Fat men are less masculine. The extra fat they carry feminises their hormone balance and reduces their testosterone levels. But the reverse is also true: if fat men lose weight, their testosterone levels rise.
Fat cells synthesise the enzyme aromatase, and aromatase converts testosterone, the male sex hormone, into estradiol, the female sex hormone. The more fat men have, the less testosterone and the more estradiol they have circulating in their body.
Because estradiol tells the brain that there are lots of steroid hormones circulating in the blood, the brain reacts to the high estradiol level by lowering the amount of LH and FSH it secretes. LH and FSH stimulate the production of testosterone and sperm in the testes. If there are lower levels of these two hormones, the testosterone level declines further and sperm quality decreases too. Some studies suggest there is a link between overweight and reduced fertility and sexual problems.
The solution? Lose weight, of course. But to be really convinced you want to see a study which shows that fat men become manlier after losing weight. So Danish endocrinologists at Aarhus University did an experiment with 43 men aged between 20 and 59, with a BMI higher than 33.
The men followed a 14-week diet and exercise programme to lose weight. Before starting the programme, the researchers measured the men's hormone levels and sperm quality. The table below shows that the higher the BMI, the worse the values were.
The researchers repeated the measurements for 27 of the subjects after they’d completed the programme. In the table above we've shaded the statistically significant effects, which show that weight loss made the men more fertile and raised their testosterone level.
"To conclude on this pilot cohort study, we observed that the altered androgen profile tended to improve following weight loss and that weight loss may potentially lead to improvement in semen quality", the researchers write.
Reprod Health. 2011 Aug 17;8:24.