Obesity impairs male fertility through long-term effects on spermatogenesis

Obesity refers to excessive accumulation of body fat, which has a negative impact on health. According to the World Health Organization (WHO), a body mass index (BMI) of 25–29.9 kg/m² is defined as overweight, while a BMI of 30 kg/m² is defined as obesity. The incidence of overweight and obesity has sharply increased [1]. Relevant statistical data show that the obese population has doubled worldwide from 1980 to 2008, and more than 10% of the population is obese [2]. The prevalence of obesity has continued to rise according to subsequent surveys, with the most recent estimate indicating that 35.2% of men and 40.4% of women are obese [3].

Obesity is well documented to be linked to diseases such as type 2 diabetes mellitus, cardiovascular disease, cancers, and sleep apnea syndrome [4, 5]. Recently, the effect of obesity on fertility has been extensively investigated. However, current studies have principally focused on the effects of obesity on the reproductive function of females or female animals, while males or male animals are poorly studied [6]. Additionally, while most reports have examined obesity in relation to reproductive function, the underlying mechanisms have not been elucidated [7, 8]. In addition, conclusions on the effects of obesity on semen parameters and reproductive hormones have differed [9] due to the many factors that may impact male fertility. For example, a meta-analysis performed by MacDonald et al. found no statistically significant association between BMI and semen parameters [10], while a study performed by Sermondade et al. found a significant J-shaped association between BMI and an abnormal sperm count [11]. This study aimed to investigate spermatogenesis in male rats with obesity induced by high-fat diet administration to minimize experimental bias and identify possible mechanisms.

Obesity is a chronic metabolic disease caused by interactions of various genetic and environmental factors. Interestingly, in the past 50 years, along with the annually increasing trend in human obesity, fertility has shown a parallel decreasing trend [18]. Several studies have indicated that a higher BMI is associated with significant decreases in sperm concentration [19‐21]. Investigators have attempted to investigate the relationship between obesity and fertility decline; however, no consensus has been reached.

Currently, no unified standard is available for evaluating obese rats. In 1929, MO Lee [22] proposed the Lee index to evaluate obesity in rats, which is the most commonly used method to evaluate obese rats. In a diet-induced model of obesity reported by Levin, rats became obesity-resistant with high-fat diet administration [23], and differences were found in their energy, endocrine, fat and glutamic acid (GLU) metabolism. Based on the relevant literature and the Lee index, one-fourth of the rats in the high-fat diet group were excluded in our study because they were obesity-resistant. The remaining three-quarters of the rats were classified as the obesity group [24]. Our results demonstrated successful generation of the obese rat model.

Currently, the effect of male obesity on sperm count, motility, and morphology in humans is controversial. A review found that up to 2015, progressively reduced motility was reported in 13 of 35 articles, while a decreased sperm count with normal morphology was reported in only 9 of 29 papers [6]. However, two recent meta-analyses including 14 and 21 studies demonstrated an increased risk of azoospermia or oligozoospermia in overweight or obese males [25, 26]. The results from this study indicated that obesity induced by a high-fat diet can change the histomorphology of seminiferous tubules, which may not have obvious effects on male fertility immediately, but the long-term effects on spermatogenesis induced by obesity may impair male fertility.

Due to gene polymorphisms in the population, many factors (e.g., smoking, alcohol consumption, and medication use) that can affect seminal parameters are often overlooked. In addition, sample selection bias can lead to inconsistent conclusions regarding the impact of obesity on sperm. This study, by controlling various confounding factors, revealed that obesity reduced sperm motility without changing the sperm count, which is consistent with previous reports [25]. The possible mechanism may be damage to the integrity of sperm cell membranes, sperm cell DNA, and sperm mitochondria induced by excessive reactive oxygen species (ROS) [27, 28].

Apoptosis is an autonomous programmed cell death process that is stimulated under specific conditions and is regulated by various genes. We found that the AI of the spermatogenic cells increased significantly in the obesity group. A recent study found that apoptosis of testicular spermatogenic cells is one of the major causes of male subfertility [29]. Cell apoptosis is predominantly regulated and controlled by the homeostasis of Bax and Bcl-2. When the Bcl-2/Bax ratio is disrupted, downstream caspase signaling pathways are activated, resulting in apoptosis. A high-fat diet has been reported to increase Bax and caspase-3 expression but reduce Bcl-2 expression in the testis [30]. Therefore, based on our results, obese rats exhibited increased spermatogenic cell apoptosis due to imbalances of Bcl-2/Bax. Furthermore, obesity resulted in lipid metabolic disorders and hyperlipidemia, which may increase the stress response of the endoplasmic reticulum. Therefore, the incidence of spermatogenic cell apoptosis is further increased [31, 32] via increased GRP78 mRNA and protein expression.

Mammalian reproductive function is predominantly controlled and regulated by the hypothalamus-pituitary-testis (HPT) axis. Male reproductive endocrinology is principally composed of three groups of hormones, including GnRH, GnIH, LH, FSH, and T. T is one of the major sex hormones in males and has an important role in the HPT axis. Hypothalamic hormones are intricately associated with obesity-induced physiological changes, indicating a mutual cause-effect relationship between obesity and gonadal hormone decline [33]. In a study with 3219 European males, Fui and other investigators found that the TT and FT levels in obese males were lower than those in normal-weight males. Another study with 314 Asian males reached the same conclusion [34]. Camacho found that obesity reduces testosterone, low testosterone levels can promote male obesity, and testosterone increases after weight loss [35]. Compared with rats fed a normal diet, rats fed a high-fat diet had lower testosterone levels. When these rats were fed a normal diet, their testosterone levels returned to normal [35]. Although biologically active serum FT only accounts for 2% of TT, it is directly involved in functional activities, such as the development of male secondary sex characteristics, maintenance of spermatogenesis and sexual desire [36]. As a diagnostic marker for male hypogonadism, the serum cFT level exhibits better sensitivity than the serum TT level [37].

SHBG is a blood transport protein for testosterone and estradiol. Its synthesis and secretion are regulated by androgen and estrogen. Serum SHBG may exert direct or indirect effects on androgen conversion and metabolism, and it regulates GLU homeostasis and fatty acid metabolism. In this experiment, our results confirmed decreases in serum TT and cFT levels and an increase in the serum SHBG concentration in the obese rats. Therefore, we hypothesize that serum SHBG is the key factor in reducing serum cFT levels, and obese rats may develop mild primary hypogonadism (reduced serum TT and cFT, increased SHBG). Primary hypogonadism in the obese rats is likely the initiating factor for alterations in the HPT axis.

Oxidative stress is highly correlated with a wide variety of inflammatory and metabolic disease states, including obesity. Oxidative stress is highly correlated with cumulative damage in the body induced by free radicals that are inadequately neutralized by antioxidants, and oxidative damage is aggravated by decreases in antioxidant enzyme activities, such as those of SOD, catalase (CAT), and glutathione S-transferase (GST) [38]. Evidence suggests that there are many sources of oxidative stress in obesity [39]. In our study, to determine the effect of obesity on oxidative stress, the concentrations of SOD and MDA in testis homogenate were determined.

SOD, which protects cells from free radical damage by ROS, is an important enzyme that protects against injuries caused by internal and external superoxide ions. MDA is an aldehyde generated in the process of lipid peroxidation caused by free radicals. MDA indicates cell membrane damage and reflects the severity of an oxygen radical attack on reactive cells and the levels of free radical metabolism in vivo. Decreased SOD and increased MDA can trigger oxidative stress, causing cell damage and even death. This study found decreased SOD levels and increased MDA levels in the testicular tissues of obese rats, demonstrating that the oxidative stress level of testis tissues in obese rats is increased and may impact sperm motility. Several possible explanations may account for these findings: obesity is associated with elevated serum free fatty acids, and unsaturated fatty acids are susceptible to attacks by ROS, producing peroxidation and subsequently resulting in decreased SOD levels and MDA accumulation, which is ultimately reflected by an increased oxidative stress level [40].

The rats with obesity induced by high-fat diet administration exhibited lipid metabolism dysfunction and altered reproductive hormone levels as well as increased oxidative stress levels in testis tissues, leading to mild primary hypogonadism. Meanwhile, the normal function of the HPT axis is maintained in the short-term through a corresponding feedback mechanism. However, the long-term effects of obesity may cause a decline in male fertility.