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ou can tell a lot about a man from his sperm cells — including his weight.

In a study published Thursday in the journal Cell Metabolism, researchers in Denmark showed that a man’s sperm cells look different if he’s obese. Those differences might be tiny changes in molecular architecture, but they can determine how genes are switched on or off, and could potentially affect the behavior of any offspring those sperm produce.

The findings suggest that fathers might want to start exercising more and eating better before they conceive, given that their ill health leaves its mark on their ejaculate.

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“If we can improve the health of the parents before conception, maybe we can reduce the risk of obesity in their children,” said Dr. Mary-Elizabeth Patti, an endocrinologist and obesity researcher at the Joslin Diabetes Center in Boston, who was not involved in the research.

According to Patti, the link between maternal weight and childhood obesity and diabetes was well established. But, “what’s not been as well appreciated is that the health of the father can also influence the developing baby,” she said.

The study looked for something called epigenetic markers. These are like Post-it notes stuck onto the DNA telling a cell whether or not it should express a specific gene, and they can be added or removed depending on what’s going on inside the cell or in the body as a whole.

A team led by Romain Barrès, a metabolic disease researcher at the University of Copenhagen, compared sperm from 10 obese men and 13 lean men. Using sequencing technology, they found that the epigenetic markers were most different in those parts of the genome associated with eating patterns.

Barrès and his colleagues also followed six obese men who were undergoing weight-loss surgery. By comparing sperm from before and after the surgery, they could see that the dramatic shift in a man’s weight also changed the epigenetic tags in his reproductive cells.

“Environmental factors or lifestyle have the potential to change the the heritable information we have in our sperm,” Barrès said. His study was funded by the Novo Nordisk Foundation, the philanthropic arm of the Danish pharmaceutical company that specializes in diabetes medicines.

The researchers didn’t look to see if the epigenetic differences in the sperm carried over into the next generation — that kind of experiment is nearly impossible to do outside of a tightly controlled lab. But these results could help explain why the chain of obesity is so hard to break. Currently, more than two-thirds of American adults are considered to be overweight or obese.

Previous studies have shown the amount of exercise a man does can influence epigenetic markers in human sperm. And research on mice has shown that eating and activity patterns can influence the weight of offspring. Barrès’s study adds to the idea that it isn’t just our genes and our parenting that determines the health of our children: it could be our lifestyle, too.

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