What you weigh affects how your sperm play. And your fertility. Overweight men tend to have lower semen volumes, less sperm and more oddly shaped sperm. The same is also true for men who are too thin. So, along with the many other health hazards associated with obesity, add poor semen quality to the list.
Obesity in both sexes is known to be associated with heart disease, diabetes, hypertension, and metabolic syndrome among other nasty conditions that can shorten your life. Typically, obesity is measured with BMI or body mass index, which looks at weight in relation to height. Not a perfect measure, but reasonably accurate. Using this tool, the ideal BMI for men (and women) is considered to be 20-25. A Danish study of 1600 men showed that overweight men with a BMI > 25 had a 22% lower sperm concentration compared with healthy weight men. Interestingly, a BMI of <20 was also associated with poor semen quality. For optimal sperm production, then, it helps to be not too fat and not too thin.
But what about fertility? Is it also affected by obesity? Yup. Another recent study showed that for every 20-pound increase in a man’s weight, there is a 10% increase in the chance of infertility. And this remained true when other factors that might influence the results were accounted for, including obesity status of the women, the man’s age, cigarette smoking, alcohol intake, and solvent and pesticide exposure. In addition, obesity was associated with infertility in both older and younger men.
So what is it about weight that influences men’s sperm production and fertility? One theory is that sex hormone metabolism is altered by changes in weight. Sex hormones are the “fuel” for the engine (testis) to make sperm. Obesity increases fat stores and fat converts male hormones (testosterone) into female hormones (estrogens). Too much estrogen in men is bad for sperm production. Another theory posits that normal 2-degree difference in testis temperature relative to the body is lost with obesity, as excessive fat provides too much insulation and results in overheating. On the other hand, when a man is too thin, he may take on a “catabolic” metabolic state. With a body in “starvation mode,” fertility is not the first thing on its mind and sperm production and fertility suffer.
So, is the epidemic of obesity the reason why sperm counts have been falling in Western countries over the last 50 years? Maybe. But this problem is unique in that it is utterly and entirely preventable. Eat well and in moderation, sleep well and treat your body like a temple. You used to it for your own health; now do it for the health of your future family.