Men who are overweight or obese may face a greater risk of infertility, as a recent study published in the journal Human Reproduction found that greater body mass index (BMI) correlated highly with measures of lower sperm count and overall poor semen quality.

The collateral damage of overweight and obesity (a BMI above 25 and 30, respectively) should concern men who fall within that range and wish to continue having children, the researchers argue. Unfortunately, many men overlook the sexually related challenges that come with a widening waistline. More typical drawbacks such as poor heart function or high cholesterol, share much of the focus — until a couple wishes to grow their family but learns they can’t do so.

"The heavier the men, the higher the chances of a low sperm count," urologist Dr. Keith Jarvi, who was not involved with the latest study, told Reuters Health. "I don't think that this message is well known or appreciated by men in general.”

Urologist Dr. Michael Eisenberg, of Stanford University School of Medicine, and his colleagues collected semen samples from 468 men who were currently in relationships. They also evaluated the semen for sperm motility, vitality, and appearance, in addition to weighing the men, measuring their waistlines, and calculating BMI.

By and large, the men who were overweight and obese ejaculated less semen than the men who were at healthy weights — 3.3 mL for men in the normal weight category compared to 2.8 among the obese. A healthy volume for ejaculation is between two and five milliliters. Beneath that threshold men could become infertile, Dr. Jarvi pointed out. The most rotund men in the study — those with waistlines 40 inches and up — had 22 percent lower total sperm count than men under 37 inches.

Indeed, semen quality must be assessed according to a number of relevant factors, Eisenberg explains. "All aspects of semen quality are important," he told Reuters Health. "Ejaculate has several chemicals that provide a safer environment for sperm. As such, if the volume is low it may be a problem."

Eisenberg’s study isn’t the first to draw a link between obesity and male infertility. Last year, a massive 14-study data pool, which included more than 10,000 subjects, found that 32.4 percent of obese men had a low sperm count, compared to men of a healthy weight, who had low sperm count 24 percent of the time. Obese men were also more likely not to have any viable sperm whatsoever (6.9 percent) compared to other men (2.6 percent).

A number of theories have been proposed to explain this effect. One is that greater fat storage can end up turning testosterone into the female hormone estrogen, leading to a slowing (or stoppage) of sperm production. Another theory posits that the hormone leptin, which is produced by fat cells and makes people feel full, might actually damage sperm cells or those that produce them. Researchers have also submitted the simple idea that the insulating fat tissue causes temperatures within the scrotum to rise, killing sperm.

Present researchers say they didn’t take into account exercise for their study, a limiting factor that makes it difficult to know whether body weight itself contributes to low sperm count or another related cause. More important, Dr. Jarvi points out, is one day discovering how low sperm count among men of a healthy weight can be avoided.

"The big question is what does reduction in body weight do to the sperm counts in men starting with a low sperm count?" Dr. Jarvi told Reuters Health. "This is the question that my overweight patients ask."

Source: Eisenberg M, Kim S, Chen Z. The relationship between male BMI and waist circumference on semen quality: data from the LIFE study. Human Reproduction. 2013.