"I Didn't Stop Hiking Because I Got Old. I Stopped Because Getting Up The Next Morning Cost Me The Whole Day."

A 68-year-old on the arithmetic nobody warns you about - and the single number that made her look at this again after six years of writing it off.

I stopped hiking at 64.

Not because I got old. Because I did the eight-mile loop on a Saturday, came home perfectly fine, and then spent the whole of Sunday in a chair.

So the next month I did six.

Then six, and cancelled the Sunday plans in advance.

Then I stopped putting it in the calendar at all - because I'd stopped being willing to spend two days on one.

That took eleven months. I did not notice a single step of it.

It never once felt like a decision. Every subtraction was sensible on its own. That's exactly what makes it invisible.

You start arranging the week around what your body will allow. One thing per day. Never two days running. You decline the evening because there was already a daytime, and you give a reason that isn't the reason - because the honest version, I can't afford tomorrow, sounds far worse out loud than it feels.

I want to be precise about this, because every leaflet aimed at women my age gets it wrong.

I was not in pain. I was not frail. I could still have done the eight miles that Saturday and the Saturday after.

I simply couldn't pay for it any more.

What shrank wasn't what I could do. It was how far from my own front door I was still willing to plan.

Eight miles to six in eleven months. Then six to none.

I sat down once and worked out what that rate does over five years, and then I stopped working it out, because the answer was a woman who doesn't leave the village.

Nobody sends you a letter about this. There's no appointment where someone says this is the year it started. You just quietly become a smaller version of yourself and adjust to each step as it happens.

That evening is the reason you're reading any of this.

Two reasons. I stand by both.

The first is that I was already trying to take fewer pills, not more. There's a drawer in my kitchen and I have spent years getting it emptier. My GP and I have had that conversation twice. Adding something back needed a better reason than an advertisement.

The second is that this industry has a habit of finding women my age and lying to them. A memory supplement was taken to court over its claims. There are "trials" that start at $19.95 and quietly become $79.95 a month with a cancellation number nobody answers. A friend of mine calls the whole shelf expensive urine, and most weeks she's right.

I'd bought an NMN capsule years ago on somebody's recommendation. Two months. Nothing. Threw it out.

It came from a woman in my walking group. She's 74, she does the full loop while the rest of us cut the corner, and she is not a person who forwards things.

She sent me one finding. It wasn't a flattering one.

A trial of 140 healthy adults, aged 40 to 60. Three doses, eight weeks, blood measured in a laboratory. The 300 mg group's NAD+ went up 51%. The 1,000 mg group's went up 142%.

And the 100 mg group was no different from placebo. At every single measurement. Not slightly weaker. The same as nothing.

I went and found the bottle I'd thrown out.

It was 250 mg. Split across two capsules.

Six years. I had never once tested the thing I thought I'd tested. I'd tested a dose that was already known not to work, decided the category was nonsense, and shrunk my life by four miles while I was at it.

I'm not a scientist. I ran a school office for thirty-one years, which mostly means I know what a document looks like when somebody has actually signed it.

First: is there anything in it at all. An independent lab put fifteen NMN products through analysis and published the contents. The range ran from nothing to slightly over the label. Three of them had no detectable NMN whatsoever. That is the real scandal in this business and nobody advertising in it mentions it.

OMRE tests every batch twice. Once on the raw powder - right molecule, how pure. Once on the finished capsule - actives, heavy metals, bacteria, mould. Purity printed as a number, over 99%, rather than the words "high purity," which mean nothing at all. GMP, FDA-registered facility in the United States. Formulated by a physician.

I asked for the certificate. It came by email. One flaw, which I'd want told to me: you can't type your own lot number into their site and pull your own batch. It's on the page and sent on request. Further than most. Not perfect.

Second: the label. Two capsules. 500 mg NMN, 500 mg trans-resveratrol, 5 mg of a black pepper extract for absorption. Every amount printed, nothing hidden inside a blend. I took that paragraph to my doctor and I'd tell you to do the same - resveratrol thins the blood slightly, and it matters if you're on a thinner, or blood pressure medication, or anything for diabetes.

Third: the trap. They do run a subscription, and it's cheaper - but nothing pushes you into it. I bought a single bottle, once, and nothing renewed and there was nothing to cancel on a Sunday evening. After the stories I've heard, that mattered more than it probably should.

It is not a joint or a pain product. Anyone telling you otherwise is selling you something this can't do.

Raising the molecule is not the same as changing an outcome. In one trial of 42 older adults, blood NAD+ doubled and the memory scores did not move at all.

Trials miss. In a 60-person walking study, the main thing the researchers set out to measure didn't improve. A couple of secondary things did.

I'd rather hand you those three than have you find them yourself and wonder what else I skipped.

It's about $2.30 a day.

For scale: there are clinics charging $300 to $1,500 a session to drip this molecule into your arm, and then booking you in for the next one.

About one review in ten is negative, and the two complaints are the price and "I didn't notice anything." Some people take it three months and get nothing. That's true and I won't pretend around it.

And the windows don't line up. Most people who notice something notice between week four and month three. The refund window is thirty days. One bottle is thirty days.

Which means a one-bottle trial is a trial built to come out negative. I'd have failed my own test if I'd bought one bottle. Nearly a third of their NMN customers take the three-pack - $207 - without anyone suggesting it, and now I understand why.

So do what I did. Two dates on the calendar the day it arrives:

Day 25 - decide. That's how you protect yourself from the mismatch. They refund an opened bottle, which is more than most of this shelf will do.

Week eight - run the two-day test. Do a full day out. Then put something small in the calendar for the following morning, in advance. Whether you keep it is your answer. It's a far better answer than how you happen to feel on a Tuesday.

There's a map in my hallway with pins in it. My husband started that. After he died I kept it going, which surprised us both, I think.

I know the exact year the pins stopped moving. I just didn't notice at the time.

I'm not going to tell you this handed me back my forties. I'm 68 and the eight-mile loop may well be finished.

But there's a pin in that map from June that wasn't there last year.

The radius came back before the mileage did. That was always the part worth having.