Functional relationship between obesity and male reproduction: from humans to animal models

BACKGROUND: The increase in the incidence of obesity has a substantial societal health impact. Contrasting reports have been published on whether overweight and obesity affect male fertility. To clarify this, we have reviewed published data on the relation between overweight/obesity, semen parameters, endocrine status and human male fertility. Subsequently, we have used results obtained in animal models of obesity to explain the human data. METHODS: Pubmed, Scopus, Web of Science and Google Scholar databases were searched between September 2009 and October 2010 for a comprehensive publication record. Available studies on adult human males were examined. The included animal studies examined obesity and fertility, and focused on leptin, leptin receptor signaling, kisspeptins and/or NPY. RESULTS: Most overweight/obese men do not experience significant fertility problems, despite the presence of reduced testosterone alongside normal gonadotrophin levels. Only a subgroup of subjects suffers from hypogonadotropic hypogonadism. Animal models offer several explanations and show that reduced leptin signaling leads to reduced GnRH neuronal activity. This may be due to decreased hypothalamic Kissl expression, a potent regulator of GnRH/LH/FSH release. As the Kissl neurons express leptin receptors, the Kissl system may participate in transmitting metabolic information to the GnRH neurons, thus providing a bridge between metabolic regulation and fertility. CONCLUSIONS: Infertility in overweight/obese males may be explained by leptin insensitivity. This implies a possible role for the KISSl system in human obesity-related male infertility. If substantiated, it will pave the way for methods to restore fertility in these subjects.