**5.1 Causes of male infertility**

There are many internal and external causes that can lead to male infertility. They span from genetic mutations or variations, medical conditions to lifestyle choices. Genetic mutations may distort e.g., hormonal levels of Follicle Stimulating Hormone (FSH), Luteinizing Hormone (LH) or testosterone or their correspondent receptor sensitivities whereas genetic variations in the number of sex chromosomes can lead to clinical syndromes such as Turner (X0) or Klinefelter (XXY). Medical conditions cover a wide range and include distorted hormonal levels of the hypothalamic–pituitary-testicular (HPT) axis, immunological infertility (production of sperm antibodies), obstruction of the ductus deference, retrograde ejaculation (i.e. ejaculation into the bladder), erectile disfunction, varicoceles (swollen veins in the scrotum that block blood drainage), sperm disorders (mostly very low numbers or not made at all, odd shape, no straight motility, not fully mature thus unable to fertilize the female egg). External factors may involve exposure to toxicants (such as pesticides, heavy metals, PCB etc.) [42], heat [43], radiation [44] or xenoestrogens etc. [45]. Lifestyle choices potentially leading to male infertility may embrace excessive alcohol, illicit drugs or medication abuse. Furthermore, medical comorbidities such as cystic fibrosis, chronic obstructive pulmonary disease, and obesity can lead to male infertility [41].

The group of medical conditions including dyslipidemia, hypertension, insulin resistance, and obesity is often referred to as 'metabolic syndrome' (MetS) [46]. Each of these disorders can affect male fertility in its own way but when combined as in metabolic syndrome, additive effects have been observed [46]. Lotti et al. [47] noticed a decline in age-adjusted testosterone levels in male MetS patients without changes in LH and FSH levels. Furthermore, after adjusting for age and total testosterone, a negative correlation between the number of MetS components and normal spermatozoa morphology as well as erectile dysfunction was found [47]. In infertile men, a positive correlation between MetS and prostatic abnormalities has been shown [48]. Sertoli cells of the testis use glucose to produce lactate that is needed by the developing germ cells [49]. The transport of glucose from the blood capillaries through the basal compartment of the Sertoli cells and then through the blood-testis barrier towards the adluminal compartment of the seminiferous tubules is tightly controlled. Furthermore, this glucose transport is also regulated by the Hypothalamic–Pituitary-Gonadal (HPG) axis [50]. Hyperglycemia conditions in testicular cells such as in diabetic men interfere with these transport mechanisms and may thus compromise spermatogenesis. For males, obesity has a severe impact on the development and function of the testicles, epididymis, prostate, and other male reproductive organs [51]. With the increase of body mass index and abdominal circumference, ejaculation volume gradually reduces, and the total sperm count in semen also decreases [52]. In addition, obesity damages sperm chromatin or inhibits chromatin condensation [43]. In men, obesity may cause a gradual decline in sperm quality and thus reduce fertility [53]. Obesity also led to DNA fragmentation, increases apoptosis and epigenetic changes that can be transferred to the offspring [54].

### **5.2 Prevention and treatment**

A healthy diet and lifestyle combined with regular physical activity can help to avoid MetS and obesity and thus may prevent male infertility. Spermatozoa are very susceptible to oxidative stress. Because of their small amount of cytoplasm, spermatozoa have very limited capacity to defend reactive oxygen species. Moreover, the plasma membrane of spermatozoa contains a high amount of unsaturated fatty acids which makes them an easy target for ROS-induced damages such as lipid peroxidation and DNA damage. Both can lead to reduced semen quality and thus affect male fertility. Food that is rich in anti-oxidants, vitamins (A, E and C) and omega-3 fatty acids such as many plant products, fish and other seafood has been shown to be beneficial for male fertility [55, 56]. The high provitamin A content of RPO may

be beneficial for fertility because vitamin A plays a role in spermatogenesis [57]. RPO also contains vitamin D which is a known regulator of enzymes involved in the production of steroids (steroidogenesis) [58].
