The short answer
Exercise changes both sides of the glucose equation at once. Working muscles pull more glucose out of the blood, and your body releases more glucose into it, mostly from the liver. Whether your line goes down, stays flat or climbs depends on which side wins, and that depends on the kind of effort, how long it lasts, what you ate, the time of day and even how nervous you are.
As a rough rule of thumb, long, steady efforts tend to pull the line down, while short, very hard efforts tend to push it up. Most real workouts are a mix of both, which is why two sessions that look the same on paper can leave very different lines.
Where the glucose comes from
At rest, your muscles burn mostly fat. As soon as you start moving, they lean more on two other fuels: glycogen, the glucose they keep stored inside themselves, and glucose taken up from the blood. Your liver keeps the blood supplied by releasing glucose from its own glycogen stores and by making new glucose.
Hormones set the pace. During exercise, adrenaline and noradrenaline (the “fight or flight” hormones), glucagon and, later, cortisol all push the liver to release more glucose. The harder you go, the stronger that push. The line on your CGM is the net result of two flows: glucose leaving the blood for your muscles, and glucose arriving from your liver.
When the line tends to fall: long, steady efforts
At an easy or moderate pace (an easy run, a long ride at conversational pace, a brisk walk, a steady swim) muscles keep drawing glucose from the blood while the hormonal push on the liver stays moderate. Over half an hour or more, uptake can outpace release and the line drifts down.
That drift is usually a slope rather than a cliff. It tends to be more marked on long sessions and when your muscles start with less glycogen, for example on the second hard day in a row.
When the line tends to rise: hard efforts, races and lifting
Intervals, sprints, a hard climb, a race, a heavy strength session or a HIIT class work differently. At high intensity the stress hormones surge and the liver can release glucose faster than your muscles use it. The line climbs, sometimes during the effort and sometimes just after it.
Race day adds its own push. Nerves and excitement raise adrenaline before you even start, so the line can climb on the start line. Strength training often shows a modest rise during the session too.
That rise comes from your body’s response to hard work, not from anything you ate. It often eases once the effort ends and the hormones settle, and it can be followed by a drop later on, as described below.
Stop-and-go sports
Padel, tennis, football, basketball, climbing and circuit classes mix bursts with recovery. Their lines often look like a blend: climbing through the hardest stretches and drifting down through the steadier ones. A competitive match usually pushes harder than a friendly game of the same sport, partly because of the adrenaline that comes with competing.
After the workout: the slow drift
The workout doesn’t end when you stop your watch. For hours afterwards, muscles keep pulling glucose from the blood to refill their glycogen stores, and they respond more strongly to the signals that tell them to take glucose in. That is why some people see their line drift down in the evening after an afternoon session, or overnight after a long or hard day.
This delayed effect is a good reason to look at the whole day and the night after, not only the workout window. Our article on glucose at night looks at the overnight side.
Other things that move the line
- What and when you ate. A meal, a drink or a gel before or during a session adds glucose to the mix. Training a few hours after a meal and training first thing in the morning before breakfast can give very different lines.
- Time of day. In the early morning, the hormones that get you ready to wake up are higher, so the same session can look different at 7 am and at 7 pm.
- Heat and hydration. Hot, humid conditions are an extra stress on the body, and stress shows up on the line.
- Sleep and stress. A short night or a tense day can change how your body handles glucose the next day.
- Illness. Fighting off a cold is a stress too, and it can show on the line before you feel it.
Why your CGM can look behind during exercise
A CGM doesn’t measure glucose in your blood. It measures glucose in the fluid between your cells, which follows blood glucose with a delay usually described as somewhere between 5 and 15 minutes. When glucose changes quickly, as it can during exercise, that delay is more visible: your sensor may still be showing where you were a few minutes ago.
Sensor makers advise checking with a fingerstick when a reading doesn’t match how you feel. Our article on CGM delay explains the sensor lag and the extra delay added by sharing services such as Dexcom Share and LibreLinkUp.
Finding your own pattern
General patterns are a starting point. Your line is yours, and the quickest way to learn it is to put many workouts side by side: easy runs against intervals, morning against evening, with and without a gel.
That is what Ando is built for. It pulls your glucose from your sensor’s cloud and lines it up with every workout from Garmin, Apple Health, Strava, WHOOP, Oura or Fitbit, without you doing anything: the curve before, during and after the session, with its time in range and average. You can add the carbs you took on a workout and see them as a marker on the curve.
This article describes common patterns. It isn’t advice on how to manage your glucose. Ando is not a medical device: it shows what happened so you can notice your own patterns, and questions about your numbers belong with your doctor.
Questions people ask
Why does my glucose go up when I run hard?
During intervals, hill repeats or a race, your body releases stress hormones such as adrenaline that tell the liver to put more glucose into the blood. At high intensity that release can be faster than your muscles use it, so the line climbs. Easy running usually behaves differently.
Why does my glucose drop hours after a workout?
After exercise your muscles keep taking glucose from the blood to refill their glycogen stores, and they respond more strongly to the signals that tell them to take glucose in. That can show up as a slow drift down later in the day or during the night after a long or hard session.
Does weightlifting raise glucose?
A heavy strength session often shows a modest rise while you lift, because short, intense efforts trigger the same hormonal response as sprints. The line can then drift down in the hours that follow, as muscles refill their stores.
Why does my CGM differ from a finger prick after exercise?
A CGM reads glucose in the fluid between your cells, which follows blood glucose with a delay of several minutes. When glucose moves fast, as it can during and after exercise, that delay makes the two readings drift apart. Sensor makers advise a fingerstick check when a reading does not match how you feel.
Is a rise during a hard workout a problem?
A rise during a very hard effort comes from your body’s own response to the effort, not from anything you ate. Whether a particular number matters for you is a question for your doctor. Ando shows what happened on each workout; it does not judge it or tell you what to do.