hormonal system is a complex one. Indeed, some studies do not show the existence of a relationship between sporting activity and hormone production [62,63], while others show either a negative [64] or a positive [65] association. A study performed on a sample of the US population indicates a reduced risk of low testosterone levels in males performing light to moderate physical activity [66]. However, this is inconsistent with the findings of a comprehensive study performed by Safarnejad et al. [67], in which two training protocols were used, i.e., a moderate intensity (60% VO2 max) and a high intensity (80% VO2 max) protocol. The study showed that after 12 weeks, the levels of LH, FSH, and testosterone, including free testosterone, significantly decreased in both training groups. This reduction in hormone levels could be connected to the exercise-induced suppression of serum testosterone, which according to Kujala et. al., is associated with suppressed stimulation of gonadotropin release caused by gonadotropin-releasing hormone (GnRH) during exercise, as well as reduced testicular ability to secrete testosterone during recovery period [68]. However, Safarnejad et al. [67] also claimed that the levels of LH, FSH, and testosterone could be restored after 60 weeks of moderate-to-high intensity training, followed by 36 weeks of low-intensity training, treated as a recovery period. On the contrary, the results of a study performed by Vingren et al. [69] showed increased serum testosterone levels in subjects who performed moderate-intensity physical exercises. Another method for maintaining testosterone levels proposed in literature is magnesium supplementation (10 mg/kg body weight), which improves its levels in both exercising and sedentary males [70]. As far as particular exercise modes are concerned, it has been shown that free-weight lifting has a more beneficial influence on testosterone levels compared to training on machines [71]. Increased testosterone levels were also found in adolescent boys subjected to 30 min. of moderate physical activity—compared to the control group, who were involved in the same amount of sedentary behavior. The latter mode of activity has also been found to increase cortisol levels in the subjects [72]. In contrast, Steeves et al. [73] found no association between physical activity and testosterone levels but pointed out that although increased physical activity was not found to be linked to testosterone levels, it may be associated with a lower likelihood of low or low-normal testosterone in non-obese men, though this was not observed in obese men. The long-term effects of physical activity, however, are less well understood, with some evidence suggesting lower testosterone levels in endurance athletes [74].

Other studies have attempted to determine the relationship between physical exercise and the HPG axis [21,75]. Vaamonde et al. [21] demonstrated that under controlled conditions, no disruption of the hormonal balance involving the HPG axis occurs; on the contrary, an improvement in semen quality in physically active individuals was observed. The HPG axis plays a crucial role in the body’s response to exercise and training, both in the short and in the long term, and has an impact of testosterone levels in men. A crucial factor that influences the proper functioning of the HPG axis is sex hormone-binding globulin (SHBG). However, studies focusing on the impact of physical activity on this protein do not reach a consensus regarding changes in the concentrations of HPG under training conditions, Safarinejad et al. [67] reported a significant increase in SHBG, both in the case of high- and moderate-intensity training. Moreover, higher SHBG levels were associated with lower levels of free testosterone in the serum of the subjects. In another study, men characterized by high levels of physical activity had 14% and 8% higher concentrations of SHBG and total testosterone, respectively, compared to men whose physical activity was lower—after adjusting for BMI—these differences amounted to 6% and 3%, respectively [76]. The cause of the aforementioned endocrinological dysfunctions following intense physical exercise appears to be the production of reactive oxygen species (ROS) in the body of the individual undergoing training [77,78], which links endocrine effects of physical exercise with those involving the immune response.

It is worth emphasizing that thyroid disorders have also been shown to have an adverse impact on fertility. Bahreiny et al. [79] showed that subclinical hyperthyroidism (SCH), i.e., a mild thyroid disorder characterized by low serum thyroid-stimulating hormone (TSH) levels while normal thyroid hormone levels are maintained, has a negative influence on levels of reproductive hormones and semen quality, with decreased LH levels, elevated FSH levels, and decreased TSH levels, among others. Moreover, hypothyroidism can affect the secretion of GnRH through increased prolactin levels, leading to hypogonadotropic hypogonadism [80]. Wu et al. [81] reported that exercising is closely related to maintaining thyroid homeostasis in patients with subclinical hypothyroidism. It should also be noted that men experiencing hypothyroidism can also display reduced libido or erectile dysfunction [82,83]. For example, Jaya Kumar et al. [82] found a prevalence of libido reduction in 37.5% of hypothyroid patients, while Carani et al. [83] reported low sexual desire, impaired erectile function, and delayed ejaculation in hypothyroid men. In addition, literature data shows that while thyroid function may be impaired by excessive physical activity [84], low-level activity such as increasing the daily number of steps can reduce the risk of thyroid hormonal imbalances [85]. These findings corroborate the that physical activity should be tailored independently to the needs of each individual also when thyroid disorders are concerned.

3. Impact of Physical Activity on Fertility Through Its Effect on the Circulatory System

Cardiovascular diseases (CVD) are a major global health concern, with their risk factors, such as dyslipidemia, hypertension, and diabetes, potentially affecting male fertility [86]. Uncontrolled dyslipidemia and the associated inflammation can impair endothelial function and hormone regulation, both of which are crucial for reproductive health [25]. In contrast, improving cardiovascular fitness through exercise may help reduce these risks and thus have a positive impact on fertility [87].

One of the most recognized associations between physical activity, cardiovascular health, and male fertility is via problems with erection. The underlying hypothesis is that physical activity can improve blood flow through the testes by inhibiting atherosclerosis, boosting vascular blood delivery, refining the lipid profile, and reducing the risk of thrombosis [88]. In this context, erectile dysfunction indeed often serves as a warning sign for underlying cardiovascular problems. For this reason, assessing patients’ individual cardiovascular risk is essential before physical exercise is considered as a viable remedial measure for problems with erection [89], as it may potentially lead to the exacerbation of cardiovascular issues. However, individuals whose problems with erection have been reliably linked with impaired cardiovascular health through proper medical assessment may benefit from performing physical activity [90]. This may enhance both their reproductive as well as cardiovascular health.

Physical exercise is generally regarded as a fundamental component of a healthy lifestyle and, outside the scope of reproductive medicine, this role is often linked to cardiovascular effects such as the reduced risk of stroke or myocardial infarction. Thus, although physical activity is recognized to have beneficial effects on health, including its cardiovascular aspects, it is important to note that intense or excessive exercise can be detrimental, which also concerns the area of male fertility [2,22,24]. However, studies do not link this negative influence directly with cardiovascular effects, as immune and endocrine processes are those that are typically indicated as key in the context of the negative association between excessive physical activity and male fertility [25,91]. Studies show that a balanced approach to physical activity, i.e., moderate and consistent, improves cardiovascular function and reduces the risk of chronic diseases, thus having a potentially beneficial influence of male fertility.

Overall, the impact of physical activity on male fertility through its association with cardiovascular health seems to be less pronounced compared to other processes. The main association is the enhanced blood flow to the reproductive organs resulting from performing physical exercises. Hence, regular exercise helps improve circulation, maintain healthy blood vessels, and regulate blood pressure, all of which are crucial for the proper functioning of the body, including the testes and other reproductive structures. In addition, improved circulation supports the delivery of oxygen and nutrients to the testes, which is essential for optimal spermatogenesis. Physical activity can also help reduce the risk of conditions associated with impaired circulation and impaired fertility such as obesity and diabetes.

4. Impact of Physical Activity on Fertility Through Its Action Related to Insulin Resistance, Diabetes, and Obesity

Numerous studies point to complex negative associations between diseases and conditions connected with glucose metabolism, i.e., diabetes, insulin resistance, and obesity, and fertility [92,93]. Due to the fact that metabolic conditions are exacerbated by lack of exercise, it can be safely assumed that increased physical activity may have a beneficial effect on male fertility, e.g., by improving glucose metabolism. Literature data shows that exercise can enhance male fertility in conditions of lifestyle-induced infertility, such as obesity and diabetes [2]. Hence, adopting a healthy lifestyle, which encompasses balanced nutrition, physical activity, and effective weight management, is the key strategy for managing metabolic disorders and promoting optimal male fertility [33]. It should be noted, however, that the precise interactions between these three aspects are not yet sufficiently studied, with the need for robust studies such as randomized controlled trials (RCTs) focused on the male population [92].

It is commonly agreed that the impact on insulin resistance, diabetes, and obesity on male fertility is negative. A systematic review performed by Service et al. [24], who analyzed 112 articles published between 2013 and 2023, indicates that obesity, diabetes, and metabolic syndrome adversely affect multiple aspects of male fertility, including semen quality and sperm DNA integrity. Specifically, research points to the association between insulin resistance and sperm motility [94]. Additionally, growing evidence suggests the existence of a negative association between male obesity and live birth rates, whether through natural conception or assisted reproductive techniques. Moreover, obesity has been shown to disturb the balance between oxidation and antioxidation in the testes, leading to oxidative stress, which activates NF-κB, initiating an inflammatory response. As mentioned earlier, this leads to a decrease in testosterone production and the worsening of sperm quality [3].

As mentioned above, obesity is largely agreed to have a detrimental impact on male fertility [19]. Most importantly, increased ROS production is commonly observed in obese men, in whom low levels of circulating testosterone can also be routinely observed. Kumagai et al. [95] showed that a 12-week aerobic exercise program raised serum levels of total testosterone, free testosterone, and bioavailable testosterone in overweight or obese men, concluding that vigorous physical activity boosted circulating testosterone levels in the aforementioned group. It has been well-documented that aerobic physical activity reduces insulin resistance associated with hypogonadism and improves erectile function [96,97,98,99]. Based on the principle that exercise-induced weight loss has a positive impact on reproductive hormones by regulating metabolic pathways, Lo Giudice et al. [14] performed a meta-analysis of RCTs, whose results show that physical activity is associated both with improved semen parameters and fertility outcomes.

It is interesting to note that despite the rather well-established finding that excessive exercise is detrimental to fertility, this may not be in fact true for obese males. Studies show that endurance exercises and high-intensity interval training have a positive influence on glucose metabolism in obese males [3]. In addition, a large study performed by Fantus et al. [100] in a group of 7,372 men found that those who exercised more intensely than the recommended level of activity had a lower risk of low testosterone levels than those who did not meet the recommended levels of exercise. Although this finding may be pointing to a beneficial impact of excessive physical activity on obese subjects, it must be also remembered that obese persons are mostly unable to perform extremely strenuous exercises, e.g. due to an increased risk of cardiovascular events, and thus what is considered excessive for them may be moderate for men who are fitter.

5. Impact of Physical Activity on Fertility Issues Associated with Its Physical Action on the Male Body

Certain types of activities may negatively affect male fertility due to overheating [114], insufficient oxygen levels during physical exercise [112,113], or pressure and trauma [115]. In terms of adequate temperature levels, the ideal temperature for spermatogenesis is approximately 2°C below the core body temperature [106]. Studies have shown that elevated scrotal temperature may increase the production of ROS, which can damage the sperm plasma membrane and lead to DNA fragmentation [101]. In addition, animal studies suggest that an increase in testicular temperature by 1-1.5°C leads to a reduction in sperm production, testicular shrinkage, as well as a predisposition to the production of morphologically abnormal sperm and a lower concentration of sperm with normal motility [102,103]. Figà-Talamanca et al. [104] found that two hours spent in sedentary posture leads to an increase in testicular temperature of approx. 2°C, which suggests that prolonged time spent, for instance, in a car may increase the risk of reproductive problems. Excessive use of hot baths or saunas has also been shown to have an adverse impact on male fertility [105]. It should be noted, however, that some studies do not corroborate the link between the increased temperature resulting from sedentary lifestyle and a deterioration in semen parameters [23]. It may also be interesting to mention that research conducted on rats indicates than males are more susceptible to the effects of exercise-induced overheating than females [106], which may point to a biological mechanism that makes overly strenuous training incompatible with male fertility.

Interestingly, literature data points to the fact that exposure to excessively high temperatures may have a detrimental effect on male fertility, in a similar manner to the effects of physiological aging [18]. Human sperm is produced even at an advanced age, but when the decision is made to start a family later in life, certain changes must be taken into account. A decline in semen quality among older fathers is already observed from the age of 40, with a reduction in the number of sperm with normal morphology [107], After the age of 45, a gradual decline in semen volume is also observed, likely due to a functional impairment of the accessory glands [108]. For this reason, it can be stated that older men may be characterized by impaired testicular function [109], decreased production of sex hormones [110], and reduced semen quality parameters [18,111]. These changes may also contribute both to a higher incidence of congenital abnormalities in fetuses and their mortality [112,113]. The results of a study by Johnson et al. [114] show that men over the age of 75 had an average testicular volume 31% smaller compared to men aged 18 to 40 years. Other studies indicate that the average total number of Leydig cells decreases by as much as half in men aged 50-76, compared to men aged 20-48 [115].

In this context, regular physical activity, when practiced over the long term, may have a protective effect against the oxidative stress associated with aging [116,117]. Zhao et al. [116] demonstrated that physical exercise leads to better regulation of the mRNA levels of the enzymes SOD, CAT, and GPx. In addition, mice subjected to exercise as part of the study exhibited improved regulation of testosterone levels. In the study by Kraemer et al. [118], after a 10-week strength-power training program, an increase in free and total testosterone levels was observed in participants around 60 years of age, which was not seen in individuals around 30 years old performing the same training program. On the other hand, Häkkinen et al. [119] observed no changes in testosterone secretion after training in either older or younger groups.

In addition to temperature, other factors can have a detrimental effect on male fertility, including inadequate oxygen intake. High-altitude sports, among others, are associated with the risk of hypoxia [25,120], which is connected with testicular dysfunction [25], as is deep diving. In the study by Aitken et al. [121], sperm motility and sperm concentration deteriorated shortly after a diving episode and remained impaired for up to 3 months.

Performing sports is also connected with increased pressure, and potentially even trauma, to the gonadal region. For example, cycling can negatively affect male fertility through both pressure and increased temperature. Furthermore, regular use of tight clothing such as racing suits during cycling and the pressure exerted on the scrotum by the saddle adversely impact the fertility of cyclists. Additionally, the increased risk of pelvic floor injuries in cycling further contributes to the negative effect of this particular sport on fertility by influencing the flow in the dorsal vein complex. With worsened blood flow in that area can lead to a weakened capillary washout of Prostate-specific Antigen (PSA) into the circulation [122,123]. It should also be noted that wearing tight underwear may have a similar impact on fertility to those described above [124].

Sgrò and Di Luigi [90] make a valid point by indicating the numerous ways in which competitive sports can cause damage and dysfunction to the reproductive and sexual tracts. These negative impacts can be either temporary—i.e., presenting as genital pain, reduced sensation, hypogonadism, erectile dysfunction, changes in sexual drive, among others—or permanent. All of this impacts can occur through direct factors such as trauma to the external genitalia such as the aforementioned saddle-related issues in cyclists, or indirect factors, including exercise-induced hypogonadism, substance abuse, doping, and stress, among others.

As far as the effects of physical activity on the male body are concerned, it can be concluded that to ensure its positive impact on male fertility, the choice of sports disciplines must be carefully considered. As certain incidental factors, such as tight-fitting apparel, excessive pressure, or oxygen deprivation, may have a detrimental effect on fertility, a hypothesis worth exploring would be whether the positive effects of physical exercise may, to some extent, be counterbalanced by the aforementioned secondary aspects of intense physical activity. As such factors are particularly common in professional sports, they may contribute to the negative effects of excessive exercise on male fertility reported in scientific literature.