Researchers found that sperm from normal weight and obese men differed in how some genes were turned "on" or "off". And the differences were seen in gene regions linked to brain development and appetite control.
The Danish scientists said the findings may offer one biological explanation for why heavy dads often have heavy kids.
Active or dormant genes
"At a basic level, most people know that overweight parents often have overweight kids," said Anthony Comuzzie, a spokesman for the Obesity Society and a researcher at the Texas Biomedical Research Institute, in San Antonio.
"That's not all genetics, of course, but a good part of it is," said Comuzzie, who was not involved in the new study.
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However, he added, it may be more complicated than the set of genes you inherit. The current study tackled "epigenetics", chemical mechanisms that determine whether a gene is active or dormant at certain times.
Research has shown that epigenetic patterns can be altered through behaviour, including diet and exercise. And like genes, epigenetic patterns can be inherited.
Scientists are just beginning to understand how it works, but research in insects and rodents shows that parents' epigenetic "marks" can affect the health of their offspring.
It all suggests that we inherit more than a "random mix of genes from our ancestors", said lead researcher Romain Barres, an associate professor at the University of Copenhagen, in Denmark.
Instead, he said, people may also receive "a biological imprint of their parent's behaviour and lifestyle".
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For the study, published online in the journal Cell Metabolism, Barres and colleagues analysed sperm from 10 men who were obese and 13 men with a normal weight. The investigators found that the two groups showed different DNA methylation patterns in their sperm including in gene regions linked to appetite control.
Comuzzie explained it this way: "You can have two genes that are structurally identical. But they will not function the same way if they have different DNA methylation patterns."
To see whether obesity, per se, could be the reason for the differences, the researchers next studied a group of severely obese men undergoing weight-loss surgery. They found that the drop in weight post-surgery caused a "dramatic" change in DNA methylation in the men's sperm.
"That finding is really intriguing," Comuzzie said.
But a big unknown, he added, is what the epigenetic patterns in sperm ultimately mean for a man's children.
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"To what extent do these patterns get passed?" Comuzzie said. "Does the pattern get 'reset' in the fertilised egg? We don't know."
"We have not investigated the offspring of the men we studied," Barres said.
But his team is now working with a fertility clinic to study epigenetic differences in unused embryos, created with sperm from men of various weights. (In Denmark, fertility clinics must discard unused embryos after five years, and they can be used for research.)
For now, Comuzzie said, the findings suggest that it's not only women who need to be conscious of their lifestyle and health going into pregnancy.
"Dad does matter, too," he said.
Barres agreed. "Our results might help change pre-pregnancy behaviour for parents-to-be," he said.
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Image: Sperm from iStock