Fish oil supplements may increase testicle size and sperm count, boosting male fertility, according to findings from an observational study.

The Danish research of nearly 1700 male military conscripts, aged 18-19, found those who had taken the supplements for most of the previous three months had 1.5ml larger testicles and 0.6ml more semen volume than those who did not take them.

A lesser effect was found in men who had taken fish oil supplements in the past three months but on fewer than 60 days — suggesting a dose-response relationship, the researchers said.

Testicle size was measured by ultrasonography and participants provided a semen sample.