Somewhere in your forties, the things that always worked just stop working as well. Same effort. Less to show for it. You cut back, you exercise, you follow the advice — and the numbers barely move, or the energy stays flat, or the weight creeps back within three months. That is not a character flaw. That is a specific biological situation, and it has a specific explanation.
I hit that wall at fifty. I was on six different medications. My HbA1c was forty-seven — pre-diabetic territory, according to my GP. He’d used the phrase “heading in the wrong direction.” I’d done everything the NHS leaflets suggested. And it hadn’t been enough.
So I fasted. Properly. Eleven days. With my GP watching the bloods throughout.
Here’s what changed — and why the blood result that surprised both of us had nothing to do with willpower.
Why Your Metabolism Behaves Differently After 40
The mechanism first, because without it the numbers don’t make any sense.
Jason Fung — kidney doctor, wrote The Obesity Code, runs a dietary management programme in Toronto — describes it in terms that are hard to argue with. The body, he says, exists in one of two states. Fed or fasted. That is it. When you eat, insulin goes up. When insulin is up, the door to your fat store is shut. Your body physically cannot access that fuel while insulin is present. When you fast, insulin drops. The door opens.
That is not a theory. That is basic physiology.
Here is where the after-forty piece lands. Imagine knocking on your neighbour’s door forty times a day. First week, they answer every time. Month two, they start ignoring you. Month six, they have stopped answering altogether. That is insulin resistance. Your cells have heard “let glucose in” so many times they have stopped listening properly. Fasting does not just lower insulin — it gives your neighbour a break. Three weeks of silence. Week three, they answer the door again.
This is what decades of three meals a day, grazing, snacking between meals does. Not damage, exactly. But the signal gets tuned out. And that is why the same eating pattern that kept your weight stable at twenty-five produces different results at forty-five. The mechanism has not broken. It has been heard too many times to keep responding.
The Switch: What Actually Happens in the First Three Days
Mark Mattson — neuroscientist, spent years as head of the laboratory of neurosciences at the National Institute on Aging in the US, published in the New England Journal of Medicine on this subject — puts a number on something most people get wrong. The liver stores enough glucose to last roughly twelve hours. After twelve hours of fasting, that store runs out. Fat cells release fatty acids. The liver converts them to ketones. Your body has changed fuel source entirely.
He calls this The Switch. And it is not a gentle transition.
Day one of an extended fast feels like running a marathon in work shoes. Day two, energy drops further and you start questioning what exactly you thought you were doing. That is not your body failing. That is your body burning through the last of the glucose — scraping the tank — and not quite finding the new fuel yet. Day three, something changes. The hunger that has been persistent since hour one quietens. Not gone. Quietened. Mental fog lifts. The Switch has happened.
The discomfort of days one and two is not a warning sign. It is your metabolic machinery learning a skill it has not used in years. Like learning to drive manual after fifteen years of automatic. First week you stall at every roundabout. Month three, you shift without thinking. The Quiet is coming. But you have to get through the stalling phase first.
The reason this matters beyond my experience is that the mechanism does not wait for a diagnosis. It operates in bodies running on chronically elevated insulin for years before anything gets flagged on a blood test. Understanding what The Switch actually is — and what it takes to get there — gives you options earlier, not just after the GP conversation has already happened.
The Blood Results — Including the One My GP Didn’t Expect
I want to show you the trajectory, because one set of readings tells you very little. Thirteen months of data tells you something real.
October 2022. HbA1c: forty-seven millimoles per mole. Pre-diabetic range. January 2025 — thirteen months after I changed the inputs — it had dropped to forty. Normal range. April 2025, thirty-seven. Optimal. November 2025, still thirty-seven. That number has not moved.
That is not a fluke. That is insulin sensitivity restoring over time. The same mechanism Fung describes. The same timeline — it takes months for cellular resistance to unwind, not days.
The ALT — a liver enzyme — went from sixty-nine in December 2023 down to ten by November 2025. I was not targeting that. It was a surprise. What it most likely represents is intrahepatic fat clearing — fat that had been sitting inside the liver cells, the fat that directly blocks insulin receptors. Gone. Visible in the blood tests.
Now, the eleven-day fast specifically. January 2025. Fifteen stone two at the start. Thirteen stone twelve at the end. One stone four pounds over ten days and fourteen hours. But the weight was almost secondary to what I was tracking on the ketone meter.
By day three, GKI was under three. Therapeutic ketosis. Not at hour forty-eight, as most of the literature suggests. Earlier. The fifty-year-old body, properly fat-adapted from eighteen months of eighteen-six fasting before this, made The Switch faster than the textbook timeline.
Then October 2025. Day eight of a reset. Twenty-six hours fasted. GKI two-point-seven. Ketones one-point-six. Glucose seventy-seven milligrams per decilitre. Therapeutic ketosis at twenty-six hours. Not forty-eight. Not seventy-two.
I want to be careful here — I cannot say that is what would happen for everyone. I can say it is what happened for me. What I think it represents is a metabolically adapted body finding the lower gears faster because it has had practice. The Gearbox, once trained, shifts more easily.
And then February 2024. First bloods after I had stopped the medications and been fasting consistently for about six weeks. My GP had been cautious when I told him what I was doing. Not obstructive — sensible, careful caution. The blood tests came back. Kidney function: excellent. Liver: improving. HbA1c: moving in the right direction. He said: keep going. Not grudgingly. Genuinely.
The Part Nobody Warned Me About
There is a piece of the science that I find genuinely interesting, and that I had not come across until fairly deep into this.
William Sears and Vincent Fortanasce — both medical doctors, combined decades of research — make a specific point about the ageing brain. As brains age, they lose some of their efficiency with glucose as a fuel. They start to struggle, slightly, to use it as reliably as they once did. But here is what they also note: the brain retains its full ability to run on ketones. Unchanged. Ketones, they write, are a cleaner-burning fuel — less oxidative wear, less metabolic exhaust.
So the ageing brain does not become less capable of running on ketones. If anything, fasting becomes more relevant as you get older, not less.
That is not a marginal observation. That is the mechanism being, if anything, kinder to a fifty-two-year-old than to a twenty-five-year-old.
One important practical counterpoint here, from James Betts at the University of Bath. After multi-day fasting, insulin sensitivity can paradoxically drop slightly in the immediate period before the first meal. Which means the first thing you eat after an extended fast matters. A high-carbohydrate break-fast, when the body is primed for insulin resistance at that moment, is a poor choice. Plan what you are going to eat before the fast ends — not when hunger is making the decision. That is not a reason to avoid fasting. It is a reason to think carefully about how you come out of it.
What I Would Actually Do Differently Starting from Scratch
If I were starting this over — specifically after forty, with what I know now — there is one thing I would do before changing anything else.
Track fasting glucose for four to six weeks. A basic meter costs about fifteen pounds at Boots. First thing in the morning, before food or drink. Not one reading — a trend. That number tells you where your insulin baseline actually sits. It is the single most useful piece of information for knowing where you are starting from.
Then, first four weeks: nothing dramatic. A sixteen-eight fasting window. Eight hours eating, sixteen hours fasted. No calorie counting. No food overhaul. Just compress the window and give the neighbour some quiet. Do that consistently for a month before changing anything else.
The reason I would start there and not with an eleven-day fast is that an eleven-day fast is only genuinely useful once The Gearbox has been trained. Jumping straight to extended fasting without building metabolic flexibility first is like sitting your driving test after one lesson. Stressful, not particularly productive, and you stall in traffic.
How to track progress: fasting glucose every morning, trend over weeks. Waist measurement weekly — more informative than scale weight for what is happening metabolically. If you can get an HbA1c at three months, do it. That is your first real data point on whether insulin sensitivity is shifting.
What the timeline actually looks like: weeks one and two will be adaptation. Energy inconsistent, hunger patterns shifting, sleep potentially rougher than usual. That is normal. Do not judge it. Weeks three and four, hunger signals start to change — the waves become more predictable, The Tantrum less alarming. Month three is worth a blood test. Six months is where a meaningful comparison becomes possible.
And one hard requirement that I am not softening. If you are on diabetes medication, blood pressure medication, or anything where food intake affects your dosing — speak to your GP before you start. Not as a formality. Fasting changes insulin levels quickly and significantly, and medication calibrated for your fed state may not be right for your fasted state. That is a clinical conversation, not a YouTube one.
🎯 Key Takeaways
- The mechanism doesn’t expire after 40: Fat mobilisation and insulin response work the same way — the baseline resistance is just higher from more years of elevated insulin. Get through days one and two and the physiology kicks in as it should.
- HbA1c dropped from 47 to 37 over 13 months: Not three weeks. Thirteen months of consistent inputs. That is what the data looks like when something is actually working.
- The Switch comes earlier once you are fat-adapted: Twenty-six hours to therapeutic ketosis after eighteen months of practice. The Gearbox, once trained, finds the lower gears faster.
- The ageing brain runs well on ketones: Glucose efficiency declines slightly with age. Ketone efficiency does not. Fasting becomes more metabolically relevant later in life, not less.
- Plan the refeed before the fast ends: Post-fast insulin sensitivity can dip temporarily. What you eat first matters. Decide before hunger makes the decision.
- Build The Gearbox before attempting extended fasting: Sixteen-eight for a month before anything else. Stalling at roundabouts is normal. Month three, you shift without thinking.
Want to Track What I Have Been Tracking?
If you want to start watching fasting glucose, GKI, and ketone trends rather than just individual readings, I have built a fasting tracker that logs all of it. Trend analysis, not single data points. Free.
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⚠️ 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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