Philosophiae Doctor - PhD / The metabolic syndrome (MetS) is a collection of various metabolic, hormonal and immunological risk factors that cluster together, closely related to poorly understood phenomena such a hyperinsulinaemia (insulin resistance), hyperleptinaemia (leptin resistance), a low grade, systemic and chronic inflammation and, in males, hypogonadism. Infertility is increasing globally, and male factor infertility accounts for a large percentage of couples who are not able to conceive. The relationship between components of MetS and male reproductive health is not clear, and requires further investigation, as does the impact of MetS on male reproductive health in a case controlled study. The impact of hyperinsulinaemia, hyperleptinaemia and inflammatory cytokines on the male reproductive tract also requires investigation. Furthermore, it is hypothesised that these phenomena negatively impact steroidogenesis cascades. In order to investigate this, a case controlled study and TM3 Leydig cell culture experiments were designed.Participants were recruited from public advertisements, and screened for strict exclusion criteria, including acute or chronic inflammation, hormonal treatments, vasectomy and leukocytospermia (> 106/ml). Following clinical diagnostics, 78 males were either placed into a control group (CG) or the MetS group, with numerous parameters compared between them. Serum was assayed for routine risk markers including HDL cholesterol, triglycerides, glucose and C-reactive protein (CRP). Saliva was assayed for free testosterone and progesterone. Semen samples underwent semen analysis for ejaculation volume, sperm concentration and motility, vitality, morphology and leukocyte concentration, in addition to mitochondrial membrane potential (MMP) and DNA fragmentation (DF). Both serum and seminal fluid were further assayed for insulin, leptin, tumour necrosis factor-alpha (TNF ) and interleukins 1-beta (IL1 ), 6 (IL6) and 8 (IL8). Glucose was also assayed in seminal fluid. Separately, hCG stimulated TM3 Leydig cells were exposed to varying concentrations of insulin (0.01, 0.1, 1 & 10 pg/ml), TNF , IL1 , IL6 and IL8 (0.1, 1, 10 & 100 pg/ml) for 48 hours at optimal cell culture conditions. TM3 cell viability, protein concentration and testosterone and progesterone concentrations were assessed.XXII Results indicated that males in the MetS group (n=34) had significantly increased body mass index, waist circumference, blood pressure, triglycerides, glucose, and Creactive protein (CRP) with decreased HDL cholesterol, as compared to the CG. Furthermore, ejaculation volume, sperm concentration, total sperm count, progressive and total motility were significantly decreased in the MetS group, and sperm with abnormal MMP and DF were increased in this group. No difference was found for morphology. Serum and seminal insulin, leptin, TNF , IL1 , IL6 and IL8 were all significantly increased in the MetS group. Both testosterone and progesterone were also significantly decreased in the MetS group. Insulin increased testosterone and decreased progesterone in the TM3 cells. TNF , IL1 and IL6 all decreased testosterone and progesterone concentrations and TM3 cell viability. IL8 increased TM3 cell viability and decreased progesterone, will no effect on testosterone. These results suggest MetS is associated with decreased fertility potential in males. Furthermore, a significant increase in seminal insulin, leptin, TNF , IL1 , IL6 and IL8 suggests local reproductive tract inflammation in the absence of leukocytospermia. Strong correlations between serum and seminal insulin, leptin, TNF , IL1 , IL6 and IL8, as well as serum CRP, imply that these systemic phenomenons are related to the reproductive tract changes observed. Therefore, the underlying pathophysiology of MetS negatively affects male reproduction, in addition to general health and wellbeing. A decrease in progesterone and testosterone suggests a collapse in steroidogenesis cascades. Additionally, inflammation, increased leptin and insulin resistance likely contribute to this collapse in steroidogenesis based on TM3 cell culture experiments. These results provide novel avenues for further investigations.