Obesity in men has been associated with reduced fertility. The impact of paternal body mass index on fertility is multifactorial, with increased weight associated with endocrine dysregulation and impaired sexual desire and function. Weight loss may improve fertility outcomes as well as conferring longer-term health benefits.
Obesity is a growing health issue that causes substantial morbidity and mortality. Epidemiological data suggest an association between increasing body mass index (BMI) and reduced fertility in men, with one study suggesting that a gain of 10 kg decreases male fertility by approximately 10%.1 The effect of BMI on male fertility is multifactorial, with increased weight associated with changes in sex hormone profile and impaired sexual desire and function. Some studies also suggest poorer semen parameters in overweight men.
Despite the prevalence and costs of male subfertility, there are few data regarding the fertility benefits of weight loss in men. Although weight reduction may normalise the hormone profile, the extent that this improves fertility and the time course of any improvement are unclear.
Factors contributing to subfertility in men with obesity
Increased BMI is strongly associated with both hormonal dysregulation and poorer sexual function in men, which can impair fertility, as outlined in the Figure.
Excess body weight has numerous effects on the reproductive hormone profile in men, leading to:
- a relative excess of oestrogen
- resultant suppression of hypothalamic secretion of gonadotrophin-releasing hormone (GnRH)
- reduced serum levels of gonadotrophins (luteinising hormone and follicle stimulating hormone)
- reduced serum testosterone levels.
Furthermore, there may be a feedback effect as there is a well-recognised association between hypogonadism and the metabolic syndrome, which itself is associated with increased weight.
Effects on testosterone
BMI is a strong predictor of low serum total testosterone concentrations in men, with a clear inverse correlation shown in many studies. BMI is also inversely proportional to free testosterone concentrations, suggesting an association even after accounting for lower sex hormone binding globulin levels due to insulin resistance.2
Effects on other reproductive hormones
Low serum testosterone in overweight men is associated with inappropriately low-to-normal levels of gonadotrophins. This reversible hypogonadotrophic hypogonadism includes hyperoestrogenism as a significant component. Androgens are converted to oestrone and oestradiol by the aromatase enzyme in adipose tissue. In overweight men, the increased adipose tissue mass and upregulation of aromatase activity lead to excess peripheral aromatisation of androgens. The resulting hyperoestrogenism exerts a negative feedback effect on hypothalamic GnRH pulses, reducing stimulation of gonadotrophin secretion. Circulating oestrogens may also have a direct adverse effect on testicular function. In addition, circulating endogenous opioids are likely to have a pathophysiological role in the development of hypogonadotrophic hypogonadism in overweight men.