Oct. 22, 2004 -- Men's as well as women's body size matters when it comes to fertility.
Danish researchers found men who are overweight or obese have significantly lower sperm counts than men of normal weight. In addition, men who were underweight also had lower sperm counts compared with normal-weight men.
Another study by American researchers showed that obese women are also more likely to suffer fertility problems, even when their embryos are fertilized in the lab and then implanted in their wombs.
Doctors have known that very overweight women have a harder time becoming pregnant naturally, but this study indicates that they are also less likely to become pregnant when they use assisted fertility techniques.
Obesity Matters in Making Babies
In the first study, researchers studied the sperm quality of nearly 1,600 young Danish men who volunteered to donate a sperm sample when they took a physical to enter military service.
The study showed that overweight men who had a body mass index (BMI, a measure of weight in relation to height used to measure obesity) over 25 had a nearly 22% lower sperm concentration and 24% lower total sperm count compared with healthy weight men.
A BMI over 25 is considered overweight and a BMI over 30 is considered obese.
Underweight men who had a BMI under 20 also suffered from similar reductions in sperm counts.
"Body mass within the ideal "normal" range was associated with higher sperm concentration, higher total sperm count, and a lower percentage of abnormal sperm," write researcher Tina Kold Jensen, MD, PhD, of the University of Southern Denmark, and colleagues.
Researchers say these findings may have important implications for the future procreation potential of men worldwide in light of rising obesity rates.
The study also showed that as men's weight increased blood testosterone levels decreased.
"It remains to be seen whether the increasing occurrence of obesity in the Western world may contribute to an epidemic of poor semen quality in some of the same countries," they write. "If so, some cases of subfertility may be preventable."
In the second study, researchers looked at more than 5,800 in vitro fertilization attempts and compared success rates with the BMI of the woman.
Researchers found obese women with a BMI over 35 had lower success rates compared with overweight [BMI of 25-30], normal weight [BMI of 20-25], or underweight women.
Obese women had a lower rate of successful embryo implantation (13% vs. 19% among healthy weight women). They also were less likely to become pregnant after in vitro fertilization (22% became pregnant vs. more than 30% of normal or underweight women).
Researchers say the findings show doctors should encourage their patients to reach a health weight before attempting in vitro fertilization.