Metabolic efficiency test results: Improving poor fat utilisation

  • Creator
  • #52040

    I’ve just been through a gas exchange metabolic efficiency test. The results were interesting and I’d very much appreciate some feedback on my thoughts on how to apply these results to my training.

    The protocol for my test used 3 minute stages with 1km/hr increments and had me fasted for the test. The test results show an aerobic threshold of 171bpm and an anaerobic threshold of 178bpm.

    This relatively high aerobic threshold HR was pleasantly surprising, as I expected the test to show my as aerobically deficient based on the amount I have to fuel in longer duration workouts, even at very low perceived intensities (even 20bpm or so below the measured AeT). However, the test showed that rather than a low AeT, I just have absolutely abysmal fat utilisation at all intensities, with 35 percent fat utilisation at *rest* (over 15 minutes lying down), and 22 percent fat utilisation rate at my aerobic threshold.

    I’ve found this forum post, where the OP’s results appear to be a less extreme version of my own.

    Based on all the above, my thoughts going forward are:
    – I should try and do as many of my runs aerobic runs as possible in a fasted state.
    – I’m not actually aerobically deficient so it’s reasonable to start adding one z3 workout per week
    – I should probably do most of my aerobic training in z1 (I’m thinking about 75 percent)
    – If I adapt my training appropriately, I should see improvements in my fat utilization relatively quickly

    Does this sound sane, or am I barking up the wrong tree?



  • Participant
    maforbes2000 on #52041

    That is a very high Aet, as measured by lactate levels. Although your fat crossover is well below this, could it be that your crossover hr is not too low in terms of %age of max hr? (eg 20 bpm below 171 bpm, is not a low hr for training).

    What is your AeT and crossover in %ge of max hr?

    I suspect the advice from here, will be to do most of your aerobic runs at intensities up to the crossover hr?

    phildawson on #52045

    I suspect the advice from here, will be to do most of your aerobic runs at intensities up to the crossover hr?

    So this is the bit that has me unsure. Despite the seemingly high AeT HR, I don’t have a crossover point. Even at rest I was only getting 35% fat utilisation.

    Rachel on #52053

    Phil, it does sound similar to the post you reference and your plan sounds good, although yo might want to hold off on Z3 until you get your fat utilization in order. You have a lot of low hanging fruit to harvest right now at these low HRs! The more time you spend in Z1/Z2 will pay off in the long run.

    In that post you link to Scott writes: “I’d say do 75% of aerobic training below 127 and 25% near 164 for now.” So you need to find your own best guess crossover point (the 127 referenced in the other post.) Perhaps a good starting point would be the bottom of Z1, or about 137? You could set an even lower HR if you want to start with or if the that HR feels too hard to do fasted.

    these posts might also be informative for you:

    Dada on #52060

    RMI expeditions sent me this email today:

    phildawson on #52067

    Thanks Rachel, those threads are really useful and give me quite a bit to digest.

    I’ll put off the Z3 work until I’ve improved my fat utilisation.

    137bpm is pretty awkward in that I struggle to get my HR up above about 110 walking on the terrain I have easily accessible midweek, but can’t keep it bellow about 145 running even slowly. Perhaps I’ll have a play with adding weight to walks and see how that goes.

    From the threads you linked and the article posted by Dada, it’s probably also worth experimenting a bit with my diet.

    russes011 on #52080


    I feel like there is something fishy is going on with your MET results. I would suggest being as sure as possible with your ‘diagnosis’ before you spend a few months with the ‘prescription”.

    Specifically, how did they determine your AeT? Gas exchange usually uses the respiratory quotient method where your AeT HR is when you’re by definition at 50% carb utilization. (am I wrong here?)

    Perhaps they gave your AeT based on the lactate method. I bring this up as a possibility because you also had a AnT test, which I think usually uses the lactate method.

    (I feel like the lactate method is less reliable for determining AeT, but am not sure. For the UA training method I believe the gas exchange approach is most useful. So if this is the discrepancy your AeT HR is perhaps much lower than you think and correlates to your experiences running in the field)

    If you have the time, I would cross check your presumed AeT HR (perhaps both the lactate method one vs the gas exchange one, if you have both) and see how it compares to the UA AeT HR drift test.

    Also, I believe performing both an AeT and AnT test on the same day may confound the results of each. I think they often suggest they do them on difference days.

    You could also attach a photo or pdf of your MET so we could see it. (scrub your personal info before doing this)

    — Steve

    AshRick on #52090

    The note that AeT is “by definition” the 50/50 fat/CHO crossover point has come up a couple times on the forum recently. But…is that actually true? I would think for an endurance-trained athlete, that crossover point can be well below AeT pace.

    It’s an important thing. Can someone point to an objective answer to this?

    Rachel on #52095

    For that in between zone, a walk/run protocol works well, or hiking uphill. I do a lot of the walk/run and if you set up your HR alarms you don’t have to stare at your watch all the time. I usually get into a routine where I know about how far I can jog before my alarm will start beeping at me. I set the alarm for a few beats lower than my upper limit, and I also have one to alert me when I need to run again. It’s pretty effective at keeping you in the right zone.

    And I agree with Steve it would be great if you could upload your results so we can take a look.

    russes011 on #52101

    Aerobic threshold definition

    AshRick, The above link partly answers your (our) question.

    AshRick on #52142

    Steve — got it. Doing some further reading…max fat oxidation (MFO) occurs just below crossover. And MFO is somewhere around <65% of VO2max in a trained athlete.

    So, if we imagine for a minute that I am “trained” – and further assume that my VO2max pace is 6:00/mile (a reasonable estimate) — then 65% of that pace is ~9:15/mile.

    Which about jibes with my recent AeT test.

    I guess I keep thinking of lactate threshold, which is more like 70-85% of VO2max. The crossover point is below that, as is AeT.

    phildawson on #52155

    Thanks all. There’s been some interesting discussion of the definition of AeT which has given me some stuff to process.

    I’ve attached my test results, for those of you who asked.

    Going back over the results myself. I’m starting to suspect the test started out too fast. I plan to do a drift test later today to attempt to sanity check the results and will report back after that.

    phildawson on #52156

    Apparently that photo was too large. Here’s my test results.

    You must be logged in to view attached files.
    russes011 on #52170


    Your test appears to be a combined VO2max and RER (%carbs/%fat) test. Optimally, these tests are done on separate days, but combining them is common.

    It measured your VO2 max and from that it calculated your HR zones based on population averages. This is about as accurate as using your MaxHR and then taking a percentage of that. This is a common and accurate method, on average, as long as you gave it 100% max effort at the end–your VO2 curve flattened out at the end but I couldn’t tell for sure.

    It also measured your RER (%carbs vs %fat burned) and presented the results by HR zone. You do not have a crossover point (you always burned more carbs than fats, in the HRs measured). Your Faxmax was about 45% and occurred in your lowest zone with a HR in the 150s. Keep in mind that you would probably burn more fat at even lower HRs, so this Fatmax isn’t so useful.

    There was a glitch at the start of the test, perhaps the tubing wasn’t sealed or connected properly, and as you mentioned you started out too quick and slow downed. If the test is repeated, they should measure your data at rest for some time, and once reliable curves are established the pace should be increased slowly, especially in the beginning. With better low HR data curves you would probably have had a crossover point, say in the 130s. (A bonus metric with measuring your RER at rest is that it can give your basal metabolic rate in calories per day via the Weir equation).

    Keep in mind that your crossover point does not determine the AeT and your AeT does not determine the crossover point. They are unrelated metrics (I’ve recently learned this).

    I will post again below regarding my recent thoughts on the following:

    1. AeT definition and determination

    2. Crossover Point, an/or METs in general

    Hope this was helpful.

    I will leave any exercise/training prescription to the moderators, UA books, other participants, and online articles.


    russes011 on #52172

    PS – Your VO2 max is about 38. You can look up how good this is based on your age. This is an excellent and traditional marker of how well developed your aerobic energy system is.

    russes011 on #52175

    This is my current understanding, which has evolved significantly over the past few weeks.

    Aerobic threshold is a nebulous, albeit useful metric. We take it to mean the HR or pace below which we are predominantly using our aerobic energy system to perform exercise (note: the aerobic energy system uses both carbs and fats, while the anaerobic system does not use fats). There are different ways to define or determine one’s AeT. One reliable and reproducible method is to correlate AeT to LT1.

    LT1, or lactate threshold 1 (not to be confused with LT2, or anaerobic threshold [AnT]), is the point where lactate starts to spill into the blood stream, often defined as a lactate level of 2. (There is always some lactate in the blood stream, the level of 2 was chosen somewhat arbitrarily as the cut-off point.) If you hold your pace constant at or just above your LT1 (or AeT) the amount of lactate in your blood stays constant, let’s say at 2: your body clears it from the blood as fast as it spills into the blood. (Note: at higher HRs, when lactate levels keeps rising despite holding a constant pace, this is defined LT2, or AnT—you can no longer clear the lactate from the blood as fast as its spilled into the blood.)

    Keep in mind we don’t know exactly whats happening in the cell, let alone the mitochondria, during all of this. We just know what we see in the blood stream. I say this because we don’t know if lactate appearing in the blood truly means the anaerobic energy system has just bypassed the aerobic system (percent wise) with energy production. This is because a well developed aerobic system can also clear byproducts of anaerobic metabolism before they enter the blood stream. To some extent, the muscle cell is a black box. This why I believe AeT is a nebulous metric. Nevertheless, seeing lactate spill into the blood likely indicates that the aerobic system is ‘overloaded’ and you are no longer at an easy, ‘endlessly’ sustainable pace, ie, below AeT.

    The theory behind a drift test being useful in estimating AeT seems to be based on a combination of the above lactate studies, logic, and the data crunching experience at Training Peaks. If your chosen HR is well below a lactate of 2 (ie your LT1), your pace will not drift down more than 3% over an hour. If you’re at a HR with a lactate much higher than 2, your pace will drift down more than 5%. If you establish a pace where your HR yields a lactate of about 2—that is, you’re at LT1—then your pace will drift down between 3-5%. Furthermore, if your AeT—as determined by this test—is within 10% of your AnT, it is thought that your aerobic energy system is well developed. These tests, however, may be confounded by the inherent variability of ones HR due to stress, environment, diet, sleep, and running economy. The reliability and accuracy of these tests have not been confirmed with scientific study, as far as I know. Nevertheless, the logic behind it appears sound, and both UA and TP have their respective expert opinions vouching for it.

    Another simple way to determine your AeT (or the top of UA Zone 2) is to determine your Max HR, VO2max, or AnT and then take a population based, predetermined percentage of these HRs as your AeT. For example ~70% of your MaxHR estimates your AeT. The MAF method, takes this a step further and doesn’t require any individualized testing: your AeT is simply 180 minus age (+/-). Although these methods are simple and easy to use, they’re plagued with the inherent inaccuracy of predicting individual metrics based on population averages.

    So which metric should one use? Honestly, it probably doesn’t matter too much because the goal is to train well below AeT, and not necessarily at AeT. An estimate of AeT (the top of Z2) based on population data using Max HR and/or AnT would be reasonable. When in doubt—based on your ventilatory rate and perceived effort—just slow down. If AeT rises with time, you don’t need to chase it, just continue running at the same easy pace—benefits remain the same. Maintaining an easy feeling during these sessions should be paramount. Training long slow distance (LSD) comfortably below AeT has been proven to build one’s aerobic energy system. This is why we spend plenty of time here. For example, if your MaxHR is 190 your aerobic (LSD) training zone would 60-70% of that (HR 114-133). If you initially choose to target the middle of this range (123) this would give you a 10 point buffer for possible inaccuracy of the AeT (133). One can then make small adjustments to this predicted aerobic training range over time based on how easy the running feels. Be honest with yourself. Chasing AeT should be avoided, however, because it often causes one to actually spend too much time in Z3 without knowing it, and if this occurs studies have shown that it negates the benefits of polarized training.

    I will post again regarding Metabolic Efficiency Tests.

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