Listen up, dads: Obesity makes your sperm weird

The new research, published Thursday in Cell Metabolism, had a small sample size of just 10 lean men and 13 obese men - so we need to take these findings with a grain of salt. But the researchers found that the sperm cells of lean and obese men had different epigenetic marks - especially in gene regions associated with controlling appetite.

"The code itself didn't change," Romain Barrès, an associate professor at the University of Copenhagen and lead author of the study, told The Washington Post. The genes themselves were still the same. But the epigenetic marks that control how a gene is expressed had changed. Similar changes related to weight loss have been seen in studies done on mice. And in an observational study published in 2005, researchers found that the famished lifestyles of Swedish grandparents could be correlated to their grandchildren's tendency for cardiometabolic disease.

"Because this happened in such a short time, it couldn't be caused by a mutation," Barrès, who was inspired by the Swedish study, explained. "But it was two generations after the famine, so it was something that persisted."

That Swedish study didn't go so far as to look for the epigenetic markers that might be responsible for the changes. Barrès' study is, in some ways, reversed: He can't prove that children born from these obese men actually eat more or have a tendency to be obese. He hasn't shown the effect, but he's found a cellular mechanism that could be a smoking gun.

The new research also offers a potential ray of hope for heavier dads: In a small follow-up of six obese men undergoing weight-loss surgery, the researchers found that 5,000 or so of 9,000 markers associated with obesity went away within a year.

"The environment itself - we don't know if its the surgery, the inflammation associated with the surgery, the change of stomach size, the weight loss - something is changing the epigenetic content of the sperm cell," Barrès said.

While further study is needed, the implications are clear.

"The environment can change what your sperm cell carries, and it's not random," Barrès said. "The implication of this is that these changes will be transmitted to the egg, and maybe change how the embryo will develop, and maybe change how the child will develop. We don't know this is so. We haven't looked."

Adelheid Soubry, a University of Leuven researcher who's worked on similar research but wasn't involved in the new study, pointed out that a larger study group would be needed to confirm the data. She also emphasized that the potential implications of the epigenetic markers had yet to actually be demonstrated.

The researchers hope to follow up with a more extensive study - one that allows them to follow the process from man to sperm to embryo to child. In Denmark, embryos created for in-vitro fertilization must be used for research after five years, by law. So Barrès and his colleagues can study discarded embryos, the sperm of the men who fathered them, and the cord blood of siblings who were brought to term. That's obviously quite the undertaking, so don't expect a definitive answer on the sperm epigenetics of obesity any time soon.

But even if their work doesn't show a causal link between paternal obesity and overeating, it's becoming increasingly clear that dads have as much to get anxious over as moms when it comes time to conceive. Studies have shown that older men may put their children at higher risk of certain diseases, for example. Now it seems that environmental factors might also come into play.