Warning: this article contains discussion of eating disorders
"If you keep this up, you'll be in a wheelchair at 30."
That is what doctors told British long-distance runner Jake Smith four years ago.
From the outside looking in, his career was flying.
By the age of 23, he had signed a professional contract with Nike and represented Great Britain on the global stage.
But at the same time, his body was deteriorating - enduring a relentless pattern of race, illness, injury, repeat.
Balancing an intense training regime with disordered eating meant the 28-year-old returned from a running camp in Uganda weighing just eight stone (51kg) at 5ft 5in (1.7m). He had such brittle bones that he was told his career could be over.
After suffering two stress fractures in the space of four months, Smith sought professional help and was diagnosed with REDs – relative energy deficiency in sport.
It is a syndrome that can affect anyone and if left undiagnosed or untreated, can have an impact on performance and cause serious, long-lasting health problems.
The symptoms are slightly better documented in women - just last month, tennis player Emma Navarro revealed her diagnosis and subsequent decision to step away from competition until 2027.
But men are just as susceptible and due to stigma and lack of obvious warning signs, it can go undetected for longer and means many, like Smith, suffer in silence.
BBC Sport explains what the condition is, how it affects men and women and why, despite its prevalence and severity, REDs remains a relatively under-reported and overlooked area of sports science.
REDs can cause bone stress injuries, disrupted sleep, fatigue and low mood. Left untreated, the condition can lead to serious long-term health problems, including osteoporosis, cardiovascular disease and fertility issues.
Dr Nicky Keay, an expert in hormone health, says many of these symptoms stem from hormonal disruption caused by insufficient energy intake.
"Hormones go to every single bit in your body. So, if your hormones are not working well for you, you're going to feel it in all sorts of different areas," she says.
"In order to have healthy hormones, they need to be fed regularly. If you're not doing that and there's an energy deficit, often some of those hormone systems will slow down or shut down completely."
Women have a monthly barometer of their hormone health with the menstrual cycle, described by Keay as a "free monthly medical check".
Put simply, she says, a woman suffering from REDs has her periods "switched off".
For men, it is more subtle – testosterone levels will drop, resulting in a reduced libido and fatigue.
The impact on a woman's period is why the condition was first referred to in the early 1990s as the "female athlete triad".
But it was renamed by the International Olympic Committee in 2014 to acknowledge the broader range of symptoms of energy deficiency and that it also harms male athletes.
"We realised we were ignoring the other half of the population," Keay says.
"It's a strange situation because normally it's women's health that is the one being ignored but, in this situation, it's the men who weren't being recognised or represented."
Twelve years later that decision appears to be vindicated, with recent research highlighting the prevalence in men.
Meta-analysis published in Australian journal Sports Medicine last year, following a study involving thousands of elite and amateur athletes around the world, indicated 44.2% of female athletes and 49.4% of male athletes had low energy availability.
So why is REDs so common?
One of the answers lies in the phrase 'lighter is faster'.
For decades, it had been an accepted wisdom in sports like endurance running, cycling, climbing, horse riding, dancing and aesthetic sports like diving and gymnastics.
And for athletes like Smith, it is how his problems started.
"I believed it for years. I thought that was the way forward," he says.
"I started thinking 'how do I improve my half marathon by 10 seconds? Is it a drop in weight?'. And that's when my head completely went."
Initially, the strategy can work.
When it comes to choosing energy to cover training demand or energy to stay healthy, the body has learned through evolution and threat of predators to always prioritise movement.
It means that athletes can drop weight and still train and exercise and achieve good results.
But eventually, the luck runs out.
Described by Keay as "fool's gold", the body will adjust to the energy deficit and cause a drop in performance and high risk of injuries.
Olympic and world champion Jonny Brownlee acknowledges under-fuelling mistakes he made in his triathlon career contributed to serious injuries.
"I didn't see the importance of it. Training came first and I think we weren't educated enough in how important fuelling was," he tells BBC Sport.
"You were seen, culturally, as a bit weak if you were fuelling on the bike. And the impact of that was definitely injuries - probably not recovering from my training quite as well as I should have done, stress responses, stress fractures. So there's that side of it and probably underperformance in races as well."
Brownlee, who recently announced his retirement, believes the culture of not fuelling on the bike has now changed but a stigma lingers around REDs and the perception that it is a "female issue".
"In females, there are more obvious signs but in males it can go under the radar a little bit and then it hits you by a really serious injury," he says.
"I think there's a stigma attached in there, or it's not looked at as the obvious answer to some of your problems, it's found later down the line. Men also don't like to talk about under-fuelling because it's just seen as 'you probably should feel better and you should be on top of that'."
It is those attitudes among male athletes that prompted Keay to conduct an experiment in 2019.
Assessing 50 male competitive cyclists, Keay found in her initial screenings that nearly half had poor bone health and 25% were suffering from REDs.
The cyclists were then split into two groups and across a six-month period, one half continued their training and fuelling habits as before while the others were tutored by Keay and offered nutrition and training advice.
"The group that I left to their own devices unfortunately went on the default and they lost even more bone mineral density, similar to an astronaut in space… And they performed OK, but relative to their functional threshold power, they didn't get the race results that you would have expected.
"For the group I took under my wing, there was an improvement in their bone health and really good race results.
"So, I had evidence that REDs definitely does exist quite extensively in my group of the male cyclists and you're going to underperform if you don't fuel sufficiently. The good news is that REDs is reversible. But if you don't take action, it's a decline."
As the syndrome and its symptoms are still not commonly known, it is often difficult to know where to find help.
After former GB runner Pippa Woolven's struggles with the condition, she set up a website called Project REDs which offers information and practical resources for athletes, coaches, parents and healthcare professionals.
Keay also helps people competing at all levels to recover from REDs. Most of her patients are women, but she is starting to see more men reach out for help who, she says, are often in "bad shape".
"I think the men are maybe more reluctant than the women, or maybe they don't realise they've got it. Sometimes they might use a different word for it, they say burnout," Keay says.
"I don't mind that though because there is definitely a psychological aspect to REDs. Why are you doing this? Why are you not eating the carbohydrates?"
Sometimes, it's hard to answer those questions because the under-fuelling is unintentional, either from not understanding or underestimating energy requirements or failing to match an increase in the intensity or duration of exercise with higher calorie or carbohydrate intake.
For others, former British cycling champion Brian Smith among them, the answer comes from intention.
Competing in the road race for Great Britain at the 1996 Olympics should have been the highlight of his career but, behind the scenes of the success, Smith was suffering with an eating disorder and serious mental health condition called bulimia – something he believes he developed from his sport's obsession with weight.
"I always felt like I was overweight. Everybody was under pressure regarding weight, but nobody really spoke about it," he says.
"I'm just under six foot and I've got large glutes and thigh muscles. As a road cyclist, I used to race at 68kg and for my height and build, that's quite skinny. I even got down to about 66kg and when I got down to that, I felt underpowered, I felt bad."
Smith, now 59, only discovered he had been suffering from REDs during his career when he started running eight years ago and developed three severe stress fractures, including one in his femur.
Such has been the toll on his body that Smith says he would now never encourage anyone to pursue a career as a professional cyclist.
"It really is not healthy," he says. "If you want to go to the highest levels of the sport, you will suffer later on in life. You're destroying your body.
"Sporting teams don't care - all they're worried about is performance. So they will nurture a rider, they will feed a rider, they'll get a rider to be a certain weight. They don't care about what happens to them later in life.
"I'm suffering now and I've so many regrets. I just wish I'd known more about this."
While Smith believes his sport has a long way to go in changing attitudes, British Cycling says a lot of progress has been made through education that starts at academy level and annual physical and psychological screenings.
"One of the big things we've done in the past 10 years is try and turn this into a performance-related conversation," Dr Nigel Jones, British Cycling chief medical officer, tells BBC Sport.
"Now, we couch the conversation with an athlete around 'if you under-fuel, you will not be able to perform at your best', whereas previously, conversations in power-to-weight ratio sports might have been 'you need to be lighter'.
"I think it would be naive to say REDs is never going to be an issue. But I would like to believe that its prevalence will reduce with the work that we're doing," he adds.
British Cycling is not alone in this approach, with World Climbing taking things a step further two years ago with the introduction of mandatory health checks on athletes.
Using a combination of pre-season questionnaires and 'random and targeted' screenings at competitions - similar to the way doping tests are enforced – climbers at World Championship or World Cup-level competitions are monitored for any indicators of low energy availability or REDs and could be prevented from competing if they are not deemed to be healthy.
The policy was drawn up by expert Professor Naama Constantini, who says she is "proud" of the move by World Climbing and believes other federations should follow suit.
"We are trying to educate and prevent. I'm not a policeman and we're not looking for cheaters. I'm a medical doctor and we want to make sure our athletes are healthy," she says.
"Prevention is the best medicine you can give. We want athletes and coaches to understand that it's not about weight. It's about strength, it's about performance and fuelling the body and you pay a price if you lose weight in an unhealthy way."
With more awareness and education, it is hoped the tide can be turned on REDs.
But many still suffer in ignorance, as well as silence. That is what motivates Jake Smith to keep sharing his recovery on social media, with hundreds of men and women messaging him for advice and support.
Smith has managed to reverse his condition and is now racing better than ever, recently completing the Berlin marathon in a personal-best time of two hours, nine minutes and 24 seconds.
The big goal is the 2028 Olympics, but he is adamant that the priority is making sure he stands on every start line as healthy as possible.
"I'd never want to see an athlete go through the years I've had. I'm 10-15 kilos heavier than I've ever been and the biggest thing I can say is I've never been so happy. You need a healthy body to do everything else.
"I'm heavier, I'm faster, I'm happier."
If you have been affected by any of the issues raised in this story you can visit BBC Action Line.