Day 11 of a 21-day fast, my ketone meter read 6.0. Deep, I thought. About as deep as a body normally goes.
Six days later, day 17, the meter didn’t give me a number at all. Just the letters “HI” on the little screen, because whatever was happening inside me by that point, the device simply wasn’t built to measure it.
I didn’t know that was even possible. blood tests going back to 2012, mine, all of it on the NHS record, and I still hit a reading nobody had prepared me for. This is what actually happened between those two dates, and why the gap between them mattered more than either number on its own.
Day 11: The Reading I Assumed Was My Ceiling
On 25 June 2025, day 11 of a 21-day fast, I logged the numbers same as any other day. GKI 0.5. Ketones 6.0 millimoles. Glucose 54. Weight 13 stone 6.
I remember thinking, right, that’s proper deep. Somewhere near the ceiling of what a body normally does.
Except I hadn’t actually thought that through. I’d just assumed it, which isn’t the same thing at all.
That assumption isn’t a daft one to make. Most of what you read about ketosis describes it as something that plateaus. You get into it, the numbers settle, and from there it’s about how long you sustain it rather than how deep it goes. Day 11 fit that story perfectly. Six months earlier, my deepest reading anywhere had been GKI 0.8, maybe 0.9. Half a point doesn’t sound like much until you remember every fraction lower gets harder to find.
Mentally, it was one of the sharper days of the whole 21. I’d written at the time that I felt sharper than I had in months, not a buzz exactly, more like interference had been switched off somewhere. Numbers usually just confirm what you’re already noticing. That’s what happened here.
Day 17: When the Meter Simply Gave Up
Six days on, same routine. Prick the finger, wait for the little screen to do its thing.
Except this time it didn’t give me a number. It just said “HI.”
Not an error message. Not the battery. The meter genuinely wasn’t built to measure what was happening. Ketones somewhere north of 8.0 millimoles, the device’s own ceiling, and GKI down around 0.30, best I could work out from the glucose reading either side of it. Weight was down to 13 stone 2, the lightest I’d been in thirty years.
Nothing had changed between the two readings. Same fast, same electrolytes, same routine. It wasn’t dramatic in the moment either. No dizziness, nothing wrong, actually felt sharper than most of the fast. Sitting there with a screen that had nothing to tell me, it wasn’t really “is this dangerous” I was thinking. More, bit of a shock if I’m honest, how did I not know this was even possible.
What “Maxed Out” Actually Means
GKI, the Glucose Ketone Index, is a simple ratio of the two. The lower the number, the deeper the metabolic shift into using fat and ketones for fuel instead of glucose. A GKI under 1.0 is generally considered deep therapeutic ketosis. Day 11’s 0.5 was already comfortably inside that range.
Day 17’s 0.30 wasn’t a small step further along the same scale. It was a reading most home meters simply aren’t calibrated to register, which is exactly why mine stopped being able to.
I sat with that a minute. Not worried. More just recalibrating what I thought I already knew. If day 11 was already deep, and day 17 went somewhere the device couldn’t even follow, then wherever I’d drawn the line in my head for “as deep as this gets,” I’d drawn it in the wrong place entirely.
The bit that actually surprised me wasn’t the number itself. It was realising the meter had a ceiling, and I’d hit it by accident, not on purpose. Was day 17 some rare outlier, or is that just where a fast this long eventually takes you if you let it run? I still don’t fully know. I haven’t deliberately gone looking for it again since, but it’s sat at the back of my mind as a genuinely open question.
The Council Round: Why This Is What Autophagy Actually Looks Like
That question sent me down a bit of a research rabbit hole. There’s a researcher called Mark Mattson, decades spent at the National Institute on Aging studying exactly this. His read: fasting is about the strongest natural trigger going for autophagy, the process that clears out damaged proteins and knackered mitochondria at a cellular level. There’s Nobel Prize-winning science behind the mechanism too, a biologist called Yoshinori Ohsumi won it for mapping out how the whole process actually works.
Which is basically bin day, if you want the version that made it click for me. Ever lived somewhere the bins didn’t go out for a month? Bags pile up by the back door, the smell builds, flies turn up eventually. That’s roughly what’s happening inside your cells without autophagy running: junk proteins, damaged mitochondria, misfolded rubbish, piling up because nothing’s clearing it out. Fast for long enough, especially past the 48-hour mark, and the council lorry finally turns up. Clears the lot. Hoses the pavement down. You didn’t even realise how bad the smell had got until it’s gone and you can breathe again.
Day 17, best I can tell, is what a proper thorough bin day looks like on paper.
This isn’t something you’ll find explained clearly in most places. Not because anyone’s hiding it, more that home biomarker tracking at this depth is still fairly niche, and the research on very deep, extended ketosis is thinner than the research on shorter fasts. The evidence on autophagy timing is genuinely still developing.
What I Took From Day 17 (And What I Didn’t)
Here’s the honest bit. Nothing actually changed for me after that reading. No new protocol, no different approach to the next fast. In my case, it was mostly just nice to know. Confirmation I really was deep into what fasting’s actually for, for me, which was never the weight loss. That’s the side effect. The real reason I do this is to clean the system out from the inside, and day 17 was about as clear a proof of that as I’ve had.
I can’t say GKI 0.30 is a target worth chasing. I genuinely don’t know if it did anything for me that 0.8 hadn’t already started doing. What I can say is it answered a question I didn’t know I had: yes, this goes further than I’d assumed. Whether it should is an entirely different question, and one I still can’t answer for anyone but myself.
If you’re already tracking your own ketones or glucose through longer fasts, and you’re seeing GKI keep dropping without the plateau you expected, that’s worth paying attention to, not panicking about. If you’re on any medication that affects blood sugar, that’s a GP conversation before you go looking for your own version of day 17.
And if you’re newer to fasting and wondering when autophagy actually starts, you don’t need day 17, or anything close to it. From what I’ve read since, and what my own body’s told me on much shorter fasts, autophagy isn’t a switch that flips at some heroic depth. It ramps up gradually, starting a lot earlier than people assume. You don’t need to find your meter’s ceiling for it to be doing something.
That was never really the point. I’ll leave that with you.
🎯 Key Takeaways
- Meters have a ceiling: mine maxed out at 8.0 mmol/L on day 17, six days after a reading I’d already assumed was near my limit
- GKI 0.30 was the deepest reading in tracking going back to 2012, well past the under-1.0 threshold for deep ketosis
- Autophagy is fasting’s strongest natural trigger, clearing damaged cellular material, most active past the 48-hour mark
- You don’t need extreme depth for autophagy to be working: it ramps up gradually and starts long before day 17
- Nothing changed in my approach after this reading. It was confirmation, not a new target to chase
Want to Track Your Own Numbers Like This?
If you’re fasting and curious what your own GKI, ketones and glucose are actually doing day to day, the 14-Day Rhythm Reset Guide walks through the fundamentals of building a tracking habit that actually tells you something, without needing a reading like day 17 to make it worthwhile.
👉 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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