Help interpreting Respiratory Exchange Ratio

I was fortunate enough to have some lab testing done recently and I’m trying to work out what it all means, specifically the use of fat and carbs as fuel. I’ve attached the outputs from the tests for anyone interested enough to analyse. The test order was 1) 10 min warm up @100W then Sub max test, 2) 10 min @ 100W then Vo2 max, and 3) 2h rest and then Time to exhaustionVo2.pdf (308.6 KB)
TTE.pdf (299.2 KB)
Submax.pdf (318.6 KB)

Regarding the fat/carb source utilization, I have a few questions:

  1. looking at test 1 (sub max), I burn primarily carbs, even at low intensities. Correct?
  2. However, test 2 (VO2 Max) relies mainly on fats…is this because the carbs have been exhausted?
  3. TTE relies on fat at low W, but then once load is applied at 470W, carb use dominates (this is what I expected to see across all test fat>carb).

I train primarily in the mornings around 6am in a fasted state (i.e., nothing from 7pm previous evening) and was surprised by the sub max test. Does this really mean that I primarily rely on carbs as a fuel source until they’re exhausted, after which I switch to fats? If so, how was I able to reach VO2 max in the 2nd test, despite pulling the fuel from fat (which is far slower chemical reaction).

I find it very hard to lose weight despite a lot of training and any insights into my results would be appreciated!! I didn’t pay for the tests (part of a trial) and RER was provided without explanation or analysis.

Bump - any takers? Anyone able to help?

I think you’ve gotten the right directional interpretation. Although it does seem odd that your Submax uses more carbs than your VO2max test. I think to accurately interpret the data, you probably need to do a few things:

  1. Ask the lab where you got the testing if the measurement methodology was designed specifically to yield reliable RER data.

  2. Bring the results to an exercise physiologist for them to review. They will be able to tell you if your results are typical/atypical, and shed further light on how much you could rely on the data, and what it means.

Here’s a couple of posts on this topic:

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Thanks @DaveWh! I’m hoping to speak to the Researcher who ran the test for a little more info.

Appreciate your input!

What was the trial for by the way? The procedure looks very interesting, I’m curious.

As to your original question - I’m by no means an expert on the matter but I’ve done a lot of research and reading, your results are not that surprising. I would definitely seek opinion from the researcher running the test, but basically you seem to be very much carb adapted, your body just taps into the most readily available energy source first.

Read this article:

Carbohydrate and fat utilization during rest and physical activity

5.3. Exercise duration and substrate utilization

The pattern of substrate utilization changes with time, even when the exercise intensity remains constant. The longer the time spent exercising, the higher the contribution of fat as an energy substrate.

During low intensity exercise lasting longer than 2 h, the substrate utilization is not significantly altered as compared to those utilized during shorter bouts of low intensity exercise. On the other hand, during higher intensity levels, there is a progressive increase in reliance on plasma fatty acids.[56](javascript:void(0):wink:

In prolonged exercise, plasma fatty acid oxidation increases in parallel with the depletion of glycogen storage in the working muscle.[52](javascript:void(0):wink: The increased rate of fat oxidation is due to an increase in circulating level of catecholamine (adrenalin and noradrenalin), and a decrease in the circulating level of insulin. The catecholamines play a role in stimulating, and the insulin, in inhibiting the process of lipolysis.

This makes sense in your case, where your first submaximal effort lasting 40 minutes, followed by the 10 minute recovery, primed your body to start using up fat stores, even though you were performing a Vo2max test. One hell of a power at VO2max by the way!

What is a bit weird though is that your RER never actually goes much higher that 1 - one would expect that at some point you would be hyperventilating but still going, tapping more and more into your anaerobic energy systems, that does not seem to be the case with your test. Maybe something you could ask the researcher too.


Thanks @konradkowara!

The test was for a berry based supplement (30 days supplementation between identical tests to assess its influence).

I reckon the gel I had before the 1st test ensured I mainly metabolized CHO throughout the test (as it was most available). Then, once this was used, I reverted back to fat.

I consider myself to be highly fat adapted (I hardly ever fuel workouts with CHO, as most of these are done pre-breakfast - long outdoor rides are the exception to this). The results of the 2nd and 3rd tests would suggest that I’m comfortable using fats over stored glycogen…I think :wink:

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My first impression was that you weren’t warmed up properly for the 1st test but we’re for the VO2 max one. I then saw about the starting gel and that could a major part of the reason

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A couple of quick thoughts:

  1. your submax test shows 50:50 ratio of CHO:fat. RER of 0.85 is 50:50 (0.70 is 100% fat, 1.0 is 100% CHO), so you tend to hover around that point. You do bump up to a higher percentage of CHO at times, but nothing too far off the 50:50. If you took a gel right before the test, this would make CHO available and maybe alter your normal RER response.
  2. Perhaps a bit high on the CHO side at low intensities, but I usually don’t worry too much about that during these types of tests as there is often nerves, anticipation, a bunch of scientists poking you, etc.
  3. In line with #2, your RER is lower in the VO2max test during sub maximal exercise levels (would be nice to see your VE, VO2 and VCO2 values for all tests if you have them too. It can provide some insight into your efficiency.
  4. I’m not sure you reached VO2max only seeing your RER data (we usually shoot for 1.10 - 1.15), but I’m sure you needed to stop! That being said, you do switch to CHO metabolism predominantly after 435W. That’s a great power number.
  5. Nice work on your TTE test, not sure if there is much to be gleaned from the RER data alone, but the high-intensity exercise would require a greater contribution from glycolysis.

I’d be happy to breakdown the tests in a bit more detail if you have all of the data… See if your gas exchange threshold(s) match well with your FTP, look at your efficiency, etc.


Hi @gdumanoir,

That’s awesome - thanks for your input. PM sent with files.

Appreciate your thoughts!!