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Exercise: down first, then up: why sport moves blood sugar twice

I’ve seen it on many days: during an intense session my value drops, and shortly after I finish it turns and climbs, sometimes above the starting point. This biphasic curve is typical: here’s why, with my real CGM data.

Author: Sascha RadermacherLast updated: 21 Jul 2026

🌐 Translated from the German original. Wording has been localised for English readers; in case of doubt, the German version is authoritative.

Individual illustration: Exercise: down first, then up – why sport moves blood sugar twice
Exercise: down first, then up – why sport moves blood sugar twice

The pattern: one curve in two phases

I’ve seen it on many days: during an intense session my value falls: sometimes markedly. Shortly after I finish, it turns and rises, sometimes beyond the starting value. Then it settles again. Exactly this arc is typical.

Exercise: down first, then up (schematic) Schematic typical curve. During exertion the value falls to about 95; right after it shoots up to about 175, about an hour later, from a start of 120 to 130. Measuring only during shows a low, measuring shortly after shows a high, yet it is the same curve at different times. Exhausting exercise: down first, then up Typical curve (schematic): from my observation & the literature Target band 70–160 150100 Exertion ~95 right after ~175 ~1 h later Start ~120–130 before during shortly after later The measurement error: measuring only right after training shows “low”. Measuring only 1 h later, reads “high”. Both are the same curve: just at different times.
Fig. 1: The biphasic curve. During exertion the value falls, shortly after it shoots up. Two measurement times, two completely different numbers: the same curve.

My own curves for this

So this doesn’t stay theory: here are two real excerpts from my FreeStyle Libre 3 that show the up and down. The curves themselves are unaltered. I only resized them for the web; the explanation is in the captions.

CGM curve from the beer-garden evening
The beer-garden evening: the value shoots up (~190), then falls below the target range (~78) and settles around ~100. Schematic reconstruction from the documented FreeStyle Libre 3 reading (n=1); not an original screenshot.
CGM curve from a Peloton evening
A Peloton evening: after the sessions the value swings up and down several times: the same up-and-down in miniature. Schematic reconstruction from the documented FreeStyle Libre 3 reading (n=1); not an original screenshot.

Why the value falls first

During exertion the working muscles use a lot of glucose: partly via an insulin-independent pathway. As long as the liver resupplies enough sugar, the value stays stable. But if use exceeds the resupply, blood sugar falls.1 In studies it sometimes fell below 60 mg/dl with moderate effort, and many felt nothing of it.1

Why it shoots up afterwards

Shortly after the end, the hormone situation flips: the body releases stress hormones (adrenaline, cortisol), and the liver releases glucose into the blood, a sensible counter-regulation so the value doesn’t stay too low.2 With intense sessions (heavy strength training, sprints, competition) this adrenaline surge is especially strong, so the value can shoot beyond the starting point.2 So anaerobic, intense exertion tends to raise the value, while calm endurance lowers it.3

Exercise: down first, then up (schematic) Two phases. During exertion, working muscles use a lot of glucose, partly independent of insulin, and if use exceeds the liver’s resupply the value falls, sometimes below 70 mg/dl without symptoms. Shortly after, stress hormones adrenaline and cortisol make the liver release glucose and the value shoots up, sometimes above the starting value; the more intense the effort, the stronger the rise. Why this happens: two phases 1 During: the value falls ↓ Muscles pull glucose The working muscles use up a lot of glucose: partly insulin-independent. If use exceeds the resupply from the liver, blood sugar falls. Can fall below 70 mg/dl: often even without feeling symptoms. 2 Shortly after: the value rises ↑ Stress hormones raise it The body releases adrenaline and cortisol; the liver releases glucose. Insulin doesn’t counter strongly enough right away: the value shoots up, partly above the starting value. The more intense (strength, sprints, compe- tition), the stronger the rise.
Fig. 2: The two phases in detail: muscle use lowers, stress hormones raise.

How I use this for myself

  • Don’t let a single value drive you crazy. After hard training a short spike is often normal and goes back down by itself.
  • Read the trend arrow, not the one number.
  • If I want to lower the value after training, an easy cool-down (10–20 minutes of walking) helps me more than another hard session.
  • If I want to lower it deliberately, I choose moderate endurance from the start instead of maximum effort.
  • I never compare “right after” with “an hour later”: those are two different questions.

Key takeaways

  • Exhausting exercise (strength or endurance) often lets blood sugar fall first, then rise.
  • During: muscles use glucose (partly insulin-independent) – it can fall below 70 mg/dl without symptoms.
  • Shortly after: adrenaline and cortisol make the liver release glucose – it can rise above the start.
  • Read the trend, not the single value; an easy cool-down lowers better than another hard set.
  • Never compare “right after” with “an hour later” – two different questions.
Related

Research → Blood sugar during training
Research → 205 mg/dl at the beer garden
Knowledge → Understanding CGM
Knowledge → Movement & blood sugar

Article ID: SWF-P-014Please quote this ID for corrections or additions.

Sources

Sources as of: 21 Jul 2026.

  1. Muscle glucose uptake during exercise (partly insulin-independent); blood sugar can fall, sometimes below 60 mg/dl unnoticed. Reference. pmc.ncbi.nlm.nih.gov (öffnet in neuem Fenster)
  2. Catecholamines (adrenaline/cortisol) raise glucose after intense exercise; can overshoot the baseline. Reference. pubmed.ncbi.nlm.nih.gov (öffnet in neuem Fenster)
  3. Aerobic vs. anaerobic/intense exercise: differing acute effects on blood glucose. Reference. pmc.ncbi.nlm.nih.gov (öffnet in neuem Fenster)

Note: Personal single-case observation (n=1) plus established mechanism, not medical advice. The screenshots are unaltered FreeStyle Libre 3 curves (German app UI). On insulin/sulfonylureas, exercise can cause hypoglycaemia – coordinate with your care team. No promise of cure.

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