Sperm counts are dropping at an increasingly fast rate across the world, and scientists are hard at work trying to understand why.
According to new research from China, excess weight and obesity could be contributing to decreases in semen quality, an overall measure of male fertility that includes sperm counts.
"The continuous decline of human semen quality during the past decades has drawn much concern globally," write epidemiologist Yingxin Li from Sun Yat-sen University and colleagues in their published paper.
"Previous studies have suggested a link between abnormal BMI and semen quality decline, but the results remain inconsistent."
Li and team conducted a systematic review and meta-analysis of 50 previous studies, covering 71,337 men in total, that measured the association between BMI and several parameters of semen quality.
"Obesity and overweight were significantly associated with semen quality decline," the team writes, "and the reduction of semen quality was more pronounced in men with obesity than that in men with overweight."
The data showed that men classified as having obesity had lower semen volume, a lower total sperm count, and lower sperm motility (the number of moving sperm) compared with men with a healthy BMI.
Those classified as overweight also had lower semen volume and lower sperm motility compared with those with a healthy BMI, though the difference wasn't as significant.
A systematic review like this doesn't prove cause and effect – that excess weight causes a decline in semen quality. What it can do is highlight patterns that researchers can then investigate further – and that would include looking at the biological mechanisms at play.
The reasons for the association weren't closely examined in this study, but in the past, multiple factors, from toxins in fatty tissue to hormonal imbalances to higher body temperature, have been suggested.
Part of the difficulty in working out what's going on in sperm is that many variables can affect it: environmental pollution, smoking, age, illness, diet, and much more. Isolating each factor from the others isn't easy.
That will require more research over a longer period, and there are limitations: in many of the studies reviewed, only one semen sample was taken, for example. In some cases, self-reporting was used to establish BMI rather than professional assessments.
However, we know there are numerous interconnected factors that affect our bodies and our health, so it would not be surprising if poor health in one area also meant poor health in another.
Previous studies have shown how lifestyle changes and subsequent weight loss can boost semen quality – something this latest research backs up.
"Our findings highlight the potential and importance of maintaining a normal weight to prevent the decline of semen quality and improve male fertility," write the authors.
The research has been published in the International Journal of Obesity.