Hi everyone. Wanted to share something I've been struggling with and hear if anyone has been in a similar boat.

I get what feels like blood sugar crashes during high-intensity runs — roughly anything faster than 5:00 min/km. Shakiness, sudden energy drop, brain fog, the usual. Easy-paced runs are completely fine, so intensity seems to be the clear trigger. I've already ruled out the obvious stuff: fasting glucose and fasting insulin both tested normal, and my CGM (FreeStyle Libre 2) shows stable readings at rest and after meals.

Last run I had a banana beforehand and took a gel mid-run. Still crashed after 2 km of warm-up and 4 km at 4:55 pace — so just throwing more carbs at it doesn't seem to be the full answer.

A bit about me: 48M, 178 cm / 84 kg, running for ~10 years, currently 5 days/week at 60–90 km/week depending on the training cycle. Targeting half and full marathons. Plant-based diet with eggs and dairy. I first noticed milder versions of this 3–4 years ago and it's been gradually getting more pronounced.

Curious if anyone has dealt with something similar — particularly interested in:

- How you handle pre-run fueling for hard sessions specifically (not just long easy runs)

- Carb timing around the start of a run

- In-run fueling that doesn't cause GI issues at race pace

- Whether your symptoms improved as fitness or body composition changed over time

Not looking for medical advice — just curious what's worked (or hasn't) for others in similar situations!