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. Unaltered FreeStyle Libre 3 screenshot (n=1).

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. Unaltered FreeStyle Libre 3 screenshot (n=1).

Why the value falls first

Evidence level: established

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

Evidence level: established

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.

Sources

Sources as of: 21 Jul 2026.

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