CGMs and blood glucose during exercise

Forum multiverse,

I had a recent experience that I sent to Ryan and he thought would make a good topic for the forum: the role of glucose metabolism, how to control it, the use of Continuous Glucose Monitors as training aids, and how to avoid “the bonk”. I’m non-diabetic, but have been wearing a CGM recently.

I’ll try to be pithy upfront with my questions, and then expand below.

  • First, during the first half of a recent race my blood glucose saw a steady climb to a (shocking) peak of 294(!). What might be some physiological reasons why that would occur? I have not seen this before or since, even with much higher intensity for same duration as when I saw the peak.
  • Second, commensurate with it starting to decline (but still being really high) I lost the ability to hold the wheel of the lead two racers. Are the two related?
  • Third, about 2/3 of the way through the race, with glucose still at 230, I was fully lights-out and it was a struggle to turn over the pedals (average 157 watts for last hour for a rider with an INCSYD AT of 262). Why with such high glucose would it feel like I had “bonked” as if my glucose was in the 50-60 range?

A bit more detail:
53 year old cyclist with in-lab INCSYD test showing me at 56 ml/min/kg VO2 max, 0.36 mmol/l/s VLamax, AT 262 watts or 3.7W/Kg. I’ll add strava link for race below, but the key moments, powerprofiles and glucose levels are in the description below. Used Skratch Superfuel, same as for a 229 mile/11 hour solo effort across the state of Wisconsin last year. Came near several climbing KOMs on that ride (so plenty of punchy efforts) but did not have any bonk experience and finished with NP of 204.

Quick description of the event:
Breakfast of starbucks smoked bacon sandwich and lemon pound cake 8:15 (note this is more than I usually eat before my 90 minute morning workouts) – glucose 95

Race start at 10am – glucose reading 121

Race fuel – Skratch Superfuel, 6 sips from Camelbak every 15 minutes

Pace was “conversational” for first 30 minutes until first key segment (avg 152 watts, max 550; glucose rose from start to 171)

First big surge – 3:22 effort at avg 332 watts, max 573 (won KOM); glucose continues rising – est. 180

Glucose keeps rising for next hour and 20 minutes of racing which has just a couple surges out of corners but primarily paceline effort (241W avg, 648W peak)

Race deciding surge – at about one hour and 50 minutes of racing the race winning move was made; 17 minute effort avg 262, max 725; glucose tops out at 294 and starts dropping corresponding with the time I could no longer hold lead 2 wheels

About three hours into race and it’s lights out. Glucose has dropped to 233. For the last hour and 25 minutes of race average 157 watts. End race after 4 hours 20 minutes of effort (202W average, 234W weighted average, glucose 130)

Thoughts or comparable experiences from group?


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@Danswenson I do have one client who has done a few 4h plus races with CGM and it was pretty interesting.

Will post his info when I get back home in a few days.