Across the animal kingdom, amorous males make a valiant effort to catch the eyes of females. Dashing peacocks shake their handsome iridescent feathers at peahens. Male blackbirds serenade mates with their exquisite flute-like songs. Red stags unleash loud guttural roars to display their virility. And in the world of homo sapiens, where Tinder, Instagram and Love Island are distorting masculine body image ideals, men endeavour to attract women by getting shredded at the gym.

But research suggests that guys who are willing to pursue extreme muscle gain in order to attract a partner may, ironically, be destroying their ‘evolutionary fitness’ – their ability to pass on their genes.  And it is a problem which could affect more than one million men in the UK alone. When researchers Dr Jim Mossman and Professor Allan Pacey began to explore this disturbing “Fertility-Fitness Paradox” – which they discussed in an article for the Journal of Internal Medicine – they were shocked.

Fitness before fertility

Muscular men, who would consider themselves exemplars of masculine physical fitness, were striding into research labs and fertility clinics and discovering they were infertile. Many had no sperm in their ejaculate at all. These men had taken anabolic-androgenic steroids (AAS), which mimic the effects of the natural muscle-building hormone testosterone, in order to sculpt the kind of muscular physiques which they thought women would admire. But they now had to tell their partners that they might not be able to have a baby together.

The researchers were certainly not surprised by the link between steroid use and infertility, which is well-established. Research ties steroid abuse to oligospermia (low sperm count), azoospermia (no sperm in the ejaculate), erectile dysfunction, reduced endogenous (natural) testosterone, testicular dysfunction, decreased testicular volume, impotence and reduced libido. What astonished them was how these fitness-savvy men had no idea that their quest for perfection was destroying their own ability to procreate.

“We’ve known for a long time that men who take anabolic steroids as part of going to the gym (do so) in order to build muscle mass, and we know it is detrimental to their fertility,” says Professor Pacey. “But men don’t know that very well. Many men come to the clinic with an apparent fertility problem, and you look at them when they walk through the door and you think: ah, there’s a guy who takes steroids. And that’s a difficult discussion to have, because often their partners are a bit mad with them.”

Evolutionary paradox

Pacey is a respected Professor of Andrology at The University of Sheffield’s Medical School who researches human sperm and male fertility. Mossman, who was studying for his PhD at the time, had similar interests, but his approach was also shaped by his background in zoology and evolutionary biology.

“Jim is interested in all of those things, like how peacocks or birds choose mates, and how females choose males based on the quality of the song, or the quality of the tail, or the colour of the plumage,” says Professor Pacey. “So when Jim was working with us on his PhD, he would see these big guys coming through the door, and then he’d find out that they didn’t have any sperm. And it kind of clicked in his brain: gosh, this is a real evolutionary paradox. The good-looking guys who were muscular should, in evolutionary terms, have better fertility, because that (strength) is the whole basis of evolution. But my goodness, they don’t.”

Whereas peacocks, blackbirds and stags know how to play the mating game, men who inadvertently wreck their own fertility in pursuit of female attention are engaged in an act of evolutionary self-sabotage unique to human males. In stark evolutionary terms, a man’s “success” is not based on the size of his biceps, but on his ability to pass on his genes. In their article, Dr Mossman and Professor Pacey emphasised this grim equation: “In evolutionarily blunt terms, no sperm = no babies = low fitness. In other words, many men set themselves an unachievable goal of being both physically and evolutionary fit when using AAS, and put their masculinity and muscularity in direct conflict.”

Love Island look

This may seem like a niche issue confined to the world of bodybuilders. But according to the UK Anti-Doping agency, more than a million men in the UK are using steroids, primarily for cosmetic reasons. A report in the journal Trends in Urology and Men’s Health suggests that this figure is likely to be underestimated, as many men don’t freely disclose their steroid use. Data suggests that the majority of male steroid users are aged 20–40, with 25% starting their use in their teens. But users range from young men seeking a TikTok-ready physique to middle-aged men striving to retain their youthful strength.

UK Anti-Doping points to the corrosive influence of social media, exposure to “body image influencers”, and the popularity of the ripped “Love Island” look. Many younger users suffer from body dysmorphic disorder – a mental health condition in which a person worries about perceived flaws in their appearance and their ability to attract sexual partners. Research in the Journal of Health Psychology found that “Intrasexual Competition” – the need to outdo other men in the dating arena – is a major cause of new steroid use among younger men.

The tricky bit is that men are not exactly wrong in their body-shaping assumptions. Many research papers have shown that men who have wider shoulders, appear to be physically fit, and have greater handgrip strength are regarded as more attractive by women. It is believed that a man’s physical strength offers an “ancestral cue” to his formidability, fighting ability and resource-holding potential.

One research paper, in which women were shown photographs of men’s bodies, found that the strength of a man’s body accounted for a huge 70% of their perceived attractiveness. But a man can be fit and strong without taking steroids. And the effects of steroid abuse on men’s health – and reproductive health – can be catastrophic.

Steroids and sterilty

Anabolic steroids fool the brain’s pituitary gland – a small, pea-sized gland located at the base of your brain – into thinking the testes (the male reproductive glands, which produce sperm and testosterone) are working extra-hard.

The glands react by shutting down the production of two key hormones – Follicle Stimulating Hormone (FSH) and Luteinizing hormone (LH) – which drive the production of sperm and testosterone. As a result, Professor Pacey believes that 90% of anabolic steroid users are likely to become sterile.

“It’s very simple and it’s very clear,” insists Professor Pacey. “Sperm are produced in response to hormones released in the brain. They’re released from the pituitary, they travel through the bloodstream to the testicles, and that’s what stimulates sperm production. The testicles then produce testosterone, which feeds back to the brain, and says: ‘I’m producing sperm. Don’t produce too much hormone here.’

So there’s this constant feedback going on between the brain and testicles. But if a man takes in testosterone through steroids, the brain says: ‘whoa, there is loads of sperm being produced, so let’s throttle back on the hormones.’ So the man produces less sperm.”

Infertility goes ignored

The wider health risks of steroid abuse are well-established, from premature death rates to increased hospital admissions. Ominous NHS warnings range from hair loss and breast development to an increased risk of prostate cancer, high blood pressure and high cholesterol. But the infertility issue is often strangely ignored.

It is not just that men don’t know about this. Many men don’t want to know about this. Professor Pacey has noticed that fit young men simply refuse to believe it could be their fault when their wives or girlfriends find it difficult to become pregnant. “When we published the paper, men taking anabolic steroids were really pissed off with us,” says Professor Pacey. “I got a lot of emails from them saying: ‘How dare you say this about us?’ So I find that interesting as well. But the reality is that men who go to the gym to look good and take steroids need to be aware that their fertility may be compromised. It’s well-known, apart from among the men that do it.”

There is certainly no reason to think that weight-training is, in itself, a problem. Indeed, a 2014 study in Human Reproduction found that moderate-intensity weight training increased men’s free and total testosterone levels and improved their insulin sensitivity, which helps to increase men’s sperm concentrations – though not necessarily their reproductive success.

Neither is there any proof that more common supplements, such as protein shakes, have a detrimental effect: one study in Fertility and Sterility found that there was “no association” between protein supplementation and semen quality.

Closing window

The message for men is simple enough: lifting weights is good for you, but don’t get sucked into the world of steroid abuse. Fortunately for those who have already strayed, evidence suggests that steroid-associated infertility may be reversible, although sperm volume may take years to recover – if at all. And that’s a big problem for middle-aged couples, given that a woman’s fertility naturally declines throughout her 30s.

“You have to tell the men to stop taking the steroids, and we stand a chance of it [the sperm volume] coming back – but it might take a couple of years,” warns Professor Pacey. “The partner is not happy if she’s getting older, because her fertility is declining, and she’s having to wait for his sperm production to come back. If your partner is 38 before you try for a baby, that means you might not have any sperm until she’s 40. And that is not a good place to be.”