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On this Father's Day, John will be thankful for what he has. He, his wife and six children will gather Sunday night for dinner and probably watch a movie or play board games.

But he will feel a twinge of sadness: One daughter passed away several years ago and John and his wife, who live in a small town in northern Utah and don't want to use their last name, have been trying unsuccessfully for years to have another.

Part of John's path to having another baby is unusual, but one that may become more common: He's trying to lose weight.

That's because research from the University of Utah and elsewhere is starting to show that as waistlines expand, men's ability to father children may decrease. Sperm count is dropping among U.S. men but not in countries where obesity is less prevalent.

"I'm trying to control my diet better. My wife tells me I drink too much Coke," said John, who is also walking more and eating fewer donuts to trim down his 250-pound-or-so frame. "I'd have 20 kids if it was up to me. . . . It's the best thing you can do with your life."

The link between obesity and reproduction problems among women has long been clear, but the more-fat-less-fertile theory for men is a relatively new field. A 2006 federal study found every extra 20 pounds may increase the chances of male infertility by 10 percent.

The U. published a study earlier this year that found obese men are three times more likely to have slower-moving sperm and a sperm count so low they are considered infertile.

A reversible problem? As men's girths grow - almost 31 percent of U.S. men are considered obese, by one estimate - so, too, do their ranks in the U.'s infertility clinic.

"We see more and more men presenting overweight, and they have unexplained low sperm count," said Ahmad Hammoud, an OB-GYN and fellow at the Utah Center for Reproductive Medicine.

As principal author of the U. sperm study, Hammoud said obesity may turn out to be one of the few causes of low sperm count that can be reversed. Infertility doctors cannot give a reason for low sperm count to half the men they see, he noted. Though clearly not every obese man has fertility problems, the clinic advises men with reproduction difficulties to lose weight.

John thought he could blame his problem on a ranching accident that happened almost two decades ago: A 500-pound cow kicked him in the groin, damaging his urethra. But he subsequently fathered two children, adding to the five his wife had from previous relationships.

When he and his wife struggled to have more, John had surgery to fix his urethra. He wore baggy pants to improve his sperm count. The couple unsuccessfully tried in vitro fertilization, in which his sperm and her egg were fertilized in the U. lab and implanted in her uterus.

'What we live for': The 39-year-old also enrolled in a U. study designed to address one of the ways researchers think obesity affects reproduction. Fatty tissue contains an enzyme that converts the "male hormone" testosterone to the "female hormone" estrogen, Hammoud explained. The U. wants to see whether the drug anastrozole - which is FDA-approved to treat breast cancer - will block production of the enzyme, decreasing estrogen, boosting testosterone and improving sperm count.

Hammoud said he also hopes to get approval to study whether the stomach-reducing gastric bypass surgery will improve sperm count. The University of Pennsylvania is conducting a similar study.

The ultimate goal is to help couples conceive naturally. Artificial reproductive technologies are expensive - $12,000 for in vitro - and are viewed by some patients as a shortcut to parenthood that doesn't fix the underlying problem, Hammoud said.

The few men who have enrolled in the Utah drug study "feel relief," the doctor added. "Many men want to feel that there's a cause for their low sperm count and we can reverse that and bring them back to a healthy state."

So far, the drug hasn't worked for John. Because he may have received a placebo, he said he will seek the drug after the study is over if it proves effective. With two years left to have a baby - that's when his wife turns 40 - John feels the pressure.

"We love kids. It's what we live for."

* Infertility is defined as inability to conceive after 12 months of unprotected sex.

* Male factors alone account for up to 30 percent of all cases of infertility in couples and contribute to another 30 percent in combination with female factors.

* 67 percent of Utah men are overweight or obese, compared with 49 percent of women.

* The percentage of obese Utah men has jumped from 16 percent in 1997 to 24 percent in 2007.

* To see if you can participate in the U.'s drug study of how to boost sperm count in obese men, call Jennilee Brown at 801-581-7330 or e-mail her at