Reactive hypoglycaemia is a drop in blood glucose 2 to 5 hours after eating, often after a high-carbohydrate meal. In people with diabetes, a reading below 4.0 mmol/L counts as hypoglycaemia. Without diabetes, doctors look for a low reading taken during symptoms that settle once you eat. A CGM can show the pattern, but it can't diagnose it.
Here is the classic shape on a glucose graph, drawn as an illustration with the mmol/L values doctors use.
On a CGM graph, reactive hypoglycaemia shows as a fast rise after a meal, a peak within the first hour, then a fall that keeps going past where you started and dips under 4.0 mmol/L somewhere between 2 and 4 hours later. Symptoms usually line up with the bottom of the dip.
That is different from what a typical response looks like. In our members' sensor data, glucose after common carb foods peaks within about 30 to 45 minutes and drifts back to where it started over the next 2 to 3 hours, without dropping below it. After Weet-Bix eaten on its own, for example, the median member's glucose sat 1.6 mmol/L above their starting level at 45 minutes and only 0.1 above it at 2 hours (65 to 72 members at each time point, meals logged from January 2023 to October 2026; observational data, not a controlled study).
So a true dip under your starting level, with symptoms, is a pattern worth noticing. A single low dot on a graph, especially overnight, often isn't real. We cover that in the troubleshooter below.
“But in those who average blood glucose levels remain in the normal range, some of those people are in fact experiencing highs and lows and this in itself can be an issue.”
Dr Michelle Woolhouse, Vively Medical Director
There is no separate Australian cut-off for reactive hypoglycaemia. Doctors read the timing, the number and your symptoms together.
| Time after eating | Usual without diabetes | May point to reactive hypoglycaemia |
|---|---|---|
| Before the meal | Within 4.0 to 7.8 mmol/L, the usual day range (Diabetes Australia) | Usually normal |
| Up to 1 hour | Rises to a peak. There is no Australian 1 hour cut-off | Often a fast, high peak after a carb-heavy meal |
| 2 hours | Usually back under 7.8 mmol/L | Falling quickly, sometimes already below the starting level |
| 2 to 5 hours | Close to where it started | Below 4.0 mmol/L with shakiness, sweating, hunger or confusion that ease after eating |
If you have a reading from a meter or CGM, the checker below shows where it sits against Australian ranges. Choose 2 hours after eating, or any time of day.
Symptoms come on a few hours after eating and ease soon after you have some carbohydrate. Healthdirect lists these common signs of low blood glucose:
Some people also notice a racing heart, a headache, blurred vision or trouble concentrating. The same feelings can come from anxiety, dehydration, caffeine or a fast heart rhythm, so symptoms alone don't confirm a low. That is why a reading taken at the time matters. Our guide to low blood sugar symptoms and causes covers lows more broadly, including in people with diabetes.
Most of the time the cause is a mismatch in timing: glucose rises fast after a meal, insulin answers late and too strongly, and glucose overshoots on the way down.
Idiopathic means no other cause is found. A 2019 review by Altuntaş places these lows at around 3 hours after eating. Large meals of fast-digesting carbohydrate, sugary drinks and alcohol on an empty stomach tend to bring it on.
The same review describes a later form, with lows 4 to 5 hours after eating. It suggests that people with these later lows who also carry extra weight or have a family history of diabetes may be more likely to develop diabetes, and benefit from lifestyle change. Fasting insulin is one way to see whether your body is working harder than it should to hold glucose steady.
After gastric bypass, sleeve gastrectomy or other stomach surgery, food reaches the small intestine much faster. An international consensus on dumping syndrome (Scarpellini 2020) describes late dumping, where glucose falls too low after a meal, and agrees that diet change is the first treatment. If you've had this surgery and get these symptoms, tell your surgical team or GP.
Medicines are the most common cause of low glucose in general, mainly insulin and sulfonylureas in people with diabetes. Hormone problems and, very rarely, an insulin-making tumour can also cause lows, usually including lows when fasting. Your GP's job is to rule these out.
No. A CGM is useful for spotting a pattern and linking it to meals, but its readings aren't used to diagnose hypoglycaemia. Sensors read the fluid under the skin, lag blood glucose by several minutes, and can read falsely low when something presses on them.
“Currently, a CGM is not a diagnostic tool”
Dr Michelle Woolhouse, Vively Medical Director
Low readings are common on CGMs even in healthy people. In a study of 153 healthy people wearing a blinded CGM (Shah 2019), the median time under 3.9 mmol/L was 1.1%, about 15 minutes a day. Our members' first sensors show a very similar picture.
Some of those lows are real dips and some are artefacts. Before you read anything into a low on your graph, run through this list.
Lying on the sensor squeezes fluid away from it and causes a sudden, steep drop that recovers when you roll over. This is a compression low, not reactive hypoglycaemia. If you weren't awake with symptoms, treat it as a likely artefact.
Real glucose rarely falls and recovers in a few minutes. Sudden V-shaped drops usually come from pressure, a loose sensor or signal loss.
Sensor accuracy generally improves after the first day. Readings in the first 24 hours are less reliable, especially at the low end.
This is the pattern worth checking. Do a finger-prick test straight away, note the reading and time, then have some fast carbohydrate. A low finger-prick reading during symptoms that ease after eating is what your GP needs to see.
Glucose can fall during and after long or hard exercise, and CGMs lag behind during fast changes. Check with a finger prick if you feel unwell.
For more on sensor quirks, see our guide to common CGM mistakes.
See your GP if you get symptoms of low glucose more than once, especially if a meter confirmed a reading under 4.0 mmol/L. See them promptly if you've fainted, had a seizure, or get lows when you haven't eaten for a while. Someone who is drowsy or unconscious needs an ambulance on triple zero (000).
Your GP will usually start with a history and some blood tests:
If you have symptoms and a reading under 4.0 mmol/L, have about 15 g of fast carbohydrate, such as 6 or 7 regular jelly beans or half a can (150 ml) of regular soft drink, as Healthdirect advises. Recheck after 10 to 15 minutes, then eat a snack with slower carbohydrate and some protein so you don't dip again.
Don't treat a low with a diabetes shake. Abbott says its Glucerna products shouldn't be used for lows, because their carbohydrate isn't absorbed quickly enough.
The aim is a smaller, slower rise, so there is less of an overshoot on the way down. Diet is the first treatment in the dumping syndrome consensus too.
| Instead of | Try | GI (University of Sydney data) |
|---|---|---|
| White bread | Burgen Wholemeal and Seeds, or a dense whole grain bread | 70 to 74, down to 39 to 52 |
| Jasmine rice | SunRice Low GI White Rice | 79 to 109, down to 53 to 54 |
| Watermelon | Apple or pear | 76 to 78, down to 29 to 44 |
| Fruit juice | A whole orange | Juice not listed; orange 42 to 45 |
A day that follows these ideas might look like this: Greek yoghurt with berries and a few nuts for breakfast; a grainy bread sandwich with chicken and salad for lunch; an apple and cheese in the afternoon; and salmon with low GI rice and vegetables for dinner, with the vegetables eaten first. For more ideas, see our low GI meals, or our members' data on how Weet-Bix affects glucose.
Vively's CGM program pairs a sensor with the Vively app, which shows your readings against a 4 to 6 mmol/L range so you can see what happens 2 to 4 hours after a meal and which meals cause it. Dietitian support through the $99 a month membership helps you change meals without cutting out carbs you enjoy. If your pattern points to insulin resistance, the Baseline Health Check measures fasting glucose, HbA1c and fasting insulin among 70+ markers, and a practitioner explains your results. See how a continuous glucose monitor works before you decide.
Wear a CGM sensor for 14 days and see how meals, sleep, stress and movement shift your glucose, with guidance on what to change. A CGM shows patterns; your GP diagnoses.
There is no single Australian cut-off. A reading below 4.0 mmol/L counts as hypoglycaemia in people with diabetes. In people without diabetes, doctors look for a low reading taken during symptoms, 2 to 5 hours after eating, that improves once you eat. One low CGM reading on its own is not enough.
Yes. Reactive hypoglycaemia mostly affects people without diabetes. It can be idiopathic, linked to early insulin resistance, or follow stomach or weight loss surgery. A GP can check for other causes and order fasting glucose, HbA1c and fasting insulin to see whether insulin resistance is part of the picture.
It can be. A 2019 review suggested that lows 4 to 5 hours after eating may predict diabetes, especially in people with weight gain and a family history of diabetes. Many people with reactive lows never develop diabetes, so ask your GP about an HbA1c and fasting insulin test. Our prediabetes symptoms guide has more.
Treat a confirmed low with about 15 g of fast carbohydrate, such as 6 or 7 jelly beans, then eat a snack with slower carbs and protein. To prevent lows, spread carbs across the day, choose lower GI foods, eat carbs with protein or fat and avoid sugary drinks on an empty stomach.
Without diabetes, glucose is usually back under 7.8 mmol/L 2 hours after you start eating, and within the 4.0 to 7.8 mmol/L day range Diabetes Australia gives. A reading under 4.0 at 2 hours is uncommon. Our guide to blood sugar levels after eating explains the full curve.
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