Men who smoked while their partners were pregnant have sons with lower sperm counts, according to a study in Sweden.
The detrimental impact of maternal smoking on sons' sperm counts has repeatedly been shown, but this is the first conclusive evidence that fathers smoking causes a lower sperm count in sons. The research, published in PLOS One, suggests that a father's smoking habit during pregnancy may in fact be more damaging to the son's sperm count than the mother's smoking.
'Regardless of the mother's level of exposure to nicotine, the sperm count of the men whose fathers smoked was so much lower,' said Dr Jonatan Axelsson of Lund University.
The research team studied 104 men aged 17-20 and their parents' smoking habits while they had been in utero. After adjusting for maternal tobacco exposure, men whose fathers had smoked had 41 percent lower sperm concentration and 51 percent lower total sperm count than those with non-smoking fathers. Even after adjusting for socioeconomic factors, the association remained.
Next, the researchers measured cotinine levels - used as a biomarker to show an individual's exposure to nicotine, in the men's blood. These levels allowed the researchers to determine whether the parents had been smokers themselves or whether they had been exposed to second-hand smoke, acting as more reliable evidence than the self-reports of parents' smoking behaviour used in previous research.
Other studies have shown strong correlations between paternal smoking and child health issues including malformations and a shorter reproductive lifespan in daughters.
'Unlike the maternal ovum, the father's gametes divide continuously throughout life and mutations often occur at the precise moment of cell division,' said Dr Axelsson. 'We know that tobacco smoke contains many substances that cause mutations so one can imagine that, at the time of conception, the gametes have undergone mutations and thereby pass on genes that result in reduced sperm quality in the male offspring.'
It is known that most of these mutations are inherited from the father and there are also multiple links between paternal age and complex diseases in the child (see BioNews 974). Furthermore, previous studies have shown that male smokers have more DNA damage in their sperm as smoking can cause breaks in the DNA strands. Children of smoking fathers have up to four times as many DNA mutations in certain repeated regions than those with non-smoking fathers, which has consequential effects on the child's health.
'We know there is a link between sperm count and chances of pregnancy, so that could affect the possibility for these men to have children in future,' said Dr Axelsson. 'The father's smoking is also linked to a shorter reproductive lifespan in daughters, so the notion that everything depends on whether the mother smokes or not doesn't seem convincing.'