Do I need a glucose monitor if I don't have diabetes?

Probably not. Continuous glucose monitors were built for people with diabetes, and I couldn’t find any NHS or Diabetes UK guidance that supports wearing one to choose food if you don’t have it. In people without diabetes, readings for the same meal vary between sensors and from one week to the next, which makes them a shaky guide.

What does a glucose monitor measure?

A continuous glucose monitor is a small sensor that sits under your skin and reads your glucose day and night. It doesn’t read your blood. It reads the fluid around your cells, which lags behind your blood by up to 15 minutes, and Diabetes UK notes the readings are not quite as accurate as a finger prick, with the gap more likely to be bigger when you are eating or exercising. Straight after a meal is exactly when you would be checking it.

For people with diabetes these sensors are valuable, and on the NHS people with type 1 diabetes should generally be offered one, along with some people with type 2. That is the job they were designed for.

What do normal glucose levels look like without diabetes?

They move, and that is the point. In a 2019 study, 153 healthy people in the US, aged 7 to 80, wore a sensor for up to ten days. Their glucose sat between 3.9 and 7.8 mmol/L for a median of 96 per cent of the time, with about 2 per cent above that range and about 1 per cent below it.

Your glucose rising after a bowl of pasta is a body doing what it should. A sensor turns that normal rise into a line on a graph, and it is easy to look at a peak and see a problem that is not there.

Can a glucose monitor tell me which foods suit me?

Not reliably, on the research so far. Three studies have tested the promises these devices are sold on.

The first two were carried out on research wards where the meals were provided, which is far tidier than a working week, and I wouldn’t expect the readings to be steadier at home. The second study first appeared as a preprint, so hold it a little more loosely, though it agrees with the first.

Will a glucose monitor explain my afternoon slump?

No, and this is where the claims around sensors run furthest ahead of the evidence. The afternoon dip in alertness is largely your body clock. It turns up even in people who have had no lunch and have no idea what time it is. A heavy or high carbohydrate lunch makes it worse, and so does a poor night’s sleep.

The study in the last row of that table often gets quoted as if it proved that a glucose dip makes you tired. It measured hunger and how much people ate, and a dip two to three hours after a meal predicted more of both. It also asked how alert people felt, and the dip did not predict that. So a sensor might show you why you want a biscuit at four o’clock. It cannot show you why your eyes are heavy at two. Several of that study’s authors were or had been employees of a commercial nutrition company, which is worth knowing when you see it quoted in an advert.

If the slump is what made you look at a sensor in the first place, the afternoon crash quiz is a better starting point, and why you crash after lunch explains what does and does not help.

Is there any harm in trying one?

For most people, the harm is in what the numbers do to how you eat. A reading every few minutes invites you to judge each meal by its peak, and plenty of good food, a bowl of porridge, a banana, a jacket potato, makes glucose rise. If you have ever found yourself anxious about food, or have a history of restricting it, I would keep well away from a live number that updates all day.

There is also the cost, which I won’t quote because it changes, but which buys you data the research says is hard to act on.

What should I do instead if I am worried about my blood sugar?

Go to your GP practice, which can do a proper blood test. The NHS lists feeling very tired, peeing more than usual, feeling thirsty all the time and losing weight without trying among the symptoms of type 2 diabetes, and says “Not everyone with type 2 diabetes has symptoms, so it’s important to get checked out.” See a GP if you think you may have it, or if you are at higher risk.

In England, if you are aged 40 to 74 and do not already have diabetes or certain other conditions, you should be invited to an NHS Health Check every 5 years, which may include a blood sugar test. If yours has not come through, ask.

If what you want is fewer mid morning raids on the biscuit tin, the plate is a better tool than the sensor: protein, fibre and a wholegrain at each meal, with breakfasts that keep you full as a place to start, and an honest look at whether the order you eat things in matters.

If you already have diabetes, none of this is about you. Your sensor, and how you use it, is a conversation for your own diabetes team.