Nutrition, Diet & Fasting - Food foundations
Continuous glucose monitors if you are not diabetic
Last updated 30 September 2026. How we write this .
Without diabetes, a glucose monitor mostly shows normal ups and downs. Healthy people spike after meals, a popular sensor read high in testing, and there is no good evidence that tracking helps.
Start here Without diabetes, a glucose monitor mostly shows normal ups and downs. Healthy people spike after meals, a popular sensor read high in testing, and there is no good evidence that tracking helps.
By the end, you will understand What does a CGM actually measure? What does normal glucose look like on a CGM? Are glucose spikes a sign that something is wrong? Key words used in this lesson Systematic review A review that searches for all the evidence on a question using a pre-declared method, rather than selecting studies that suit an argument. The method is what distinguishes it from an ordinary review article. Full plain-English glossary
Where this information comes from Early human evidence. Small or preliminary human studies exist. Findings need confirmation in larger, better-controlled research.
This lesson links the specific studies and guidance used for its central claims.
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Report an error or outdated claim .
If you do not have diabetes, a continuous glucose monitor (CGM) will mostly show you a body doing its job. Healthy people's glucose rises after meals, sometimes briefly into ranges that sound alarming, and comes back down. In a recent trial, a popular sensor also read higher than finger-prick tests, and so far there is no good evidence that wearing one improves health in people whose glucose is already normal.
UK clinical guidelines recommend CGMs for people with type 1 diabetes and for some people with type 2 diabetes who use insulin. The wellness market sells the same kind of sensor to everyone else, with a very different promise.
What does a CGM actually measure?
A CGM is a small sensor worn on the skin. It does not sample blood. It measures glucose in interstitial fluid, the fluid that surrounds your cells, and takes a reading every few minutes. Those readings are turned into a continuous line on a phone or reader.
That line is an estimate. The sensor uses the glucose in interstitial fluid to estimate the level in your blood, so there is good reason to expect the two to differ. Small errors matter less when the job is catching dangerous highs and lows in someone with diabetes. They matter more when someone without diabetes is reading meaning into every small rise.
What does normal glucose look like on a CGM?
The best answer comes from a 2019 study in the Journal of Clinical Endocrinology and Metabolism led by Shah. Researchers at 12 US centres fitted 153 healthy people aged 7 to 80, none with diabetes and none with obesity, with a CGM for up to 10 days. The sensors were blinded, so participants could not see their readings and change their behaviour.
Average glucose was 5.4 to 5.5 mmol/L in every age group except the over-60s, where it was 5.8. People spent a median of 96% of the day between 3.9 and 7.8 mmol/L. They also spent a median of about 30 minutes a day above 7.8 and about 15 minutes a day below 3.9. In other words, short spells above and below the textbook range are a normal part of a healthy day.
Are glucose spikes a sign that something is wrong?
This is the question behind most CGM marketing, and it rests heavily on one study. In 2018, Hall and colleagues at Stanford University gave CGMs to 57 adults who had no prior diabetes diagnosis and tracked them for two to four weeks. Participants also ate three standard breakfasts: cornflakes with milk, a peanut butter sandwich and a protein bar. The cornflakes and milk pushed 80% of participants above 7.8 mmol/L, the level the study used to mark the prediabetic range.
The team grouped people into what they called glucotypes, based on how much their glucose swung. About a quarter of the people with normal standard blood tests fell into the most variable group, and they spent up to 15% of their recorded time in the prediabetic range. The paper was widely reported as showing that many apparently healthy people have hidden glucose problems.
In 2021, researchers from Denmark and the Netherlands, led by Hulman, published a critique in the same journal. They pointed out that Hall's study rested on only 57 people, and they tested the glucotype idea in two independent groups. Glucotypes offered no more insight than a person's plain average CGM glucose. In a group of 116 people with prediabetes, average glucose alone correctly predicted the glucotype of 107.
So here is the honest uncertainty. Nobody yet knows whether brief after-meal rises in people with normal blood tests predict future disease, or what a healthy pattern should look like beyond averages. A 2024 review in Diabetic Medicine, the journal of Diabetes UK, found no agreed clinical benchmarks for judging CGM data in people without diabetes and a lack of consistent, high-quality evidence for the uses the wellness market promotes.
Can a CGM tell you how you react to a specific food?
Less reliably than it seems. In a 2025 University of Bath trial in the American Journal of Clinical Nutrition, led by Hutchins, 15 healthy adults wore a FreeStyle Libre 2 sensor while also giving finger-prick samples after a series of test drinks and fruit. The sensor read about 0.9 mmol/L higher than the finger-prick on average, both fasting and after eating. It overestimated time spent above 7.8 mmol/L about fourfold. Adjusting for each person's starting reading cut that to about twofold, but did not remove it.
The size of the error also varied from person to person and from food to food. For one smoothie, the sensor gave a noticeably higher glycaemic index, a ranking of how quickly a food raises blood glucose, than the finger-prick did. The authors advised caution when using CGMs to judge responses to foods, and said finger-prick sampling should be preferred for accurate measurement.
Does wearing one improve your health?
The best summary so far is a 2026 systematic review in the European Journal of Medical Research by Liao and colleagues. It pooled 23 studies with 1,074 people without diabetes from 11 countries, though only 7 were randomised controlled trials. CGM use was linked with a modest fall in average glucose compared with controls, and with people sticking to diet changes more closely. It made no significant difference to body mass index. The glucose benefit appeared in people with prediabetes, and no appreciable benefit was seen in people whose glucose was already normal.
Most of those studies paired the sensor with a structured lifestyle programme, so the device was one part of a wider package. The Diabetic Medicine review also flagged a risk that has barely been studied: that wearing a CGM could have harmful effects on some people's eating habits. Its authors called for commercial claims about CGMs in people without diabetes to be labelled misleading.
Who are CGMs actually for, and how should you check your blood sugar?
NICE recommends flash CGMs for adults with type 2 diabetes who take multiple daily insulin injections and have recurrent or severe low blood sugar, cannot sense when their sugar is low, cannot do finger-prick tests, or would otherwise need to test at least 8 times a day. NICE limited its recommendations to people on insulin because it found evidence of cost effectiveness there and none for people not using insulin.
If you want to know whether your blood sugar is healthy, the NHS route is a blood test that checks your blood glucose level. People aged 40 to 74 without an existing health condition can ask their GP surgery or local pharmacy for a free NHS Health Check. See a GP if you have the common symptoms of type 2 diabetes: feeling very tired, peeing more than usual, feeling thirsty all the time or losing weight without trying. Not everyone gets symptoms, so a check is worth having if you are at higher risk.
This lesson is general education, not medical advice. If you are worried about your blood sugar, ask your GP or pharmacist about a blood test.
Related in this classroom Related in other classrooms The studies behind this page Continuous glucose monitoring profiles in healthy nondiabetic participants: a multicenter prospective study - Journal of Clinical Endocrinology and Metabolism, 2019Glucotypes reveal new patterns of glucose dysregulation - PLOS Biology, 2018Towards precision medicine in diabetes? A critical review of glucotypes - PLOS Biology, 2021Innovative solution or cause for concern? The use of continuous glucose monitors in people not living with diabetes: a narrative review - Diabetic Medicine, 2024Continuous glucose monitor overestimates glycemia, with the magnitude of bias varying by postprandial test and individual: a randomized crossover trial - American Journal of Clinical Nutrition, 2025Continuous glucose monitoring in non-diabetic populations: a systematic review of observational and interventional studies with meta-analysis - European Journal of Medical Research, 2026Type 2 diabetes in adults: management (NG28), Blood glucose management - NICESymptoms of type 2 diabetes and how it's diagnosed - NHSSources for this classroom
Educational content only. Not medical advice, diagnosis or treatment. Always consult a qualified healthcare professional before changing your health regimen.