I started running to lose weight. I never learned how to fuel.
This one's personal
On Monday, BBC Sport published a piece called Race, illness, injury, repeat: REDs and why men are suffering in silence. It tells the story of Jake Smith, a British marathon runner who signed for Nike at 23, then spent years stuck in a loop of racing, getting ill and getting injured, because he wasn't eating enough for the training he was doing.
Doctors told him he could be in a wheelchair by 30. He turned it around and ran 2:09 in Berlin this year. His line at the end really brought it all home for me:
I'm heavier, I'm faster, I'm happier.
I'm not a Nike athlete. I've never run 2:09, and I never will. But I read that article and recognised almost all of it. So this post is about the version of this story that doesn't make the news: ordinary runners, like me, who came into running to lose weight and never quite learned how to feed it.
One thing to be clear about before we start. I've never been diagnosed with REDs. I'm a Level 3 Personal Trainer, not a doctor or a dietitian, and this isn't a diagnosis, of me or of you. It's what I see when I look back at years of my own training, now that I know what the warning signs are.
What REDs is, in plain English
REDs stands for Relative Energy Deficiency in Sport. It's a mouthful, so here's the short version: your body isn't getting enough energy from food to cover your training and all the everyday jobs it needs to do to keep you healthy, like repairing tissue, making hormones and fighting off bugs.
When that happens, your body does what it always does under threat. It keeps you moving and starts switching other things off. Hormones, bone repair, recovery, your immune system. You can keep training, and for a while you might even feel fine. Dr Nicky Keay, a hormone health expert interviewed by the BBC, calls that early bit "fool's gold". Eventually, the bill arrives.
It used to be thought of as a women's issue. It isn't. The BBC reports a 2025 study in the journal Sports Medicine that found low energy availability in 49.4% of male athletes, slightly more than in women. In men, the signs are just less obvious, so it tends to get spotted later, usually when something breaks.
How it starts: running to lose weight
My first run was in July 2007. I was overweight, and I wanted that to change. Over the next three months I ran 683 kilometres and lost over two stone. It worked. It felt brilliant.
Here's the problem. When you start running to lose weight, you learn a whole set of habits that make perfect sense for that goal:
Eat as little as you can get away with. Hunger means it's working.
Food has to be earned. Run first, eat later. Maybe.
Don't eat before a run. Why put back what you're about to burn off?
Gels and sports drinks are cheating. All that sugar, for a long run? No thanks.
Then, one day, the goal changes. You're not running to lose weight any more. You're training for a half, or a marathon, or a time. The mileage goes up. But the habits don't change, because nobody tells you they need to. You're still eating like someone on a diet while training like someone who's trying to get faster.
That was me for years, and I went round the loop more than once.
In 2014, friends started picking stills out of my ice bucket challenge video and pointing out my double chin (thanks, everyone). So I started running again to lose weight. Within a few months people were saying things like "where have you left Paul?" and "you're disappearing". If you've ever dieted, you'll know that's music to your ears.
By March 2015 I was running 50 to 60 miles a week, and I wrote this on my old blog:
"I know that I have to eat more to keep up with the 50-60 miles a week that I'm running, but the original goal still stands. I want to be a lean mean fighting machine. I don't really know what to eat, or when to eat, and how to keep the mind games in check."
I ended that post with a question: "How do you balance weight loss with serious training? Does anyone else get my struggle?"
Eleven years later, this post is my answer. And judging by the BBC article, yes. Plenty of people get it.
Years after that, I tried a different fix: no sugary treats at all. No biscuits, cakes, sweets or chocolate, for two whole years. I posted about it at the time, and admitted it had given me "a certain smug satisfaction" about my self-control, a badge of honour I wore with way too much pride. That post was also the first time I said out loud what was really going on. For many years, I'd defaulted to disordered eating: crash diets, fasting, cutting things out completely. It wasn't about sugar at all. It was something much deeper.
(I marked the end of it with a sticky toffee pudding at my gym's Christmas party. No regrets.)
The clearest example? In the build-up to the Manchester Marathon in 2023, I regularly did my long runs fasted. Then I stayed fasted until dinner, because I was still a bit heavier than I wanted to be and I believed it would help shift the weight. My longest run of the week, and then nothing until the evening. I thought that was discipline. Looking back, it was the exact opposite of what my body needed.
"You don't really have the physique for it"
Here's something else I've never hidden. In pretty much every marathon training block I've ever done, I've talked about wanting to lose weight. Not just to myself, either. I've said it out loud in my videos.
Part of that comes from a conversation about ten years ago. I told a fellow runner, a marathoner, that I really enjoyed distance running. His reply, word for word:
That surprises me... you don't really have the physique for it. You look more like a sprinter.
I've never forgotten it.
And I understand where the thinking comes from. Running a marathon means carrying your own body weight against gravity for 42 kilometres, so less weight can mean less work with every stride. I'm a touch taller than Sabastian Sawe, who ran the first ever sub-two-hour marathon in London this April, and he's reported to weigh around 60kg. It's very easy to look at that and draw a straight line from the scale to the stopwatch.
But here's my own story. I ran 2:59 at Manchester at 76kg, and I was not lean. I ran 2:47 in Valencia at around 73kg. Yes, I was a bit lighter for Valencia. I was also nearly three years fitter, stronger and more experienced. The scale was never the whole story, and chasing it is exactly what left me running on empty.
Stereotypes don't help. Telling someone they don't have the body for running is the kind of comment that sends people off to diet when they should be eating. Your body is the one doing the running. Feed it.
Race, illness, injury, repeat
When I read the BBC's headline, I laughed, because that was basically my Strava for a decade.
In 2015 I set myself the challenge of 12 marathons and 52 half marathons in a year. I had no idea about fuelling. I got injured, ran on empty all summer, and managed seven marathons before it fell apart. I wrote about that year in my story.
In 2024 I had a London Marathon place. In the build-up, I got a stress fracture in my foot. I collected my bib and stood there gutted, because I'd been strongly advised not to run. I can't prove that was down to fuelling, and nobody ever told me it was. But bone stress injuries are one of the classic warning signs of REDs, and when I look back at what I was eating against what I was running, I know which way I'd bet.
Then there's the bit I never talked about, because I thought it was normal.
In every marathon block I've ever done, my legs hurt every single day. Not injured. Just empty. Some days my recovery runs felt less like running and more like waddling. I had nothing to give. My paces were all over the place: one day I'd feel great, the next I'd be crawling at a heart rate that made no sense.
I thought that was just what marathon training felt like. Tired legs were the price of the miles. Everyone's tired, right?
I thought tired legs were the price of the miles. They were the price of not eating enough.
What changed
What finally opened my eyes was my Level 3 Personal Trainer course. Working through the course materials, I started applying what I was learning about nutrition and energy to my own training, and the picture didn't look great. But I also noticed that one idea barely came up: energy availability. In simple terms, that's how much energy your body has left over for everything else once your training has taken its share. The more I realised it wasn't really being talked about, the more I wanted to understand it, so I started doing my own research. That's how I ended up reading about REDs, and recognising myself in almost all of it.
At the start of September I started fuelling my training on purpose. Not "eating clean". Not a new diet. Just eating enough, every day, to match the training I'm actually doing, and treating food as part of the plan instead of a reward for finishing it.
The biggest change is the simplest one. For years I ran most of my runs fasted. Now I eat before I run. I post my breakfast every day too, as a way of holding myself accountable. It turns out a bowl of porridge is a lot easier to skip when nobody's watching.
I've also been using daily targets to keep myself honest, because after years of under-eating, my own sense of "enough" can't be trusted. It's like a petrol gauge that's always stuck on full.
And I'm tracking everything, every day: my resting heart rate and my weight, alongside the training. I want to be really clear about the weight bit. I've largely moved on from the days when I was obsessed with the number on the scale. Now it's just a data point, and an incredibly useful one. I'm not trying to make it go down. I'm using it, along with my resting heart rate, to work out what my baseline fuelling needs to be and to see how my body is responding to the training. The numbers tell me whether I'm keeping up with the work, instead of me guessing.
The key is that I look at the trend, not the day. Daily weight swings around massively depending on things like how much salt you've had and how hydrated you are, so a single reading tells you very little. I look at weekly averages instead. It's also why, in my opinion, weekly weigh-in programmes can be misleading: you're judging yourself on one snapshot of a number that moves every single day.
A word of warning, though. If tracking your weight would send your head somewhere unhelpful, don't do it. For some people, the number on the scale is the problem, not the solution. A registered dietitian can help you find a way of checking in that works for you.
Nearly two months in, here's what's happened:
The heavy, waddling legs have gone. Not once since September. Not one day.
My paces make sense. Easy runs feel easy at the same pace and heart rate, week after week.
My resting heart rate is consistently lower. A sign my body is recovering between sessions rather than limping from one to the next.
I'm getting faster. Without training any harder.
I wish I'd done this ten years ago. I'd run 2:45 for a marathon and I still didn't see it. It took a PT course and a lot of reading to join the dots, which is exactly why I wanted to write this.
Which leaves me with one question I can't stop asking. I ran 2:45 at Loch Ness on legs that, looking back, were running on empty half the time.
If I could run 2:45 under-fuelled, what could I do with some gas in the tank?
I don't know the answer yet. But I'm going to enjoy finding out.
Carbs aren't the enemy
If you've ever tried to lose weight, you'll know the drill: cut the carbs, pile on the protein. Ditch the pasta, live on chicken and eggs, and watch the scale drop.
And it does drop, fast. That's why it feels like it's working. But a lot of that quick drop isn't fat. Your body stores carbohydrate in your muscles and liver as glycogen, and every gram of glycogen is stored with roughly three grams of water. Cut your carbs and you empty those stores, and the water goes with them. The scale cheers. Your legs don't.
For a runner, that's a terrible trade. Glycogen is the fuel your muscles lean on for longer and harder running. Empty the tank on purpose and then wonder why every run feels like wading through treacle? I've been there.
So these days I'm doing the opposite of what my dieting brain wants. I'm keeping my protein at a good level for my body weight, because it matters for recovery and keeping hold of muscle. But I'm eating a lot more carbs than I used to. A lot more.
Which means I'm carrying more water. My weight goes up and down day to day as my glycogen stores fill and empty. And I'm completely fine with that, because that's not fat. That's fuel in the tank. It's another reason I only look at weekly averages. I won't be conned again by cutting carbs and suddenly "losing" a load of weight that was mostly water.
How much protein and carbohydrate you need depends on you, your body and your training, which is exactly what a registered dietitian or sports nutritionist can help you work out.
The signs it might be you
You don't need to be an elite athlete for this to apply to you. If you run a lot and recognise a few of these, it's worth taking seriously:
You're tired all the time. Not just after hard sessions. Heavy legs on easy days, every day.
Your paces are all over the place. Good day, terrible day, no obvious reason.
You keep picking up niggles. Especially bone stress injuries or stress fractures.
You're always getting ill. Colds that linger, one after another.
Your sleep is rubbish. Even though you're exhausted.
Your mood has dipped. Flat, irritable, not enjoying running the way you used to.
You've lost your drive. In men, low testosterone can show up as low libido and fatigue. Men often call it "burnout".
You're training harder but not getting faster. Or you've stopped improving altogether.
None of these on their own means you have REDs, and this list isn't a way to diagnose yourself. Plenty of things cause tiredness. But if several of them sound familiar, especially alongside a history of dieting or restricting food, it's worth a proper conversation with your GP.
If you'd rather tap than read, try this quick check. It isn't a test, and it won't tell you what's going on. It's a nudge to take note of…
Could I be under-fuelling?
Tap anything that sounds like you over the last month or so. Nothing is saved or sent anywhere. It stays on your screen.
This isn't a medical test or a diagnosis. It's a nudge to notice the signs. Only a doctor can tell you what's going on.
What to do about it
The good news, according to the experts in the BBC piece, is that REDs is reversible. Jake Smith is 10 to 15 kilos heavier than he used to be, and running faster than ever.
Here's where I'd start:
Talk to your GP first. Tell them how much you're training and what you've noticed, including any stress fractures. They can rule other things out and decide whether you need any tests or a referral.
Get advice from a qualified nutrition professional. That means a registered dietitian (check they're registered with the HCPC) or a registered sports and exercise nutritionist (look for SENr). They can work out what you actually need, which may well be more than you think. I'm deliberately not giving you a number here. Working out an individual fuelling plan is their job, not mine, and a number from a blog post is the last thing anyone with a complicated relationship with food needs.
Read up. Project REDs was set up by former GB runner Pippa Woolven for athletes of all ages, genders, sports and abilities. Their message is a good one to remember: anyone can experience REDs, and it's entirely preventable. They also have a list of specialists if you need one.
Listen. The Physiology of Endurance Running Podcast, hosted by Dan Nash and Owain Zerilli, turns sport science research into plain English, and it's covered this topic brilliantly. Start with REDS & Running Social Media, with Sian Secc, then Weight & Performance: Finding the Balance and Understanding body composition, with Dr James Fleming.
If food feels complicated for you, say so. If eating more feels scary, or food rules are running your life, that's not a fuelling problem to fix with a spreadsheet. Talk to your GP, or contact Beat, the UK's eating disorder charity.
Stop seeing tired legs as a badge of honour. Hard training should make you tired. It shouldn't make you empty.
So, 2015 Paul, here's your answer
You asked how to balance weight loss with serious training. Here's what I'd tell you now.
Stop trying to do both at the same time. When you're training hard, food has one job: to fuel the training and keep you healthy. That's it. If your weight is something you want to change, do it with proper help, at a time and in a way that doesn't fight your training.
And that lean mean fighting machine you wanted to be? It's no use to anyone if it's broken.
As for the mind games? You don't win those on your own. Tell someone. Your GP is a good place to start.
Does anyone else get my struggle?
That's how I ended my blog post in 2015, and I'll end this one the same way. If any of this sounds like you, I'd love to hear from you. Leave a comment below, drop me a message or find me on Instagram at @pksruns. And if you know a runner who's always tired, always injured and always "fine", send this their way.
Eat your breakfast. Your legs will thank you.
Have a great week, folks.
Paul
