Carb utilisation and ingestion on activities

As an ex software developer, I quickly scanned SessionMesg class definition for the calories/fluid consumed in FIT SDK release 21.141.00.

If I’m reading it correctly, 177 and 179 aren’t defined in that SDK release.

Couldn’t find anything in under 5 minutes, but I don’t have time to download the latest and take a closer look.

Lots of undocumented sdk fields are in use, and its use in Garmin Coneect decoded and available in custom streams, interval fields, custom activity fields.

Cheers

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Yes, I’m aware of that, was hoping to see it as a documented field.

I have updated the calculation with new work from Jeff. The continuous/interval flag has been replaced by variability index (normalised / average power). Intervals.icu calculates grams from two models (both of which only work for cycling):

  • Total work (kJ), variability index, male/female, prior day training load
  • TSS, duration, vo2AtVT2 (derived from FTP), variability index, prior day training load

The lower value is chosen. For my data the two models produce similar values for 90 minute rides but kJ model is a lot lower on longer rides.

If you want to see the new numbers just re-analyze one old ride and activities from that point onwards will be updated.

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I just took a look at a very very very easy session. It was probably almost fat burning, as I never ever reached even Z2. 2 hours at 100w (<40% intensity). It calculates 229g carbs. With an average of 100w for 2 hours that would mean, 100% carb burning. That can‘t be true. Or am I missing something?

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@R2Tom what was your total work in kJ?

I think “probably almost fat burning” is an misunderstanding of the reported science and empirical data.

Some numbers from Brooks/SanMillan 2018 study “Assessment of Metabolic Flexibility by Means of Measuring Blood Lactate, Fat, and Carbohydrate Oxidation Responses to Exercise in Professional Endurance Athletes and Less-Fit Individuals”

Power Carb burn grams/minute Fat burn grams/minute
World class cyclists 130W 1.5 g/min 0.55 g/min
Moderately active individuals at lowest cycle ergometer power output 130W 2.0 g/min 0.4 g/min

So for a world class cyclist, at really low power relative to ftp, they are burning 90grams/hour of carbs, and fat is only 33 grams/hour. And these are some of the best fat burners in the world. Absolutely less than 40% intensity.

Moderately active individuals are burning 120grams/hour of carbs, and only 24g/hour of fat oxidation.

People look at those charts and miss the fact that carb and fat oxidation rates are on different scales - carb burning scale is 10x the fat burning scale.

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Wow. I was one of those that missed it. All similar diagrams I’ve seen have been on coaching-type websites or videos, rather than from the original literature, and they don’t tend to have the numerical axes. Thanks for pointing this out.

“Mostly burning fat” :crazy_face:

I didn’t read the paper. What is moderately active? They compare absolute power, better would be percentage of VO2max, to compare between highly trained people and ‚moderately‘ trained people.

Then you see, for a wide range of people fat oxidation stays high up to 70-75% of vo2max. Somewhere around 0.4 to 0.5 g/min.
Of course it is not 100% fat burning, but there is no doubt that there will be significant fat burning for LIT sessions. And then you have to remember that 1g fat has more than twice the energy of 1g carb.

So for this ride I would calculate this way:
IMG_1475

For 120min very low Z1 intensity, assuming low 0.4g/min fat oxidation = 48g fat. That’s 370kJ energy (assuming 20% efficiency). There were a total work of 780 kJ. Missing 410 kJ from carbs, that would be 120g. The field says 229g. Where is the mistake?

But I must admit, if I look at this 1h Z2 ride, it is more plausible. That’s a value I would understand.
IMG_1477

I’m trying to help, but you can read the paper or someone else can answer.

The conversation is about CARB oxidation, not fat oxidation.

Your body is oxidizing carbs even at rest.

And for that one world class cyclist, he was burning 90 grams per hour of carbs at 130W at arguably at or below 40% Intensity Factor.

So world class at 130W is 90g/hour, and moderately active was 120g/hour. Without pulling up the paper, and assuming its accurate info, it’s reasonable to put upper zone1 carb burning at 90-120g/hour around 130W.

Here is one of my rides in the ~2hour and low intensity:

  • 912kJ of work (roughly 900kcal)
  • 37C / 98F so less carb burning due to heat and heat adaptation

and Garmin:

Just trying to help by offering up some info.

Estimates from Intervals is not changing my fueling strategy. I’m not confused or concerned about fueling, because I know the fundamentals and apply that info to properly fuel myself both on and off bike. A big part of my strategy is “winning in the kitchen” so I don’t have to train my gut, or guzzle sugar water on the bike.

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To add on this topic, there is (are) Garmin Apps available on GarminIQ, like “Fat Burner”, that records the fat (in g.) and carbs (in g.) burnt during an activity, purely based on HR.

Ex. for one of my training:


For this activity, CHO consumed as per Interevals is 143g.

I still say: not using your own carb-burning-model doesn´t give you right data.
Here you can see how it works: https://analytics.sentiero.de/

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Worth a read

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Could you share the numbers for that session? (TWD, your FTP, duration, NP, avg power, etc)

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Hello, can someone tell me if the diagrams of Garmin iQ are also shown in Intervalls.icu.

Yes you can using custom activity streams:

Look at the fit file on fitfileviewer. com and look at the record messages. If it is a simple field you don’t need to write any JavaScript. You can just choose the field from the custom stream dialog.

Thanks for the information

But I have no idea about JavaScrip
LG Helmut

Do the calculations include a person’s Basal Metabolic Rate?

Yes, for that time during exercise, it is all-inclusive.

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I still get the same weird results like last week by the way, 153g CHO for a 17 minute commute ride. I guess related to the quick 500W sprint across the bridge?

I reprocessed/reanalysed the file but no difference.