Endurance Skating

Training, Racing & LCHF Fueling For Skating And Endurance Sports

Lab Ratting: Maximum Aerobic Heart Rate & Substrate Usage

I almost didn’t write this blog post because when I got the results they were not quite as good as I was hoping for. However after mulling on it for a while, I thought it’d still make a good post and also help me to know where I stand and how to improve going forward..

fat burning tshirt

Am I there yet?

Lab Testing vs 180 Formula

Even though I’m a firm believer in the 180-formula for approximating your Maximum Aerobic Heart Rate (MAHR) – ie the heart rate that will give you the maximum absolute amount of fat oxidation – it is a still statistical “best-fit” of data, and there is no way to determine its precision for your own particular n=1 case.

Even if you assume the formula is accurate, it still doesn’t really tell you just how good a fat burner you are — ie what your fuel substrate profile looks like. Depending on your body’s ability to use fat for fuel, you could just as easily be burning a 40:60 or a 90:10 mix of fat:carb, and both could qualify as your Maximum Aerobic Heart Rate.  Both the MAHR and the substrate usage profile are important to know for endurance athletes in particular, as it will give them a good idea of what intensity to train at to develop their aerobic engine, and also what macronutrients they need to be replacing to avoid a catastrophic glycogen depleted bonk during a racing.

In order to get an accurate substrate profile, you need to book yourself in for a Laboratory RQ Test.  RQ stands for “Respiratory Quotient” – the test basically works by comparing the Oxygen:CO2 ratio of the gas you are breathing in and the ratio that you are exhaling.  It so happens that the 3 different macronutrients produce different amounts of CO2 when oxidized by your body, so by comparing the Respiratory Exchange Ratios before/after, it’s possible to quite accurately calculate how much fat, carbs, and even protein that you are using. Clever stuff, indeed.

Getting Tested

So, in the name of Science, I booked myself in for my own RQ Test a couple of months ago.  The testing protocol basically consists of running on a treadmill where the speed is gradually ramped up, with a gas mask strapped onto your face so that the exhaled air can be collected and analyzed.  Heart rate data was also collected so that my substrate profile could be correlated with changes in heart rate.

Oh, so you think darkness is your ally...

Oh, so you think darkness is your ally…


All in all, the test took about 25 minutes. The treadmill was initiated at a low warm-up pace and gradually ramped up by 1km every 4 minutes.  I have to say that it felt like the treadmill was not very well calibrated, as it felt 10-15% faster than it should have – I run on gym treadmills pretty regularly, and I’ve got a pretty good feeling for a certain pace and what my heart rate should be doing at that pace, and this felt way too fast.  Having said that that, the actual pace that you are running at is completely irrelevant for what we are looking to achieve with this test, which is substrate usage vs exercise intensity. It doesn’t matter if 160bpm equates to 9km/hr or 12km/hr for the purposes of the test, what matters is what my heart rate was at the time. Because of that, I’ve left off the actual km/hr paciing from the discussion, as they’re either inaccurate and/or irrelevant.

OK, so what did this test actually tell me?

It’s numbers time…


detailed energy & substrate profile usage breakdown

detailed energy & substrate profile usage breakdown

substrate chart

Key numbers:

Maximum aerobic heart rate measured at 135bpm

Fat oxidation at MAHR: 384hcal/hr (43g/hr)

Carb oxidation at MAHR: 115kcal/hr(29g/hr)

Protein oxidation at MAHR: 52kcal/hr(13g/hr)

What’s Going On?

OK, this was tres interessant.

First thing to note is that my recorded MAHR of 135 is actually 8 beats lower than given by the formula (at time of writing). You’d think that given my relatively high maxHR (I top out around ~200bpm) AND my relative health/fitness status, I would have expected my MAHR to have been higher than the formula if anything, but No. The test numbers are what they are. I have some more thoughts on this later on..

Low Intensity

My substrate usage at my MAHR was 70/20/10 Fat/Carb/Protein. In absolute terms this worked out to a fat oxidation rate of 43g/hr, or 0.72g/minute.  Remember back to the FASTER Study which demonstrated that the LCHF elite ultra runners group was averaging 1.54g/minute of fat oxidation and it’s clear that I’m nowhere near that level; that shouldn’t come as a surprise, as I’m far from elite… I do however manage to match or even slightly exceed the average of the high carb group (0.67g/min)… woopie doo.

What is equally interesting is how my substrate profile changes with increasing intensity. You can see that my absolute ability to use fat for fuel remains high up around 40g/hr right up to 175bpm. This is a good thing – it means I have an ability to push up to a high intensity and still be engaging near-maximum lipolytic power. However at this higher intensity the additional energy requirements is now being sourced from more carbohydrate oxidation; I have become more glycotic, and my substrate profile usage has changed to 46/48/6 Fat/Carb/Protein, which equates to 92g/hr of carbs and 14g/hr of protein. As you can see, it’s the carb requirement that has massively increased.  92g/hr of carb usage equates 369kcal/hr.  If we assume that I can only carry 1,600kcal of glucose in muscles & liver, then we are looking at a potential bonk scenario somewhere between 4 – 4.5hrs at this intensity. in the real world, there are various other factors in play which might extend this to beyond 4.5hrs. Also, 175bpm is actually above marathon race intensity for me, which is why I can complete a full marathon using zero exogenous carbs. Also if I go longer, I have to slow down, which means less carb requirement anyway – it’s the central governor doing what it’s meant to do and limiting intensity in relation to distance in a well balanced metabolism.

Threshold Intensity

Above this level, somewhere between 175bpm and 182bpm I hit my lactate/anaerobic threshold. Physiologically speaking, this is where my blood lactate level is no longer at a steady state and is accumulating faster than I can clear it. The reason is that the body has switched over so heavily towards carb usage, and one of the byproducts of CHO oxidation is lactic acid. 182bpm is about 92% of my maxHR, which is well above any sort of intensity that I can maintain for more than about 30-45mins.  Although it’s much debated if lactic acid is actually a bad thing, or if it’s the hydrogenation that actually results in the “burn”, it undoubtedly correlates well with the exercise intensities and durations that can be sustained.

Thoughts & Interpretation

The testing protocol left a little to be desired, in my opinion.

Firstly, I would have liked a longer warm up. The body gradually switches to fat oxidation during exercise, and doesn’t fully  reach this point for 60-90 minutes. So it’s possible that had I had a better warmup then I could have produced some better fat oxidation numbers. I guess this is the difference between Lab and real world scenarios. If I ever do this test again (and I’m sure I will at some stage) then I’ll be sure to give myself a proper 45 minute warm up even ahead of stepping foot onto the treadmill.

Additionally, in retrospect, I did this test while still recovering from running a marathon, and was still quite fatigued at the time.  It’s quite likely that my aerobic function still impaired and not optimal going into the test.  That’s totally my fault, and nothing to do with the testing protocol.

Also It would have been nice to have had smaller incremental steps in treadmill speed. As the treadmill ramped up, I went from 135bpm  to 150bpm, which is a big difference.  Given my suspicions about the accuracy of the treadmill in the first place, it left a bit of a hole in the data at very important exercise zone…

It left me slightly disappointed that my recorded MAHR was as low as 135bpm. This is only about 68% of my maxHR, and about 50% of VO2max. I suspect that if more data points had been taken between 135bpm and 150bpm then they might have found that my real MAHR was a a few beats higher, but the treadmill test was increased in discrete 1 km/h increments.  Off of this (lack of) data, I’m going to continue to assume my MAF range is 140-145bpm.

We tend to think of fat vs carb, and aerobic vs anaerobic in discrete terms, but in reality the body works as a single unit and the various system interplay with each other and operate simultaneously; the important thing is “which one are we stimulating and targeting in any given situation?” Looking at my substrate usage profile, it becomes clear that I can safely ramp up my intensity quite a lot higher and not worry about “going anaerobic” – or more concisely, switching primarily to anaerobic muscle fibre stimulating carb usage. So long as I respect the crossover point (looks like somewhere around 170bpm) then my body will still be mainly aerobic – ie using fat as fuel and be stimulated as such.  In fact, some higher intensity work around the 160-170bpm (up to 79-84% of maxHR) range should help develop my aerobic function as my body will mainly upregulate lipolysis in response to increased energy demand; fat is still the primary fuel right up to 170bpm, after all.  You see.. MAF and MAHR is not the be all, end all.


The recorded MAHR of 135bpm is right around my intended  Le Mans target heart rate… At this sort of intensity, at a minimum I have to worry about consuming 29g/hr of carbs and 13g/hr of protein (remember, fat stores are already plentiful) – that should be easy…


Final Conclusion?

Becoming a fat-burning endurance machine is very much a multi-year endeavour. Increasing the size of the fat burning engine should be the primary goal of the endurance athlete. At the moment I would say that I’m more of a “fat-smouldering assembly-line work in progress”.

Related Posts

Fat Adaptation for Athletes

The Big Book of Endurance Training & Racing – (Maffetone)

Ben Greenfield’s Podcast: Five Simple Steps To Turning Yourself Into A Fat Burning Machine

MAF Training – Train Slower (And Smarter) To Get Faster



12 comments on “Lab Ratting: Maximum Aerobic Heart Rate & Substrate Usage

  1. runnerD
    November 18, 2015

    Thanks for an interesting read and sharing your test data. I recently started following the 180/MAF formula and am thinking of getting tested to determine how accurate it is for me (have yet to find a place in town that offers such a test, though). My understanding was that the 180-formula gives a target heart rate that is approximated by RQ = 0.85 and represents a 50/50 fat/carb burning – i.e. the threshold between mainly aerobic to mainly anaerobic. It sounds like your interpretation of MAHR is maximal fat oxidation, which may explain why it is lower than your 180-formula target?


    • Van
      November 18, 2015

      Yes, indeed. MAHR is the point of max fat burning and can happen at (almost) any RQ number because it depends on total energy output as well as percentage coming from fat, while the crossover point (RQ 0.85) is a fixed point that can be thought of as when you go “mostly anaerobic”. The best fat burners will have a large AND efficient engine.


      • Ben
        November 26, 2015

        I think runnerd has it right. Here is Phil Maffetone’s definition of maximal aerobic heart rate from his book:
        “The training heart rate that reflects optimal aerobic training, and a number which when exceeded indicates a rapid transition to more anerobic training”.
        This would imply a heart rate near the crossover point as the MAHR. In your case you were still burning 40g/hr of fat at 167bpm. Not too much less than at 135bpm. Above 167 is when you transition “rapidly” to anerobic metabolism.
        Also I would not be dissapointed with your reaults. You seem quite fat adapted with a crossover point at around 167. Many athletes have noncrossover point and necer burn more fat than carbs at any heart rate.


  2. Van
    November 26, 2015

    The MAHR is not the same as the crossover point – Dr Maffetone speaks about how MAHR (max fat burning) can occur at different RQ values, but is nearly always below 0.75 in good fat burners:
    (@41m, great overall interview)

    Liked by 1 person

    • Ben
      November 26, 2015

      I disagree. I just listened to the inrerview with Phil Maffetone that you referenced. He says nothing about RQ of .75. He says that MAHR is somewhere below RQ of .85 which represents 50/50 fat to sugar burn. He says good fat burner will be in the 60’s as far as fat % burn vs carbs. This is an RQ of .8x.
      So it is still somehat unclear as to how the 180 formula relates to actual metabolic efficiency testing. His definition from his book that I cited in my previous comment implies it is at some heart rate just before the crossober point is reached.


      • Van
        November 26, 2015

        No… listen carefully… he says that the *percentage* fat at the MAHR is in the 60s, which implies an RQ < 0.85.


      • Van
        November 26, 2015

        and sorry.. I meant 0.85, not 0.75 (just to unconfuse things)


  3. Van
    November 26, 2015

    Incidentally, if you ever listen to the Natural Running Network podcast with Richard Diaz, he does lab testing on runners extensively, and I have heard him mention that the 180 formula is a good approximation in “about 70% of cases” that he tests. So make of that what you will!


  4. runnerD
    December 6, 2015

    Well it looks like my metabolic test results indicate that I’m an outlier of the 180 formula like you, Van. I hit my 50/50 fat/carb ratio at a HR of 190bpm. This seems a little silly since using the MAF formula, I’d be about -3 to -10 years old.

    Unfortunately, although my body seems to be happily burning fat, I’m only going at about 6 min/km at 190bpm, so although my aerobic engine is dominant, it doesn’t produce any steam!

    According to your conversation with Ben above, I should be training at about a 60/40 fat/carb ratio as my target HR, ~183bpm. I’m going to try that going forward, but wary about getting injured, since maintaining 183bpm on a regular basis seems a little strenuous…

    You said in your post that you plan to stick to a MAF range of 140-145bpm – shouldn’t you be training at a much higher HR based on your test results if the interview you referenced defines MAHR as 60/40 fat/carb ratio?


    • Van
      December 7, 2015

      I think you’re still confusing the 50/50 crossover point with the Max Aerobic Heart Rate. All you should be concerned about with figuring out the MAHR is the absolute amount of fat that you are oxidizing at that particular heart rate – the amount of carbs your are oxidizing is almost incidental, and can either be very low, or moderately substantial. Keto athletes will inevitably have very small amount of energy coming from carbs until very high heart rates, thereby producing a very high crossover heart rate.

      Have a read of Zach Bitter’s blog post here where he clarifies his own MAHR:
      “Analyzing the data was wonderfully reassuring to me. My fat metabolism peaked at 1.57 grams/minute. At this point in the test, my VO2 uptake was at 49.4. By dividing this number by my eventual VO2 Max of 66.1, I can calculate at what intensity I burn the most fat: 74.4%. At that intensity, I was burning 98% fat 2% carbohydrate”

      In the comments, he then says that point lines up with a HR of somewhere around 140-150bpm.


      • runnerD
        December 7, 2015

        Ok, I think my confusion is not with the difference between 50/50 vs MAHR, but what the HR derived from Maffetone’s 180-age formula refers to. Is it meant to approximate your 50/50 crossover point (or just below), or is it meant to approximate your MAHR?

        I care less about what my MAHR is because I don’t intend to train at a HR where I burn 98% fat. According to Maffetone, I need to train at a HR close to the 50/50 crossover point in order to make the best improvements.


      • Van
        December 9, 2015

        180-age is a statistical derivation from the datapoints on the numerous athletes that Maffetone worked with following extensive lab testing to determine the MAHR. Hence if you want to know your own MAHR the only way to know for sure is to do your own Lab test. Maffetone came up with the formula as a “best fit” catch-all for general advice,but it was only AFTER he had collected the data that he was able to backfit the formula to the data. If you listen to the whole Primal Endurance interview then you will hear the whole story of how he came up with the formula.

        Being statistical, there are of course inevitable outliers – hence why I make the point of Richard Diaz saying it works in “about 70% of cases”.


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This entry was posted on May 27, 2015 by in Nutrition, Science, Training and tagged , , , , , , .

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