What Does a Continuous Glucose Monitor Actually Show You? #064
What a CGM Actually Is, and What It Shows You
By Leah Vachani, Certified Nutritionist, Menopause Specialist and Metabolism Coach
Last reviewed: September 2026
Part 4 of 8 in The Blood Sugar & CGM Series — an eight-part series on blood sugar, continuous glucose monitors, and taking charge of your own metabolic health. Each post stands alone, though they build in order.
Quick Answer
A continuous glucose monitor is a small sensor worn on the upper arm for around two weeks. A filament just under the skin reads glucose in the fluid between your cells, which trails blood glucose by roughly 5 to 15 minutes. What it shows is how your own body responds to your own meals — and research on more than a thousand people found that identical meals produce very different responses in different people, with less than a third of the variation explained by the food itself.
Where to Listen:
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Chapters
00:00 The question I keep leaving you with
02:26 What a CGM actually is
03:50 What it measures, and why it lags behind
04:54 Two days of data, painted out loud
06:28 The spike, then the crash, then the craving
07:50 The four things worth watching
11:01 Why a number on its own tells you nothing
13:12 The one that matters most, and it is not the peak
14:52 Two people, one muffin, different results
16:07 How much of this is genetic
18:19 What to do this week
What is a continuous glucose monitor?
Physically, it is a small disc, somewhere between the size of a nickel and two stacked quarters depending on the brand. It adheres to the back of the upper arm and stays in place for about two weeks.
Underneath sits a filament about a quarter of an inch long and roughly as thin as an acupuncture needle, resting just below the skin. It does not enter a blood vessel and it is not drawing blood. Women often describe the filament as feeling like the world’s thinnest dental floss. Most people report that insertion is painless, and that within a day they forget it is there. Showering, sleeping, swimming and exercising are all fine.
The sensor takes a reading every few minutes and transmits it to an app on your phone. What you get is a continuous line rather than a single point.
Does a CGM measure blood sugar?
Not exactly. It measures glucose in interstitial fluid — the fluid that sits between your cells, which is present throughout the body rather than only where the sensor happens to be.
Glucose reaches your blood first and then moves out into that fluid, and the movement takes a few minutes. So the sensor is not reading something different from your blood sugar — it is reading the same thing, a few minutes further along. That gap is called lag, and it is typically 5 to 15 minutes.
Practically, that means if you eat and immediately watch your phone, you may think nothing is happening. Something is happening — you are watching it on a short delay. It also means comparisons between a CGM and a finger stick are least reliable when glucose is changing fast, such as just after eating or during exercise.
What does a day of CGM data look like?
On a settled day, the line is low and flat overnight. Breakfast produces a gentle rise — a hill rather than a mountain — that returns to roughly the starting point without much drama. Lunch does something similar. An afternoon walk nudges it down. Dinner produces another modest rise, and overnight it settles again. Viewed as a whole, it resembles rolling countryside.
On a difficult day — same person, different choices — a skipped breakfast followed by a sweet coffee mid-morning produces a fast, high rise, then a fast fall that overshoots below the starting point. That drop is when shakiness, irritability and sudden hunger appear. Eating something quick produces another rise and another fall, and by mid-afternoon the pattern repeats. That day looks more like a mountain range.
The useful part is the order of events. The spike comes first, the crash follows, and the craving comes after the crash. What has often been interpreted as a failure of willpower is a chain of events with an identifiable cause, usually about two hours earlier.
What should you actually look at?
How high it goes after eating. The most watched, and the least informative of the four.
How long it takes to come back down. The one that matters most — see below.
Where it sits when you have not eaten, especially overnight. Effectively a fasting glucose reading available every morning rather than once a year — and revealing precisely because food is not involved.
How much it moves across the day. Known in the research as glycemic variability, and increasingly thought to matter separately from peak height.
One benchmark is worth knowing, because practitioners in this field broadly converge on it: back to roughly your starting point within about two hours of eating. That is a working rule of thumb rather than a validated cut-off. If an ordinary meal keeps you elevated for three hours or more, it is worth attention.
One warning applies to all four. A number alone means nothing. A rise from 95 to 150 means one thing after a peanut butter sandwich and something entirely different after a salad with grilled chicken. The sensor supplies the effect; you have to supply the cause. Two weeks of data with nothing recorded alongside it tends to produce anxiety instead of answers.
Beyond that, this article deliberately gives no target numbers. There is no scientific consensus on what a non-diabetic 24-hour glucose curve should look like, and anyone offering a single universal threshold is oversimplifying. Part six of this series covers that properly.
Why does recovery matter more than the spike?
Because food raises blood sugar. That is what food does, and it is not a malfunction. A healthy metabolic system is not one that never rises — it is one that rises and then returns, reasonably promptly, without dramatic overshoot on the way down.
Two people can reach an identical peak after the same meal. One returns to baseline promptly. The other is still elevated hours later, or overshoots on the way down and feels shaky. Same peak, very different situations — and neither is visible on a standard lab report.
Why do two people respond differently to the same food?
This is the central finding. The PREDICT study, published in Nature Medicine, recruited 1,002 participants in the UK — including a large number of identical twins — and validated its results in a further 100 people in the US. Participants ate identical, carefully measured meals while their responses were tracked.
Inter-individual variability was substantial. And when the researchers examined what drove it, only around 29% of the variation in glycemic response was explained by the macronutrient content of the meal — the very thing printed on the label, and the basis of essentially every diet plan in circulation.
Because the study included identical twins, the researchers could estimate how much of the response is genetic. For glucose, the unadjusted genetic contribution was around half. Which is less discouraging than it sounds: if roughly half is inherited, roughly half is not, and that half is modifiable. As the lead researcher put it, that leaves substantial room for lifestyle to produce pronounced effects.
Equally important, and less often quoted: variability within a single person was much lower than variability between people. Your own response to a given meal is reasonably consistent from day to day. That consistency is what makes personal measurement useful rather than noise — if your body did something different every time, there would be nothing to learn.
What does this mean for standard nutrition advice?
Most nutritional guidance describes what happens to most people on average. If responses to identical meals vary as much as this research suggests, then average is the wrong unit for an individual decision.
The PREDICT researchers put it plainly: this degree of variation in healthy people is exactly why one-size-fits-all nutritional guidelines are problematic.
Which offers a different explanation for a familiar experience. If you have followed good advice carefully and not got the expected result, the advice was not necessarily wrong. It may simply have been written for a distribution rather than for you, and nobody ever checked which part of the distribution you occupy.
What can you do without a sensor?
Notice the sequence. The next time you hit an afternoon crash or a sudden craving, work backwards roughly two hours. You are looking for the same relationship a CGM would show, at lower resolution.
Write down the meal you suspect — the breakfast that leaves you starving by eleven, the lunch that makes you sleepy at your desk.
Write down the meal you are confident about, too. In practice, the surprising result is often the one nobody suspected.
Continue the series
This is part 4 of 8. Next: how to actually get a CGM and wear one — what it costs, whether you need a prescription, where it goes and what the first two days are like. See the full series.
Resources
The Metabolic Five — my free guide to every metabolic test worth asking for, including the fasting insulin nobody offers you. Download it here.
Clarity — my program for reading your own real-time glucose data. On your own, in a small group, or one-on-one with me. See current availability.
The Inner Circle — my monthly membership for women who want ongoing support. Join here.
Free Community — come and join us, no cost. Join here.
Fullscript — my practitioner dispensary, 15% community discount. Shop here.
AG1 — the greens powder I use daily. See it here.
AFFILIATE DISCLOSURE: Some of the links above are affiliate links, which means I may earn a small commission if you purchase through them, at no additional cost to you. I only recommend products I genuinely use and trust.
Sources
Berry SE, Valdes AM, Drew DA, et al. Human postprandial responses to food and potential for precision nutrition. Nature Medicine, 2020;26:964–973.
Berry SE, et al. Predicting Personal Metabolic Responses to Food: The PREDICT I Study.
Hall H, Perelman D, Breschi A, et al. Glucotypes reveal new patterns of glucose dysregulation. PLOS Biology, 2018.
Kennedy K. Hack Your Blood Sugar. Chapters 2 and 8 — device mechanics, interstitial fluid and lag time.
Medical disclaimer
A quick note from me: I’m a certified nutritionist and menopause specialist, but I’m not your doctor — and this article isn’t a substitute for personalized medical care. The information here is meant to inform and empower you, not replace a conversation with your healthcare provider. Always check in with your team before making changes to your diet, supplements, or lifestyle. Take what serves you, leave what doesn’t, and trust yourself.
