updated 10/21/2004 5:45:05 PM ET 2004-10-21T21:45:05

Beer bellies may take a toll on men below the belt, not just around it. Men who weigh too much are more likely to have poor sperm quality, research on nearly 1,600 young Danish men has found. Being too thin is a problem, too.

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Women don’t get off the hook. Though it’s long been known that very overweight women have trouble conceiving naturally, a large new study confirms they also are less likely to become pregnant even when embryos are fertilized in lab dishes and placed in their wombs.

“Among the severely obese, we saw significantly reduced implantation and pregnancy rates,” said Dr. David Ryley of Beth Israel Deaconess Medical Center in Boston. He presented results of the women’s study this week at a meeting of the American Society for Reproductive Medicine.

The sperm study was done by doctors at various hospitals and universities in Denmark and published in the October issue of the reproductive society’s journal, Fertility & Sterility.

It involved 1,558 men, average age 19, who volunteered to give a semen sample during mandatory exams to determine their fitness for military service in two cities, Amsterdam and Aalborg.

Study focused on body mass index
Sperm counts, sperm concentration, semen volume and other measures of sperm quality such as shape and motility were measured, along with testicle size and hormone levels. Researchers also calculated each man’s body mass index, a measure of obesity that takes into account height and weight.

Scores for men with healthy BMIs — between 20 and 25, or 139 to 174 pounds for a man who is 5-foot-10 — were compared to those of men above and below that range.

Sperm counts and sperm concentration were 28.1 percent and 36.4 percent lower respectively in underweight men. The same measures were 21.6 percent and 23.9 percent lower respectively in overweight men.

Why this may be happening is unclear.

“Low BMI can result from a 'healthy lifestyle' but may also be due to many chronic diseases,” the authors write.

The biological explanation or mechanism also may be different in underweight and overweight men, they note.

Hormone imbalance to blame?
“It may be an alteration in hormonal values,” said Dr. Anthony J. Thomas Jr., a Cleveland Clinic urologist who is president of the Society for Male Reproduction and Urology and was not involved in the study.

Men produce and need a certain amount of the female hormone estrogen. Fat cells produce estrogen, so too much or too little of it may be a problem.

“There’s a balance, and that balance is the milieu in which sperm develops,” Anthony said.

Other research suggests that smoking and heavy alcohol use also harm sperm production, he said. The new study is a reminder that doctors should always check a man for signs of infertility when couples are having trouble getting pregnant because the problem is just as likely to involve men as women.

“It’s not uncommon for a man to come in after his wife has had a million tests” and then discovered to have sperm problems, Thomas said. “It’s probably one of the first things a doctor should do.”

Obese women also face fertility problems
The other study on women involved 5,847 attempts at in vitro fertilization in which embryos are fertilized in a lab dish, at Beth Israel’s infertility clinic, Boston IVF.

Obese women — whose BMIs were 35 and over — had little more than a 1 in 5 chance of becoming pregnant through IVF; the odds were better than 1 in 4 for women with healthy weights. Embryos were also less likely to implant in the fat women.

Overweight women have irregular periods and lower ovulation rates, making it harder for them to get pregnant naturally, Ryley said. When given fertility treatments, they often need higher doses of drugs and for a longer time to spur ovulation, he said.

His study didn’t find that very thin women had more trouble conceiving as some previous research has suggested.

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