For the past 20 years scientists have been fretting over the decline of sperm counts in the West. The most recent alarm came late last year, when a study found that sperm counts had fallen in French men from an average in 1989 of 73.6 million sperm per milliliter of semen in a 35-year-old man to 49.9 million per milliliter in 2005.
While nowhere close to threatening our fertility as a species—that number would need to drop below 15 million per milliliter—it is not a comforting trend given that lower sperm counts make it harder to father children. But the cause has been unknown. Did the lower sperm counts stem from high-fat diets, being overweight, or trace amounts of chemicals in the environment and their effect on the body’s hormones? The search for a smoking gun produced a lot of intense speculation but little firm evidence.
Now, a deceptively simple study published in the British Journal of Sports Medicine has found a new and surprising culprit: television.
“Men who watched more than 20 hours of TV every week had 44 percent lower sperm counts compared to those who watched almost no TV,” says lead author Audrey Gaskins, a doctoral candidate at Harvard’s School of Public Health. “And then men who exercised for 15 hours or more per week at moderate to vigorous levels had 73 percent higher sperm counts than those who exercised less than five hours per week.”
As Gaskins says, she and her co-authors found the strength of the effect “pretty surprising,” especially as their subjects were all young, healthy men aged between 18 and 22, and almost all had baseline sperm counts within the World Health Organization’s definition of a normal range. They also filled out comprehensive food questionnaires, which the researchers then used to create two control markers: a more Western, processed diet and a healthier, grains-and-veggies diet. Neither diet influenced the outcome. “We think,” says Gaskin, “that our results are independent of diet and BMI [Body Mass Index] and smoking.”
Of course, a new finding like this must be replicated. But even noting that and other limitations in its design, reaction has been positive. “What a great paper!” emails Tim Church, professor and director of the Preventive Medicine Research Laboratory at Louisiana State University’s Pennington Biomedical, and a leading expert on activity. As he notes, we are all simply less active than we were 50 years ago. “Whether it is physical-activity-induced improvements in blood flow, mental health, autonomic balance, or just general well-being, there are plenty of reasons why these findings makes sense.”
“I suspect,” he adds, “the findings would be even more dramatic in 30-year-olds, where they have had more time to accumulate damage.” We’ll know in a year or so. Gaskins is in the middle of a followup study looking at older men in greater clinical detail.
Trevor Butterworth is a regular contributor to the Financial Times, Forbes, and The Wall Street Journal.