Training interment fasting and CGM

Hi everyone,

A little boring background first.
So basically after a long while I got back to training.
So in July 21 I started unstructured and very low volume.
Also started IF 16-8 no breakfast at the same time.
Started trainerroad in October. Did all rides fasted in the morning because of work and kids. Normally without any major issue.
So now I dropped from 93kg pandemic weight in july to 77kg. So now my weight is fine as my height is 1.90m.
My FTP at 185 is still a very long way off from my all time best.

So after this boring part. My question or better discussion topic is this.

I want to keep training early in the morning and keep IF. But from what I read training every day fasted is not very productive. But for all other aspects of life fasted mornings seem to help alot.

So I thought I could exploit continuous glucose monitoring during a workout to know when I really need to fuel up during a workout. But I could find any really good source of info. Or if this is at all feasible.
I learned a lot from CGM health-wise but not much during training.

So anyone really use CGM for training or follow a plan doing all fasted rides?

I found this article from the science of ultra really well written that takes a look at fasted morning workouts. I would think that your CGM could occasionally read higher depending on the intensity of the workouts because your liver would push any last remaining glycogen stores into into the blood as glucose in response to the high intensity exercise (if you did a sweet spot workout for example)

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I used to work for a company that did research on CGM and therefore got to wear them 3 separate times for 2 weeks. I did not specifically do fasted rides, but I definitely under fueled a few rides. Here is what I learned while training:

  1. If I started to feel tired and grumpy on a ride, my CGM would chirp at me for low blood sugar.
  2. If I could pedal at endurance watts but had no upper end, my blood sugar was below fasting levels by 10-20 units (fasting levels for me are between 85-90)
  3. If I under fueled a ride, I could eat whatever I wanted afterwards and my blood sugar wouldn’t go above 95-100
  4. For TR specific rides, it had little effect on the first interval, but subsequent intervals were increasingly more difficult.
  5. The lower my blood sugar was at the end of the ride the worse my legs felt the next day.

The first learning taught me that grumpiness and sluggishness was from blood sugar below fasting levels. I have not gotten grumpy on a ride since.

The next three learnings taught me I have much better repeatability and can hold power for a lot longer when I am eating sugar. For 1-2 hour hard rides, I can get by on 40 grams of sugar no problem. Any longer and I need 60-80g. For easy rides, I can get by on 40g sugar nearly indefinitely.

I still have at least 60g for long easy rides due to the last learning.


Would getting a RER test or using the CGM be more beneficial? Will either of those tools give you a number on your fat vs CHO utilization?

This really seems like another marginal gain type of deal. Not needed for 99% of athletes (meaning all of us).

I’d think with all this “fasted” and “train low(CHO)” stuff that you’d really want no shit lab data to be as precise as possible since the benefits seems negligible.

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Almost all my rides would fall into the “IF” category…not by design, but because I’m not a breakfast eater and tend to have my first meal in the late afternoon or early evenings. My rides are a combination of TR and longer, unstructured outside riding. I have no issues I’m aware of completing 60 to 90 minute trainer rides with no breakfast or 3 to 7 HR outdoor rides. I’m on an endurance plan, so most rides are threshold and below, with an occasional VO2 Max thrown in or some hard surges during outdoor rides. I rarely eat on my outdoor rides, but will occasionally snack on something at a rest stop.

If your training is progressing and fasted mornings help with your other lifestyle aspects, why change things? Sometimes good enough is good enough.

Personally, I would give it a try and see how your body responds.

I perform most of my training in the morning (5 am) and have neither the time nor the desire to eat that early. I do however try to carb up the night before if I have an intense workout planned - especially VO2 work.

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Normally, I always able to complete the workouts. My concern is that I am also burning muscle on the more intense rides. I don’t want to get to an overtraining state.
Also I really want to see if correlations between blood glucose and a correctly fueled workout. My normal response is a spike at start then the blood glucose stays mostly stable. From what I’ve read this is the normal response.