Levels have dropped dramatically over the past 40 years. By Christa Sgobba

As guys are getting bigger, one thing is getting smaller: their sperm counts. Over the last four decades, men’s sperm counts have been declining, a new review and meta-regression analysis from the journal Human Reproduction Update concludes.

Researchers crunched the data from 185 studies containing nearly 43,000 men who provided semen samples from 1973 to 2011. During that time period, they found that both total sperm count and sperm concentration significantly declined for men in “Western” areas, including North America, Europe, Australia, and New Zealand.

In particular, sperm concentration declined about 1.4 percent each year, for a total drop of 52 percent over that time period. Total sperm count declined 1.6 percent each year, for an overall drop of 59 percent.

Related: 5 Ways You Can Make Your Sperm Stronger Right Now

This shows that there was no “leveling off” of the decline in recent years, the study authors write. As a result, more and more men are meeting sperm count criteria for poor fertility or infertility, meaning that conception can be more difficult.

And poor fertility issues are just one possible effect of this: Recent studies have linked lower sperm count to overall health issues, including disease and premature death, the study notes.

“A decline in sperm count might be considered as a ‘canary in the coal mine’ for male health across the lifespan,” the researchers write.

Related: Does Your Sperm Have an Expiration Date?

So what’s to blame? This study didn’t delve into the possible causes, but a few jump to mind: Environmental influences—including endocrine-disrupting chemicals—come to mind, as well as a Westernised diet, smoking, and growing rates of obesity.

Cleaning up your diet may help. In fact, a 2015 study from Taiwan found that the more than men followed a Western diet—high in animal protein, processed foods, and fat and sugar—the worse their sperm concentration and the normally-shaped sperm they tended to have. Adding omega-3s, found in things like walnuts, may help sperm quality.

Originally published on menshealth.com