Young men who are obese may have a lower sperm count than their normal-weight counterparts, a new study suggests.
The findings, reported in the journal Fertility and Sterility, add to evidence tying obesity to relatively poorer quality sperm.
A number of recent studies have found that compared with leaner men, obese men tend to have lower sperm counts, fewer rapidly mobile sperm and fewer progressively motile sperm, which refers to sperm that swim forward in a straight line rather than moving about aimlessly.
But age is a "confounding" factor in examining the relationship between obesity and sperm quality. Older men tend to have lower sperm quality than younger men, and they also tend to have more body fat.
However, among the more than 2,000 men in the current study, obese men between the ages of 20 and 30 generally had a lower sperm count than normal-weight men in the same age group.
What all of this might mean for an obese younger man's chances of becoming a father is unclear. Studies have so far come to conflicting conclusions as to whether obesity actually impairs a man's fertility.
And these latest findings do not reveal whether the difference in sperm count between obese and normal-weight men would be enough to also make a difference in their fertility, according to lead researcher Dr. Uwe Paasch, of the University of Leipzig in Germany.
For their study, Paasch and his colleagues used information from a database on men who had come to their fertility clinic for a semen analysis between 1999 and 2005. The 2,157 men included in the study were 30 years old, on average, and had no known infertility problems.
Overall, obese men had a relatively lower average sperm count than normal-weight men, but were still within what's considered the normal range. That range is between 20 and 150 million per milliliter of semen, according to the National Institutes of Health.
In an email, Paasch told Reuters Health that "we do not know in detail" whether the difference in sperm count between obese and lean men would affect their fertility. But, he added that the relationship between weight and sperm count offers young men another reason to try to maintain a normal weight.
It is not entirely clear why obesity is related to sperm quality. Some studies have found that obese men tend to have altered levels of testosterone and other reproductive hormones compared with thinner men. In this study, though, hormone levels correlated with age, but not with body weight.
In other research, Paasch noted, he and his colleagues have found that high levels of body fat are associated with changes in the collection of proteins that allow sperm to survive and function.
The current study had a number of limitations, including the fact that the men were patients at a fertility clinic rather than a sample from the general population.
The researchers also point out that weight categories were based on body mass index, or BMI, a measure of weight in relation to height. The problem is that BMI does not precisely reflect a person's level of body fat.
Other studies have suggested that body fat, and abdominal fat in particular, is more closely related to sex-hormone levels than is BMI.