THE year was 1974. As newspaper columnists were busy wringing their hands over the development of in-vitro fertilisation and the prospect of “test-tube babies”, an alarming discovery about male fertility completely escaped their notice.
Comparing contemporary semen samples with historical data, two doctors in Iowa named C. M. Kinloch Nelson and Raymond Bunge showed that there had been over the previous two decades. In 1951, each millilitre of semen contained 107 million sperm; by the 1970s, that figure had dropped by more than 50 per cent – to just 48 million. The average volume of semen from a single ejaculation had fallen too.
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By the 1990s, the issue started catching considerably more scientific attention, although some researchers were still sceptical. They blamed differences in techniques, or the fact that studies were mostly on men already having treatment for infertility. Such doubts are now shrinking. “There is a huge body of scientific evidence showing this decline,” says at the University of Rovira i Virgili in Spain.
For researchers like Salas-Huetos, the big question is no longer whether this so-called “spermageddon” is really happening, but why and what to do about it. Studies are beginning to shed light on environmental toxins that may be to blame, as well as other lifestyle factors contributing to the problem. With a better idea of the prime suspects, we may finally be able to put the brakes on this trend, or even reverse it.
Sperm count decline
Globally, around , according to a recent report by the World Health Organization. There are many potential causes, but between 30 and 50 per cent of cases are linked to problems with the quantity and quality of semen. It may be that the total number of sperm is simply too low, or that the tadpole-like cells struggle to swim – a problem called poor motility – which vastly reduces the chance that sperm can reach the ovum, or egg cell. Some may have genetic defects within the chromosomes they are carrying, known as DNA fragmentation.
, a reproductive epidemiologist at the Icahn School of Medicine at Mount Sinai, New York, has led many of the most eye-catching studies. Her interest began in the 1990s, when she was asked by the US National Academy of Sciences to a reporting rapid sperm decline. Swan was initially sceptical: she suspected that the researchers might have missed some confounding factor in their analysis. When she crunched the data, however, she kept finding the same rate of decline predicted by the Danish team. “We didn’t change the slope at all – not down to the second decimal place,” says Swan.
Her conviction has only increased over the subsequent decades. In 2017, she and her colleagues published of more than 42,000 men between 1973 and 2011, making it the largest of its kind. Swan’s team examined two different measures: the concentration of sperm in a millilitre of semen and the total number of sperm in the sample. In North America, Europe, Australia and New Zealand, both figures seemed to be falling at a rate of around 1.5 per cent per year on average, resulting in a 50 to 60 per cent drop over the whole period.
If the decline continued at this rate, the median sperm count would reach zero by the mid-2040s. Within a generation, “we may lose the ability to reproduce entirely”, the magazine .
At the time of this analysis, Swan and her team didn’t have enough data to draw strong conclusions about sperm counts in the rest of the world. They have now filled this gap in knowledge with additional data from South America, Asia and Africa. The ensuing paper, published in 2023, .
Such studies do have some limitations. Meta-analyses can be skewed by differences between datasets. Counting sperm is a fiddly job and the technology used to do it has changed over the years, which may bias the reported numbers. Nevertheless, and the pattern remains, says at the University of Nottingham, UK.
Then there is comparative evidence from man’s best friend. Vets regularly test stud dogs’ sperm quality, which is carefully recorded for controlled breeding programmes. This provides an excellent source of data to study potential changes in dog fertility over time. Lea and his team recently between 1988 and 2014. Crucially, the lab’s methods had remained the same across the 26-year period, eliminating any possibility that methodological changes would bias the results. “The decline in sperm quality we saw in dogs parallels that seen in humans,” says Lea.
Male infertility
There is one caveat that is still taken seriously. It isn’t yet clear how important a decline in sperm is for overall fecundity. We see large variation in sperm count between men who don’t have any significant health issues, but the absolute figures don’t seem to make a big difference to the chances of conception until they dip below a very low threshold. “And all of the average sperm counts are still above the level required for fertility,” says at the University of Edinburgh, UK. It is possible, she says, that we are simply witnessing , rather than an endless decline.
Even so, scientists investigating the trend are concerned with the speed of the decline, which doesn’t show signs of slowing. Indeed, Swan’s research suggests it may even be accelerating. “If you see a 50 per cent decline in 50 years, that sets off an alarm,” says Salas-Huetos.
Lea, meanwhile, points out that the reduced sperm counts coincide with a . Increasing numbers of children are being born with genital malformations, for instance, and rates of testicular cancer in young men have also been on the rise. “That data’s really robust,” he says.
What causes sperm decline?
The suggestion is that there may be some underlying environmental or lifestyle factors driving all these different trends. This is now the primary area of interest for Swan, and she says there is a growing body of evidence that chemicals may play a major causal role.
For decades, pollutants called (EDCs), have been on our radar as the prime suspects behind infertility in men and women. These are thought to interfere with hormonal signalling and include common chemicals found in some plastics and
The drop in sperm count today could be the result of EDC exposure during the earliest months of development. We know that fetal growth is, in part, governed by levels of sex hormones, and . Animal studies have shown that exposure to EDCs during of fetal development can result in genital malformations and a reduced ability for males to produce sperm later in life. Likewise, research examining people who have been has also shown links between exposure and decreases in sperm count, viability and motility.
Environmental hazards
Common pollutants can make their way into the testes through the diet and the surrounding environment. To test the effects that this may be having on sperm quality, Lea’s team recently incubated samples of dog and human sperm with different concentrations of diethylhexyl phthalate and polychlorinated biphenyl 153. The former is used in cosmetics, personal hygiene products and furniture materials. The latter, once widely used in industrial products such as paint and rubber, was banned internationally in 2001 after being deemed potentially carcinogenic, but it persists in soil, water and building materials. The result was and higher rates of DNA fragmentation in sperm. Swan’s research has also . Other studies have found similar results for bisphenol A, or BPA,a chemical used to make rigid plastics, including food storage containers and refillable drinks bottles. For instance, one showed that men who were occupationally exposed to BPA were , erectile dysfunction and decreased sperm quality.
Although the UK’s Food Standards Agency says that the levels of BPA typically detected in food , it has banned its use in items intended for infants. The agency says that it is reviewing the latest evidence and may revise restrictions on BPA in the future.
In any case, some manufacturers have replaced BPA with other chemicals, such as bisphenol F (BPF) and bisphenol S (BPS). A . It found that BPA was the worst offender, but BPF also decreased sperm motility, energy and capacity to fertilise an egg. BPS didn’t have any significant effects.
Diet and sperm count
It isn’t just environmental toxins that are in the crosshairs. The role of diet in sperm decline has also come under scrutiny. From 1980 to 2019, the number of men and boys in England aged 16 and over who were obese , with similar trends across many other countries.
Multiple studies have found that , have a lower overall sperm count and have reduced sperm motility. This, in turn, can influence the outcome of fertility treatments. In 2022, a team of researchers, including Salas-Huetos, tracked the success rates of 176 couples visiting Massachusetts General Hospital Fertility Center and of the men. A 5-centimetre increase in a man’s waist circumference was associated with a 6.3 per cent drop in sperm concentration and a 9 per cent reduction in the chances of a live birth for each cycle of treatment.
Salas-Huetos suggests that inflammation is to blame. Excess body fat produces inflammatory molecules that can wreak havoc on other tissues, including those of the genitals, and reduce the production of testosterone. “The hormonal [balance] that you have to maintain to produce sperm – it is totally disturbed,” he says.
The nutritional quality of food may also be key. A 2019 study found that men who , vegetables, nuts and fish have higher sperm concentrations with greater motility than those with less balanced diets. These foods tend to be high in antioxidant compounds, which help to neutralise rogue molecules called free radicals. The production of semen seems to be , so any nutrients that mop up these molecules may contribute to sperm health. The presence of omega-3 fatty acids – which are known antioxidants with anti-inflammatory properties – appears to be particularly important in predicting sperm quality, according to . High sugar consumption, in contrast, is linked to reduced sperm quality.
Since the covid-19 pandemic began, more attention has also been given to the impact of viruses on sperm. Several studies have shown how infection with covid-19 reduces sperm counts and motility, , or fever upsetting the delicate homeostasis needed to grow healthy sperm.
This decline usually reverses as the infection wanes, although one study of men visiting a fertility centre showed that .
How to improve your sperm count
There is some good news. If obesity, diet and infection are affecting sperm production, lifestyle interventions and vaccinations may be able to offset a fertility crisis – and a handful of randomised clinical trials over the past few years already provide grounds for optimism.
at the University of Copenhagen in Denmark and his colleagues asked 56 men who were obese to reduce their calorie intake to just 800 calories a day for eight weeks. Publishing their results in 2022, Andersen and his team found that on average, and their sperm count increased by more than 40 per cent. Tracking the men for a year after the diet showed that these improvements continued for as long as they kept the weight off, but disappeared if they regained the lost kilos.
Salas-Huetos and his colleagues, meanwhile, examined the benefits of adding 60 grams per day of walnuts, almonds and hazelnuts to men’s diets for 14 weeks. These nuts are all high in antioxidants, which the team suspected would help reduce genetic damage to sperm. Sure enough, the men adhering to this programme , and reduced DNA fragmentation, while those continuing with their standard diet didn’t show any improvement.
There may be many different routes to reaping these benefits. An offered 137 men an individualised regime emphasising increased physical activity and an adherence to the “Mediterranean diet“, which incorporates many of the components thought to benefit fertility. After four months, they experienced significant improvements in sperm quantity and quality compared with people in the control group
The effect of exercise on your sperm count
Given that diet and exercise often go hand in hand when it comes to lifestyle interventions and good health, you might think that exercise would be firmly linked to better sperm. In reality, however, it isn’t so straightforward.
Small studies have shown , such as 30 minutes of cardio, three times a week. Likewise, a compared with those who had less physical jobs. They also had higher testosterone and oestrogen levels, which have been associated with better general hormonal balance. However, other studies suggest that you shouldn’t overdo it. For instance, , such as trekking up a high mountain for 6 to 8 hours a day for five days, or after intensive cycling training for 16 weeks. A larger study of cyclists found no link with fertility (see “What causes sperm decline? Fact and fiction”, below), but more evidence is needed to say for sure.
So, where does that leave us? “If you think of what a doctor would tell you to change to improve your heart health, you’re also going to improve your sperm health,” says Swan.
It has been 50 years since Nelson and Bunge first announced their findings. While today’s researchers may still debate the specific causes, consequences and cures of sperm decline, they are united in one opinion: the urgent need for a greater understanding.
“It deserves a lot of attention,” says Boulicault. “Reproductive health is a really important part of people’s lives and that means that the way we do the science has to be extra vigilant and robust.”
Swan compares it to the climate crisis. “We’re following the same path,” she says. “You have the first warnings, then you have the big wave of denial, then you have more people agreeing that it is getting worse.”
Next comes the hard part: taking action. The clock is ticking – and future generations may judge us for the decisions we make today.
What causes sperm decline? Fact and fiction
Mobile phone use
A identified an association between mobile phone use and reduced sperm motility, viability and concentration: the greater the use, the worse the outcome. The electromagnetic waves used to transmit data are hypothesised to contribute to genetic damage. The evidence is often criticised for not being robust enough, though. Shanna Swan at the Icahn School of Medicine at Mount Sinai, New York, is sceptical. "It's potentially extremely important, but I haven't seen convincing evidence yet," she says.
Tight undies
There is a commonly held belief that tight underwear has an affect on sperm quality, possibly due to scrotal heating. A handful of studies . However, these studies are small, haven't been linked to lower fertility and are mostly carried out on men already attending fertility clinics.
Cycling
Several small studies appeared to support the idea that pressure from saddles could disrupt blood flow or cause tissue damage that reduces sperm health. However, in 2014, , the largest of its kind, refuted this idea, finding no relationship between cycling and infertility. Some small studies have suggested that extreme cycling training may harm sperm quality (see main story), but these findings haven't been tested on larger numbers of people.
David Robson is a science writer and the author of The Expectation Effect: How your mindset can transform your life. He is based in London
