Previous studies had shown that smoking causes DNA damage to sperm and mitochondria – the energy-producing centres in cells – but this detailed analysis shows how that may occur.
Dr Ricardo Pimenta Bertolla of Sao Paulo Federal University, Brazil, lead author on the study, said: 'More and more studies are demonstrating a harmful effect of smoking on male fertility. Our results point in the direction of important semen alterations: semen of smokers presents [with] an inflammatory nature, associated with decreased capacity of sperm to achieve fertilisation and generate a healthy pregnancy.'
The researchers carried out functional and protein analysis on sperm from 20 smokers, who smoked at least 10 cigarettes per day, and from 20 non-smokers. They compared 422 different proteins between the two groups, and found that the proteins in the smokers' semen indicated an inflammatory state.
Sperm from the smokers had more DNA fragmentation and fewer active mitochondria. The cap structure on sperm called the acrosome, which helps the sperm penetrate an egg, was more often damaged in smokers' sperm.
The authors suggest that substances such as nicotine and cadmium in cigarette smoke cause oxidative stress, which triggers an inflammatory response, leading to the observed sperm damage.
In the light of this cumulative evidence of harm, Dr Bertolla recommends that men pursuing fatherhood quit smoking: 'The whole process of producing a mature sperm takes around three months, so if a man wishes to quit smoking before attempting fatherhood, I would recommend quitting three months ahead of time,' he told Reuters.