Hypogonadotropic hypogonadism and metabolic syndrome: insights from the high-fat diet experimental rabbit animal model.

The etiology of metabolic syndrome (MetS) is complex and involves the interplay between environmental, lifestyle and genetic determinants. MetS in men can be associated with a biochemical pattern of partial hypogonadotropic hypogonadism (HH). A similar pattern has been noted in both men and women with a variety of acute illnesses and chronic diseases, and there is ongoing debate regarding whether this phenomenon might adaptive (e.g. diverting resources from reproduction into survival), or maladaptive (e.g. anemia, sarcopenia, osteopenia and fatigue of androgen-deficiency amplify and widen the adverse consequences of the original disease-trigger). In women with hypothalamic amenorrhea (HA-HH secondary to chronic bioenergetic deficit from dietary restriction and/or intensive exercise), a genetic link to congenital HH (CHH) was recently established; women carrying monoallelic CHH gene mutations will typically not develop CHH, but are significantly more susceptible to HA. However, the male reproductive axis seems to be more resistant to similar environmental insults. In contrast, MetS-associated HH (mHH) is specifically a male phenomenon; the reproductive phenotype of females with MetS tending instead towards hyperandrogenism, rather than hypogonadism. The underlying pathogenic mechanisms responsible for mHH have not been clearly identified and, as yet, there has been no investigation of a potential role for CHH mutation carriage in its etiology. Over the decades, the use of either genetic- or diet-induced obesity and/or MetS animal models has greatly helped to illuminate the complex etiology of metabolic dysregulation, but the strong relationship between obesity/MetS and mHH in males has been largely neglected, with little or no information about the regulation of reproductive function by metabolic factors under conditions of bioenergetic excess. However, the pathogenic link between MetS and HH in males has been recently investigated in an animal model of high fat diet (HFD)-induced MetS, which perfectly recapitulates the human phenotype. Interesting insights derived by these studies have added novel information about the causative role played by hypothalamic alterations driven by metabolic disturbances in mHH. In particular, it appears that HFD-induced inflammatory injury at the hypothalamic level negatively affects GnRH neuron content, with the reduction of circulating gonadotropins and sex hormones being related to MetS severity.