Take all the blood out of a grown person. Five litres of it, in a bucket. Now answer me this: how much actual sugar is dissolved in all of it, swimming through every vein, the moment your blood sugar is perfectly normal?

I would have guessed a cupful. A small bowl, maybe. The real answer is one cube. A single teaspoon. That is the whole budget. That is how finely the body holds the line, because glucose is the strangest pair of things at once, the fuel you cannot live without and a poison if there is a teaspoon too much. One cube in five litres, regulated minute by minute, all day, your whole life, without your permission or your thanks.

I heard that figure this week and I have not been able to put it down. Not because of the chemistry. Because of what it does to the story I tell about myself.

I have spent most of my life with a story and almost no measurement. The story said: I eat to quiet something. The story said: I want to be thinner, and I want to disappear, and somehow those are the same wish wearing two coats. The story was true, as far as it went. But a story is a soft thing. You can edit it in the dark. You can tell it generously on a good day and cruelly on a bad one, and either way it answers to you, not to the facts. For twenty years I instrumented other people’s systems for a living. I put a number on the thing so the arguing would stop. It never once occurred to me that I had left my own life entirely unmeasured, running on story alone.

The doctor I have been studying says the same thing in plainer words. See yourself as an experiment, he says. Do not be frightened of experimenting. But if you are going to experiment, measure something. And then he holds up the small device on his arm, the monitor that tells his phone what his blood sugar is doing, right now, in real time. He says the thing that has stayed with me all week: once you have seen it on your phone, you cannot unsee it.

You cannot unsee it. I keep turning that over, because it sounds at first like a threat and I have come to hear it as a mercy.

Here is the cruelty he describes, and you should brace yourself for it, because it is a true story about a real man and it is the most honest picture of a human being I have read in a long time. A successful man, intelligent, wealthy, sick. He needed surgery he could not have, because his blood sugar was too high for the surgeon to touch him, and his blood sugar was too high because he could not stop eating bread. His wife found him getting up at four in the morning to eat it from the fridge. So she started throwing it in the bin. So he ate it from the bin. So she poured detergent over it. So he ate it anyway. In the end she sprayed bleach on the bread and left the can standing beside the bin so he would know, and the doctor tells you all of this not to shame the man, who consented to the telling because he wanted to help someone, but to make one point land: this is not weakness of character. This is a thing that calls. It says eat me, eat me, and intelligence is no defence against it, and self-control is no defence, and the only thing that ever begins to work is the thing nobody wants to do first.

Honesty. He says it flatly. Step one is not a diet. Step one is to be honest about what the problem actually is, even if you can only be honest with yourself and no one else on earth. Which foods. Which hour. What is really happening at four in the morning. Because if you are not honest, he says, how could you ever sort it out? You would be solving the wrong equation. You would be optimising a system you refuse to look at.

And this is exactly where the measurement comes in, and why I have stopped hearing it as surveillance. The story can lie. The number cannot. The monitor does not call you weak. It does not convene a courtroom. It simply shows you the line on the screen, the spike after the thing you swore was harmless, the flat calm after the meal you doubted. It takes the argument out of your own head and puts it on the glass where you can finally see it. The bleach on the bread is what a life looks like when the story is all there is, when honesty has been refused for so long that another person has to stand guard over a bin. The quiet line on the screen is what it looks like when you finally let yourself be measured.

There is a detail in his account I cannot stop thinking about. He says he can see, on his own monitor, when a conversation stresses him. The number lifts. He told the man interviewing him, near the end of three hours together, my blood sugar stayed flat the whole time. I felt safe with you. That is your feedback. The body had kept a record the mind would have been too polite to admit. The instrument knew before the man did. It always does. The body is the most honest witness in the house, and I have spent decades trying to argue it down.

So this is what I am taking, on a Tuesday I am fasting, with the chocolate I actually love in the drawer and no shame attached to it. I am going to stop running my life on story alone. Not because the story is worthless, but because it answers to me, and I have proven, over thirty years, that I am an unreliable judge in my own case. I am going to measure something. Pick one true number and watch it without flinching. Let the screen say what the story will not.

And I am going to be honest first, before any plan, about which things call to me and at what hour, because a problem you will not name is a problem you have agreed to keep. The man at the bin was not a fool. He was a man who could not, for years, say the true sentence out loud. The bleach was the price of the silence.

You cannot unsee it, the doctor says. He means it as the good news and I have finally heard it that way. Once you measure the thing, you can no longer be fooled by your own kind editing. The dark place where the story gets rewritten goes light. It is the end of a certain loneliness, the loneliness of being the only witness to your own life and a dishonest one at that.

Measure something. Tell yourself one true thing. Then begin.