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This is a sequel question.

The hunger trigger is all but obscure over long bike rides. It is possible, for example, to keep on riding until muscle glycogen, blood sugar, and liver glycogen are exhausted (perhaps in that order), and not even know the impending "hitting the wall".

And so it's refreshing to see the potential for a technical solution. Rather than guess when to eat; wait until it's necessary.

There are apparently devices on the market that measure blood glucose, and it is possible to transmit the data in realtime to a head unit (to Wahoo Elemnt Roam, at least; though this is perhaps not an exclusive feature).

Is it possible to use a glucose monitor to determine when to fuel on long bike rides?

Do pros do it, for example?

Oh, and to state the obvious, unlike, say, blood oxygen, there is no known way to measure blood glucose short of inserting a needle; is that accurate?

Sam7919
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    CGM is a thing, people do it. Worth a read. https://www.bicycling.com/health-nutrition/a38539834/continuous-glucose-monitors-for-athletes/ – mattnz Aug 04 '23 at 03:19
  • @mattnz Perfect! Thanks – Sam7919 Aug 04 '23 at 04:13
  • “Oh, and to state the obvious, unlike, say, blood oxygen, there is no know way to measure blood glucose short of inserting a needle; is that accurate?” No, it’s possible to do spectroscopy through the eyes or thin layers of skin. But it looks like pretty much all devices are only in tests and not really on the market yet: https://www.labiotech.eu/best-biotech/blood-sugar-monitor-diabetes/ – Michael Aug 04 '23 at 06:54
  • It can be possible to train yourself to hit the wall in a softer way that gives a little warning before the real bonk (at least at endurance pace - it yr be harder if everything is faster). Fasted training did that for me, with the sign on the day being that I'm burning more fat, evidenced by breathing harder, needing more water, and sweating more for the same effort. That's normally a fairly small hill, and if I tried to smash up it, I couldn't. That's a little too late to take on carbs on a hilly ride, or probably one faster than mine. – Chris H Aug 04 '23 at 07:06
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    Unless you are a pro trying to get to the podium, or have some other serious medical condition, then this is all completely ridiculous. Your body is capable of telling you you're hungry, even if you are exercising. Wild animals don't need electronic devices to know them they're hungry, neither do you. – whatsisname Aug 04 '23 at 20:04
  • @whatsisname If your body is capable of telling you you’re hungry even during an intense workout, then how do you explain bonking at all? To bonk is, quite simply, to be starving. In a starvation state you’d imagine the body would be screaming hunger, and no one would ever bonk—or, at the very least, would know that it's approaching. That said, it’s unclear how an intense workout suppresses hunger. A bit of googling points to hormonal secretions. – Sam7919 Aug 05 '23 at 18:01

1 Answers1

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It does seem possible to use continuous glucose monitors for this purpose. By way of background, CGMs are used by diabetics, and are only just starting to be used by non-diabetics. You are basically putting a patch with a very small flexible needle onto your arm for two weeks. The needle goes just under your skin. It measures glucose in the interstitial fluid.

I think that at least some pro roadies are using them off the bike. For better or worse, the UCI banned their use in UCI-regulated competition (meaning that this doesn't apply to amateur athletes unless your federation says so). This includes having one attached but not transmitting. I'm not sure how well integrated they are with head unit software, e.g. how many CGM wireless protocols exist, have Wahoo and Garmin implemented the fields in their computers, etc.

The cost relative to the alternatives is the main downside. The patches are single-use, last a couple weeks, and are rather costly. For amateurs, you could simply eat on a schedule. If you pay attention to your body, it is possible for many people to get at least some sense that they may be close to bonking. I should have been more explicit in the other question: you would simply eat and drink on schedule. Many head units can be programmed to send custom alerts on a timer, e.g. here are the Wahoo instructions. Pro road racers have their nutrition schedules printed and taped to their stems. As an amateur, you can aim for 30g per hour and see if that does it for you. I think 30g is sufficient for a lot of group rides, 60g is both achievable and should cover even more use cases. It is possible to hit or exceed 90g per hour with some planning (mainly relying on drinks), but that's probably not necessary for all training or racing scenarios.

In addition, I don't know how much of a learning curve exists for amateurs interpreting CGM data. For example, glucose under 72mg/dl is considered hypoglycemic in the medical setting. Is that the correct threshold in a sports setting to detect impending bonks? Does your own personal threshold vary significantly from the average? Are the data noisy and it's hard to see trends? Basically, too much data can be a curse - you might wind up ignoring the data, or if you have only some idea how to interpret and use it, you might wind up misusing it.

Weiwen Ng
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    The thing to do is to find a friend who has diabetes and uses a CGM. They might even have an old CGM they could lend you. – Daniel R Hicks Aug 05 '23 at 21:54
  • @DanielRHicks I don't know if those will connect to cycling head units. So, for monitoring glucose while riding, that could be a problem. You could naturally use a phone to, at minimum, record the data for later analysis. – Weiwen Ng Aug 06 '23 at 00:30
  • Most reasonably modern CGMs can connect to an iPhone, and should be reasonably flexible with the connections. – Daniel R Hicks Aug 06 '23 at 01:50