You’ve heard the warning before, probably more than once. Fast for any real length of time and the muscle goes with the fat. Every “protect your gains” article says roughly the same thing: eat every three hours or your body starts breaking itself down.
So when I strapped 28kg to my back, the exact weight I’ve lost since Boxing Day 2023, and did a hundred squats with it fasted, I half expected my legs to argue back.
They didn’t.
That surprised me enough to go looking for a proper answer, not just my own experience. Then I did the test again, six and a half weeks later, to check it wasn’t a fluke. Here’s what actually happened, and what the research says about the muscle loss warning nobody seems to have tested on themselves.
The Advice I’d Been Following For Years
The advice is everywhere. Eat little and often, especially if you’re trying to hold onto muscle as you get older. Six small meals a day. Protein every three hours or the body starts breaking down what you’ve built. It’s in bodybuilding forums, it’s what personal trainers say, and if you’ve ever searched “muscle mass vs body fat men’s health check” you’ll have landed on some version of it.
And on the surface, it makes complete sense. If muscle needs protein to repair, and you’re not eating for sixteen, eighteen, twenty hours at a stretch, surely something has to give.
I believed it too, for years. It’s part of why I was nervous the first time I loaded a rucksack and squatted with it mid-fast.
What’s Actually Happening To The Muscle
Here’s what I didn’t know until I went looking properly. A team at the Norwegian School of Sport Sciences, led by Kristoffer Kolnes, ran a proper trial. Seven days, water only, tracking leg strength the entire way through.
Maximal leg strength: completely preserved. Even though the group had lost 4.6kg of lean mass by day seven.
I read that twice. If you’re losing mass and not eating, why does the strength hang around?
Turns out your body isn’t sentimental about muscle. Professor Brad Schoenfeld at Lehman College, a proper muscle scientist, describes it as the body’s core survival logic. It doesn’t know you live near a supermarket. It’s still running roughly the same calculation it ran for our ancestors: muscle is expensive to maintain, so it only keeps what’s earning its keep. Stop asking anything of it, and the instruction changes from “keep this for later” to “strip it for parts, we need the budget elsewhere.”
Which meant the hundred squats with the rucksack weren’t just a stunt for the camera. They were the instruction. Every rep was me telling my own body, in the only language it actually listens to: we’re still using this.
A second study took it further. Van Proeyen and his team, published in the Journal of Physiology in 2010, found that training fasted actually increased the body’s capacity to deposit protein into muscle. Not just hold onto what’s there. Build.
The “You Can’t Build Muscle While Fasting” Myth
So does fasting actually cause muscle loss? On the evidence above, not in any meaningful way, provided the muscle is still being used.
But there’s a second myth sitting underneath the first one: that you can’t build muscle while fasting at all. I’d assumed that too, for a long time.
Building muscle is a bit like building a house. You need bricks, that’s protein. You need builders, that’s the training stimulus. You need time, that’s recovery. The myth says the builders down tools if they’re not fed every three hours, which is roughly where the six-small-meals advice comes from.
The reality is closer to this. The builders work a shift, your eating window, then go home. They don’t need feeding overnight. As long as they get enough bricks during that shift, roughly 0.8 to 1g of protein per pound of bodyweight, and you give them work to do, the house still gets built. Meal frequency turns out to be close to irrelevant. Total protein plus a consistent stimulus is what matters.
Can you build muscle while fasting, then? The evidence suggests yes, as long as those two things are in place.
Two Tests, Six Weeks Apart
I wanted to know if this held up outside a lab, not just inside one.
15th September 2025: 25kg rucksack, a hundred squats, deep into a fast. I wasn’t even close to failing, which surprised me at the time, because I’d half expected to be crawling by the end.
31st October 2025, day ten of the Rhythm Reset: 28kg this time, the exact weight I’ve lost, a hundred squats again, fasted the whole way through.
Two separate tests, six and a half weeks apart, both mid-fast. Not one lucky day.
It wasn’t all straightforward, and I want to be honest about that bit too. After a 21-day fast last summer, day one back on sit-ups, I couldn’t manage a single rep. My body just wouldn’t respond. I panicked slightly, wondering if the fast had wrecked me after all. Day two, twelve reps, no problem. Nothing had actually changed overnight except the neural pathways reconnecting. Muscle preserved doesn’t mean instant coordination. Give it a day or two before you judge anything by how it feels.
What This Doesn’t Prove, And What I’d Do Differently
I want to be straight about the limits here, because a tidy story isn’t the same as an honest one.
A Polish research team found VO2 max, your aerobic capacity, dropped noticeably over an eight-day water fast. Strength and your aerobic engine aren’t the same system, and they don’t respond to fasting the same way. What I tested was strength. That’s not the same as saying every kind of performance is unaffected. It isn’t.
There’s also a genuine cost here, just not the one people assume. You cannot fit a new kitchen without ripping the old one out first. The MDF units and the laminate worktop have to come out before the solid oak and granite go in. That’s what happens every time you train seriously, fasted or not. The old, less efficient muscle protein gets broken down first. The soreness, the temporary weakness, that demolition phase isn’t damage. It’s the prerequisite for the upgrade. Skip it, and you keep the old kitchen.
None of this is really about willpower or discipline, and it never was. The “eat every three hours” advice wasn’t dishonest. It just came from a time before anyone had tracked strength through a proper water fast, in a lab, with someone actually measuring it. The guidelines haven’t caught up with the mechanism yet.
If you’re on medication that affects your heart rate, blood pressure, or blood sugar, or you’ve got joint issues, that’s a GP conversation before you add loaded training to a fasting pattern, not after. All of my own testing has been self-monitored, with regular GP reviews alongside it.
So here’s what I’d leave you with. If the muscle loss warning were as absolute as it’s usually delivered, none of what happened on that dry stone wall path in October should have been possible. It was. Twice. What would you actually find out about your own body, if you tested it instead of just believing what you’d been told?
🎯 Key Takeaways
- Strength held, even under loss: A controlled 7-day water fast preserved maximal leg strength despite 4.6kg of lean mass lost, per Kolnes et al.
- 28kg, twice, six weeks apart: Neil’s own rucksack squat tests, fasted both times, backed up what the research suggested.
- Meal frequency isn’t the driver: Total protein and a consistent training stimulus matter more than eating every three hours.
- Not every system responds the same way: Aerobic capacity (VO2 max) can drop over an extended fast even where strength holds.
- Use it or lose it, whatever your starting point: The mechanism applies whether you’re years into this or just starting to ask the question.
Curious What Your Own Body Could Handle?
This post covered the strength side of fasting, but it sits inside a bigger system: sleep, food and fasting, movement, and mindset, in that order. If you’re just starting to build your own fasting rhythm before testing anything loaded, the 14-Day Rhythm Reset Guide covers the fundamentals: eating windows, what to eat, and how to build from there safely.
👉 Get the 14-Day Rhythm Reset Guide (free)
💪 Want ongoing support? Join Rhythm Club for £30/month (includes FREE app access, community, and weekly accountability calls)
⚠️ Important: This is educational content based on evidence and personal experience. It is not medical advice. Speak to your GP before making changes to your fasting pattern, diet, or medication.
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