Both obesity and male factor infertility have coincidently been increasing globally over the last few decades . Studies reporting on any relationship between obesity and male fertility have been generally inconsistent and inconclusive [5, 12, 14, 15]. Obesity is associated with various metabolic changes, including glucose intolerance, insulin resistance (hyperinsulinaemia), hyperleptinaemia, chronic inflammation and, in males, hypogonadism [3–5]. Insulin and leptin are present in seminal fluid, modulating sperm function post ejaculation in addition to regulating male reproductive pathways centrally and peripherally [18–21]. Changes in serum concentrations of glucose, insulin and leptin may be associated with changes in seminal concentrations, previously unreported in the literature. Therefore, in addition to a potential negative influence of sperm function in obese males, seminal fluid quality may also be negatively influenced. This pilot study aimed to investigate seminal fluid concentrations of glucose, insulin and leptin in relation to serum concentrations and sperm function in 19 non-obese (nOb) and 23 obese (Ob) males.
Obesity and semen parameters
The results generally agree with literature demonstrating that obesity has a negative impact on standard semen parameters, although other studies indicate no relationship [5;12;14;15]. This is particularly evident by decreased sperm concentration and vitality, with BMI correlating negatively with sperm concentration total sperm count (TSC) and vitality. Furthermore, WC and HC correlated negatively with sperm concentration and vitality, but not TSC. In addition, there were lower mean values for TSC, total and progressive motility and morphology in the Ob group, although these did not reach statistical significance. WC and WHR correlated negatively with total motility, however, there was no correlation between morphology and BMI, nor any other sperm parameter, in this study. Details of previous studies are available in the referenced reviews and meta-analyses [5, 12, 14, 15].
Based on the WHO (2010) criteria for semen analysis , both groups had mean results above the recommended cut-off values for ejaculation volume (<1.5 ml), sperm concentration (<15 × 106 million/ml) and TSC (<39 × 106 million/ml). However, only the Ob group had mean percentages below the recommended cut-off values for total and progressive motility (<40% and < 32%, respectively) and vitality (<58%). Although the Ob group have higher percentages of the cohort presenting with oligozoospermia, asthenozoospermia and/or necrozoospermia, this difference was not significant (which may be due to the small sample size). Teratozoospermia (<4% normal morphology) was diagnosed in 78.6% of the cohort, and 68.4% of the nOB cohort and 86.9% of the Ob cohort. This high percentage in both groups may explain the lack of correlation between morphology and all other parameters in this study. Interestingly, 21.4% of the cohort had a history of recent couple infertility as defined by WHO (2010) . However, it is important to note that no males or female partners had received any medical investigation for this complaint at the time of the clinical consultation. Although not statistically significant, a higher proportion of the Ob cohort (30.4%) reported this history, in contrast to the nOb group (10.6%). It must be clear, however, that this is defined as couple infertility and no further data on male or female factor infertility is available, and this should not be interpreted as male factor infertility percentages which were not established in the study. Furthermore, no males had consulted a medical or health care professional for infertility related complaints at the time of consultation.
Fewer studies have assessed the impact of obesity on DF and MMP. Both of these parameters were significantly increased in the Ob group compared to the nOb group. Furthermore, MMP and DF both correlated with BMI, WC, HC and WHR. The negative impact of BMI on DF and MMP confirms similar findings in previous studies [5, 14, 29]. A negative impact on MMP and DF in obesity appears to be a consistent variable in the literature when included for analysis. Damage to the sperm mitochondria function is suggested to negatively affect oxidative phosphorylation, reducing ATP synthesis and thus energy availability for motility . MMP is negatively correlated with vitality and total and progressive motility in this study, supporting this well defined relationship. Although it can be hypothesised that obesity related phenomena, such as inflammation, may mediate damage to spermatozoa mitochondria and DNA integrity, the mechanisms of these relationships require further investigation. Serum and seminal insulin and serum glucose (but not seminal glucose) correlated with MMP and not DF, with a negative correlation between QUICKI and MMP.
Serum and seminal insulin and leptin
The concept of insulin resistance, closely associated with abdominal obesity and increased WC, is used to describe the process whereby target tissues develop impaired sensitivity to the action of the hormone, particularly in adipose tissue, liver and skeletal muscle [30–32]. The QUICKI is a useful assessment of insulin sensitivity, correlating closely with Euglycaemic Hyperinsulinaemic Clamp (a gold standard assessment of insulin resistance) across a wide range of glucose and insulin concentrations . The predominantly adipocyte-derived polypeptide hormone Leptin regulates body weight, appetite and energy expenditure via hypothalamic modulation, in addition to modulation of the immune, endocrine, metabolic and reproductive systems [32, 33]. Leptin has been strongly associated with a role in the pathophysiology of obesity and metabolic syndrome, although this role has not been well understood or described . Mean serum insulin and leptin was significantly increased in the Ob group as compared to the nOb group, with a significantly decreased QUICKI. As expected, serum insulin correlated positively with BMI, WC, HC and WHR. QUICKI correlated negatively with these same parameters. However, serum leptin did not correlate with these parameters as expected, possibly due to the relatively small sample size.
Various studies have assessed leptin concentrations in male serum, although an adequate reference range appears elusive. Raised serum leptin is also associated with sperm function changes in males [35, 36]. The serum ranges of leptin in this study generally agree with concentrations found in healthy, obese and infertile male cohorts reported in the literature [35–38].
Insulin and leptin are important regulators of male reproduction via the HPT axis both centrally and peripherally , in addition to being present in human semen with important regulatory roles for sperm function and fertilisation [17–20]. Furthermore, both hormones are synthesised and secreted by ejaculated spermatozoa in an autocrine manner [18–20]. Exogenous addition of insulin and leptin to ejaculated semen has been shown to increase total motility, progressive motility, acrosome reaction and nitric oxide production in human spermatozoa . Sertoli cells too have been shown to synthesise and secrete insulin .
The results show that obese males have significantly increased seminal insulin and leptin as compared to the nOb group. Strong correlations between serum and seminal insulin (r2 = 0.823), as well as serum and seminal leptin (r2 = 0.517), are reported. The negative correlations between both seminal insulin and leptin with BMI, WC, HC and WHR may indicate that these parameters are potential predictors of seminal changes of these hormones. Seminal insulin and leptin further correlated negatively with QUICKI (r2 = -0.782 and r2 = -0.311 respectively). These correlations indicate an important relationship between pathophysiological mediators of obesity and a decrease in male reproductive potential that requires further investigation.
The source of seminal insulin and leptin is not clear. The correlations between serum and semen concentrations may suggest that insulin and leptin in the semen gains access to the reproductive tract via the blood testes barrier (BTB), seminal vesicles or prostate. Since insulin and leptin are strongly associated with increases in obesity, it is plausible that there is a cross over from peripheral circulation to the reproductive tract. Insulin and insulin-like peptides in human semen have previously been suggested to be secreted by the seminal vesicles [40, 41], and insulin appears to freely cross the BTB into the reproductive tract . No apparent data on the source of leptin is available in the literature. In contrast, as insulin and leptin are synthesised and secreted in an autocrine fashion post-ejaculation, and Sertoli cells too secrete insulin within the testes , at least a local reproductive tract source of these hormones is also plausible. The source of these hormones in the reproductive tract requires further investigation.
Seminal and serum insulin was negatively correlated, and QUICKI positively correlated, with sperm concentration. The mechanisms for this are unclear. Type-1 diabetes mellitus, in which insulin is absent, is associated with a collapse of spermatogenesis and increased germ cell apoptosis . Insulin resistance may mimic to some degree the loss of insulin in T1DM, and insulin resistance in Sertoli cells may hypothetically be associated with a decrease in spermatogenesis. As increased seminal insulin is associated with insulin resistance and abdominal obesity, increased insulin exposure during spermatogenesis may potentially develop insulin resistance in the Sertoli cells.
Although an acute in vivo increase in insulin and leptin exposure may increase motility and acrosome reaction in the spermatozoa [20, 21], this study did not show significant correlations between seminal insulin and leptin with sperm motility and vitality. Increased insulin exposure during spermatogenesis may plausibly develop insulin resistance within the spermatozoa themselves. Evidence to support this hypothesis may be found in the intracellular molecular cascades associated with insulin receptor stimulation in these cells. Insulin, as well as leptin, exert its effect on spermatozoa via the PI3K/Akt intracelleular signalling pathway, leading to protein kinase B (PKB) phosphorylation , which may mediate beneficial effects on ejaculated spermatozoa . This pathway ultimately increases cellular nitric oxide production [17;20]. In human tissues, this intracellular pathway is negatively influenced in insulin resistance . Hypothetically, over the spermatogenic cycle, it is conceivable that spermatozoa may develop insulin resistance in a manner similar to other tissue cell via a breakdown of the PI3K/Akt intracelleular signalling pathway. This hypothesis would provide an explanation as to the potential negative association between increased seminal insulin and reduced motility of ejaculated sperm. Although Lampiao & du Plessis  found an increase in motility of ejaculated spermatozoa exposed to leptin, this was not found by Li and colleagues . However, if this intracellular pathway does breakdown in spermatozoa, we would also expect to see a reduced or even negative correlation between seminal leptin and ejaculated sperm function. A model in which insulin and leptin resistance is induced in spermatozoa and Sertoli cells is required to further investigate this potential relationship.
Based on the seminal-to-serum insulin ratio, insulin was found to be highly concentrated in human semen in both groups. The concentration of insulin in human semen is somewhat supported by a limited number of studies across numerous groups, including fertile and infertile normoglycaemic subjects, carbohydrate intolerant subjects and excretory and secretary azoospermic subjects [42, 45, 46]. No plausible explanation for a physiological concentration of insulin in semen is apparent on a search of the literature. As a result of the insulin concentration, the seminal glucose-to-insulin ratio was lower than the serum glucose-to-insulin ratios. Both the serum and seminal glucose-to-insulin ratios where significantly lower in the Ob group due to a significant increase in serum and seminal insulin compared to a more subtle increase in serum glucose and a non-significant decrease in seminal glucose. This is likely due to the exclusion of participants with a high fasting glucose.
Serum and seminal glucose
Glucose has been identified in human semen, with more than half the sugar consumed by ejaculated spermatozoa being in the form of glucose glycolysis [47, 48]. Reports on normal concentrations vary widely, from 1.02 mmol/L – 5.7 mmol/L . Seminal glucose concentration ranges in this cohort (0.69 – 3.69 mmol/L) were within these reported ranges. Although there was a lower mean in the Ob group compared to the nOb group, this did not reach statistical significance. A small sample size may be the reason for this not reaching statistical significance. Sampling indicates possible significance with n = 44 in the nOb group and n = 54 in the Ob group.
Serum glucose correlated positively serum insulin and negatively with QUICKI as would be expected, and further correlated negatively with spermatozoa motility and positively with MMP, indicating an association negative relationship between serum glucose and energy production in spermatozoa. Seminal glucose correlated negatively to BMI only. There was no correlation between serum and seminal glucose either.
Seminal glucose concentrations were relatively lower compared to serum levels, as indicated in the seminal-to-serum glucose ratio. This may be due to a tight control mechanism for glucose to pass from the peripheral circulation into the reproductive tract through the BTB, reducing glucose concentrations in order to optimally support and maintain spermatogensis. Testicular cells have glucose sensing machinary which enable them to react and adapt to hormonal fluctuations and counteract hyper- or hypoglycaemic events, as spermatogenesis maintainance in vivo is dependent on adequate glucose metabolism . Glucose transport across the BTB is mediated by various glucose transport molecules (GLUT’s), such as GLUT1, GLUT3 and GLUT8, and are sensitive to various hormones (including insulin), inflammatory cytokines and growth factors . With changes in glucose or insulin, glucose transport machinary adapts in order to maintain lactate production . Insulin deprived Sertoli cells in culture show decreased glucose uptake via the BTB barrier . Therefore, it may be biologically pausible that insulin resistance in the setting of obesity may be associated with a decrease in glucose uptake across the BTB. This is however hypothetical, and further research in the physiology and pathophysiology of the BTB in relation to glucose is required. However, there was a non-significant trend for seminal glucose to be decreased in the Ob group, whereas serum glucose was significantly increased, which may warrent further insight.