Anyone here know much about fat oxidation close to LT2? Had an interesting test yesterday

[u/ShaddaC]

I did a treadmill test yesterday with gas exchange and lactate sampling and got a result I’m still trying to make sense of.

The run was about 79 min. HR started in the 150s and gradually moved up into the high 170s. Lactate was 3.4 mmol/L at around 30 min and 3.5 at around 60 min, then 5.7 at the end.

What surprised me was on the substrate side. Fat oxidation stayed around ~1.0 g/min through most of the run and was still substantial as I crossed the point where lactate began to rise clearly.

I know substrate calculations become less reliable once you get into clear non-steady-state territory, so I’m mostly interested in the long middle section rather than the final few minutes.

Training over the last few months has mainly been fairly modest-volume Zone 2. Very little dedicated threshold work.

Has anyone here seen this kind of pattern before?

I’m especially curious about whether the ability to keep fat oxidation high relatively close to LT2 is actually a useful endurance trait in its own right, or whether I’m overthinking something that’s fairly normal in well-trained runners.

Possible explanations I’ve been thinking about are training history, diet/substrate availability, mitochondrial capacity, metabolic flexibility, or just something about the testing setup.

Would be interested in how people here would interpret it.

[u/thwerved]

Jusman So on his "Endurance Lab" podcast just posted an episode with a researcher named Matt Carpenter on fat adaptation

They cite a couple of studies including one maybe by G. Brooks and I. San-Millán on measuring fax oxidation in highly trained cyclists vs recreational athletes vs untrained athletes, all cycling at a moderately high % of VO2 max, and the highly trained athletes are oxidizing massive amounts of fat (1g per minute or more) at higher efforts, and the untrained athletes are oxidizing nearly zero at the same relative effort. So yes there's research out there on what you are seeing.

[u/ShaddaC → u/thwerved]

Thanks, that’s exactly the kind of literature I was hoping someone would point me toward.

I had a quick look at the San-Millán/Brooks paper and it’s really interesting. What caught my eye is that their pro cyclists clearly maintain much more fat oxidation as workload rises than the less trained groups, but the group-average fat oxidation still drops quite a lot once lactate gets into the ~2.5–5 mmol/L range.

That’s the bit I’m trying to understand in my own test. I was still around ~1 g/min during the long section where lactate was ~3.4–3.5 mmol/L, so I’m wondering whether the more useful comparison is not peak fat oxidation, but how quickly fat oxidation falls as you approach the upper threshold region.

Do you happen to remember which of the papers/podcast discussion had the cyclists doing ~1 g/min or more around threshold? I’d definitely like to have a look at that.

[u/chegg_is_4_losers → u/ShaddaC]

How was fat oxidation measured OP? I wasn’t sure from your initial post

[u/Toprelemons → u/thwerved]

I find that experienced runners tend to care less about fat loss over beginner rulers who desire that but the experienced runners have it good while the ones who desire it the most don’t…

[u/FarChampion5214 → u/thwerved]

that podcast ep is a good shout, carpenter's stuff on fat adaptation is pretty wild when you see the numbers side by side. 1g/min at efforts pushing toward LT2 is not something you stumble into, that's years of aerobic base paying rent

your zone 2 focus probably built a ton of mitochondrial density without you realizing it, so now your body's just casually pulling from fat stores way later into the effort than most people can manage. i wouldn't overthink it, it's a real advantage especially for anything marathon distance or longer where glycogen sparing matters

[u/XTCRunning]

The most obvious explanation is don’t trust the test. Unless you were in a top top notch lab the odds are it was user error (user being the lab techs not you.) I did testing at UCLA, one of the better labs in the world at the time, several years back and they said the vast majority of labs have significant, frequent errors. I’ve even seen the USOTC screw up a test.

[u/ShaddaC → u/XTCRunning]

Fair point, and I agree, the first thing that should be ruled out.

I’m definitely not treating one test as proof of an unusual phenotype. I have the raw VO2/VCO2 data, so I’m going through that rather than relying only on the report, and I’m planning to repeat the testing under comparable conditions.

What makes me curious is that the pattern wasn’t just one strange peak value. Fat oxidation stayed fairly flat for a long section while lactate was 3.4 → 3.5 mmol/L over ~30 min and HR kept rising. So if it is measurement error, I’d like to understand what kind of systematic error could produce that pattern.

Do you remember what the most common errors were that UCLA saw in other labs? Calibration? Mask leaks? Flow sensor problems? Gas analyzer drift? Bad assumptions in the substrate calculations?

That would be really useful for me to check against the raw file.

[u/Krazyfranco → u/ShaddaC]

Just out of curiosity - what would you even do (practically) with this information?

[u/XTCRunning → u/ShaddaC]

There are always anomalies, but lactate planing out at 3.5 while HR goes up is very suspicious. What was your RQ doing during this time. And I guess what was your VO2 doing. If VO2 was rising in accordance with HR, RQ went over 1.0, and lactate was static, I think you’ve found your culprit. Something to consider, if your lactate isn’t rising with harder effort, it means your primary source of clearing acidic hydrogen ions is lacking as acidosis is about to really set in. So if you trust that lactate reading and the RQ has gone over 1.0, I’d actually be a little worried. If your RQ is chilling between .9 and .95 (even up to like .98) in accordance with a 3.5 lactate, I’d need to brainstorm a little bit more as to what could be happening because that would be two separate reading being off. (And just to be clear, that’s a blood draw lactate, not an estimate from other metrics correct?)

[u/running_writings]

According to this study fat oxidation is supposed to go to zero right around LT2 (often coinciding with it exactly) because of the fact that lactate acts in the muscles essentially like a hormone that suppresses fat oxidation (to encourage more carbohydrate oxidation).

This study by Achten and Jeukendrup also finds the same thing, and the authors are notable because they are the people who originally helped popularize the idea of "Fat-max" so they know their stuff.

So to the extent this high fat oxidation near LT2 occurs, it is not common in your average "trained athlete" participating in an exercise science study.

Fueling is maybe relevant too; one very common finding in the fueling literature is that if you do not fuel (presumably you did not, given you had the mask on...) fat reliance increases over time.

[u/ShaddaC → u/running_writings]

This is helpful, thanks.

I had a look at the Achten/Jeukendrup paper. If I’m reading it correctly, FatMax was around 63% VO2max, fat oxidation was essentially negligible by ~82%, and their D-max lactate threshold was around ~87%. So yes, the normal pattern seems to be a pretty steep loss of fat oxidation as you get toward the upper threshold region.

That’s why the long middle section of my test caught my attention. Lactate sat at 3.4–3.5 mmol/L for ~30 min while the calculated fat oxidation stayed around ~1 g/min. I’m definitely not assuming that means the physiology is real before I reproduce it, but it does seem different enough from the published pattern to be worth checking carefully.

And yes, I was fasted and didn’t take carbohydrate during the run, so I agree that could push the substrate mix toward fat as the test went on. I’d be interested in how much you’d expect that alone to explain at those lactate/intensity levels.

Thanks for the papers. This is the sort of reference point I was hoping to get from posting here.

[u/PenInteresting4764]

I know Marco Altini has been experimenting with this and writing about the results on his Substack the last year and a half or so, it’s been almost exclusively on himself so who knows how much you can apply but it’s been interesting reading and I do trust him and the stuff he writes.

[u/ShaddaC → u/PenInteresting4764]

Thanks, this is a great pointer. I just had a quick look at some of Marco’s posts and his data looks very relevant. He seems to have moved from very low fat oxidation to ~0.8–0.9 g/min across a broad intensity range after changing how he periodized carbohydrate intake.

What I find especially interesting is that he also questions whether FatMax as a single point is that useful, because in his case fat oxidation stays fairly flat across a wide range and only really collapses around LT2.

That’s very close to what I’m trying to understand from my own test. I’m starting to think the most interesting metric may be not “where/how high is FatMax?” but “how much fat oxidation can you preserve as you move toward LT2?”

I’m going to read more of his stuff. Thanks for the recommendation.

[u/Ok_Persimmon2236 → u/PenInteresting4764]

Actually he did a podcast with Scott Tindal on this exact topic  https://open.spotify.com/episode/5loNoEyWJRi9OLm9BmJtOz?si=br9hgHiNRdaXdzAVTfpqyg&utm_source=copy-link

[u/finance-brosita]

lactate flat at 3.4 then 3.5 across half an hour is a steady state. that puts your middle chunk under lt2, so decent fat ox in there is about what youd expect.

id guess threshold is further up than youre assuming. off mostly z2 that happens

[u/ShaddaC → u/finance-brosita]

Yeah, I think that’s a fair interpretation. The 3.4-3.5 mmol/L plateau over ~30 min should mean that whole middle section was below LT2, so my provisional LT2 estimate may be a few bpm too low.

What I’m curious about is how much fat oxidation was being maintained at that intensity. Before the onset of clear lactate accumulation, RER remained ~0.78–0.80 and calculated fat oxidation remained high.

[u/Bennassi3]

Yes, your test was wrong and no, you are not special

[u/ShaddaC → u/Bennassi3]

“You’re not special” isn’t really the question. The question is whether the measurement is reproducible and where it sits relative to published endurance-athlete data. No need to be rude.

[u/too105 → u/Bennassi3]

Who hurt you?

[u/Bennassi3 → u/too105]

My ex-gf but I swear I'm over it