Reduced libido and erectile dysfunction
Although hormonal dysregulation may affect semen quality, as described below, the primary reasons for the reduced fertility rate observed in men with obesity may be decreased sexual desire and impaired sexual function. Potential contributing factors include androgen deficiency, organic causes of erectile dysfunction (ED) and psychosocial factors.
Men with obesity report poorer sexual desire and satisfaction compared with control subjects, which may contribute to avoidance of sexual encounters.3 Overweight individuals are more likely to have chronic medical comorbidities and to be of lower socioeconomic class with resulting financial or domestic problems that can reduce libido. Depressive illness is also more common, and correlates with both low libido and ED. Low libido ultimately results in reduced coital frequency, lowering fertility rates.
In addition, sexual dysfunction is an important contributor to subfertility, with the presence of ED being correlated with lower fertility rates in men. ED is more prevalent in men with higher BMI. Its aetiology in this setting is multifactorial, including androgen deficiency and vasculogenic dysfunction.
Medical comorbidities such as the metabolic syndrome are significantly associated with ED. Both diabetes and hypertension are associated with vascular dysfunction, and diabetes may also lead to neuropathic complications. Furthermore, obstructive sleep apnoea (OSA) is more common in men with obesity and is strongly independently linked with both ED and androgen deficiency.
Reduced sperm quality
Effect of BMI on semen parameters
The relationship between BMI and subfertility is not completely explained by obesity-associated sexual dysfunction and reduced frequency of intercourse. Changes with increased BMI, including hormonal imbalance and increased testicular temperature, may impair semen parameters.
A recent systematic review and meta-analysis demonstrated an association between increased BMI and modest decreases in sperm motility, morphology and concentration,4 whereas earlier reviews have failed to establish this correlation.5 The association between BMI and sperm DNA fragmentation has also been investigated. DNA fragmentation may correlate with poorer sperm quality and integrity, and consequently be associated with reduced fertility and an increased incidence of subsequent miscarriage. Although some studies have demonstrated increased fragmentation in men with severe obesity,6,7 other studies have not reliably reproduced these findings.6-8 Additionally, mouse models suggest that paternal obesity may have metabolic sequelae and lead to subfertility and other metabolic effects in offspring through epigenetic effects.9
Effect of BMI on assisted reproductive technology outcomes
With the increasing use of assisted reproductive technology, the impact of paternal BMI on their outcomes is pertinent. Increased paternal BMI has been associated with reduced chemical and clinical pregnancy rates, as well as lower live-birth rates.8,10,11 This may be a result of sperm dysfunction that is not detectable by standard analysis. In addition to effects on implantation and pregnancy rates, there have also been suggestions that paternal obesity may have an impact on blastocyst development, possibly as a consequence of sperm DNA damage.