Why a dad's smoking habits could stop him becoming a grandfather

Put down the cigarette - sons of smokers have poorer sperm quality.
Put down the cigarette - sons of smokers have poorer sperm quality. Photo: SHUTTERSTOCK

Men, if you're planning to have children and you're struggling to quit smoking, here's another good reason to kick the habit in 2019. The cigarettes you're smoking today may have an impact on your kids' ability to have their own children one day.

That's according to a new study, which found men whose fathers smoked had about 50 per cent lower total sperm count than men of non-smoking dads.

"I was very surprised that, regardless of the mother's level of exposure to nicotine, the sperm count of the men whose fathers smoked was so much lower," says lead author Jonatan Axelsson of the findings, which have been published in the journal PLOS ONE.

While maternal smoking has long been associated with lower sperm counts in sons, the link between paternal smoking and sperm quality has been less clear. To examine this, Dr Axelsson and his colleagues at Lund University in Sweden, studied 104 men aged between 17 and 20.

Men were questioned about their father's smoking habits and provided sperm samples. The research team also had access to information about the amount of nicotine their mothers were exposed to by examining levels of nicotine in blood samples obtained from rubella screenings during their pregnancy.

"Those levels are closely correlated with the number of cigarettes smoked and objectively reflect the body's internal dose of exposure," the authors explain. This was important as it meant the researchers could account for how much nicotine the mother was exposed to, when determining the impact of dads' smoking on their sons' sperm counts.

And the results were clear.

When maternal smoking and socioeconomic status (which has also been linked to lower sperm count) were taken into account, men of smoking fathers had 41 per cent lower sperm concentration and 51 per cent lower total sperm count.

There are some caveats, however.

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"The association we found does not prove a direct effect of paternal smoking since we cannot rule out other yet unidentified factors associated with both paternal smoking and lower sperm counts," the authors write. "Nonetheless, there seems to exist a clear evidence that smoking does induce sperm DNA damage and possibly also mutations in sperm."

In addition, data on dads' smoking habits were based only on questionnaires answered by the sons. The team had no information about how long fathers' smoked for and how many cigarettes they smoked per day. And, the relatively small number of men studied means caution is required when interpreting the findings.

But while further research is needed, the authors believe the findings have important implications. "In view of the other studies reporting associations between paternal smoking and various negative outcomes in the offspring, these different findings taken together seem to suggest that paternal smoking poses an underestimated environmental and life-style related hazard to the offspring."

According to Dr Axelsson, while the precise mechanism remains unclear, there are some theories. "Unlike the maternal ovum, the father's gametes divide continuously throughout life and mutations often occur at the precise moment of cell division," he says. "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."

As such, these men may experience fertility problems in years to come. 

"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," says 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. Future research could perhaps move us closer to a causal link."

Current guidelines from the College of Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG) also highlight that paternal tobacco smoking pre-conception has been associated with sperm DNA damage and increased risk of malignancy in their children. 

"Counselling and pharmacotherapy should be considered for either or both parents when relevant," RANZCOG advises.

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