Fatmax vs AeT

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  • #47757


    In training videos and blogs for triathlon or cycling often FatMax is being used as the threshold at or under which low intensitiy training should be performed. As far as I understood that is the HR where your absolute consumption of fat is highest.
    At Uphill Athlete, we use AeT, which is defined as the HR at which the ratio between fat consumption and glycogen consumption is 1:1.

    To me, the FatMax value kind of makes more sense for the goal of optimizing fat metabolization. What is the reasoning for UA using AeT anyways? Or are FatMax and AeT so close that it doesn’t really make a difference?

    Thanks in advance,


  • Participant
    Dada on #47780


    Scott Semple on #48038

    My understanding of FatMax is that it can only be pinpointed in a lab. And even then, I suspect most techs will give you a blank stare if you use the term.

    We use a drift test to determine AeT because it gets reasonably close to the true value and it’s easy to test repeatedly. Because we are all dynamic, changing systems, it’s more practical to use a test you can repeat often.

    If you have access to a regular Fatmax test, then by all means use it. Lactate is another example. FOr example, I never do drift tests because I have a lactate meter.

    frnkr on #48211

    Scott, I’m curious, how do you use lactate meter (your protocol, frequency, etc)? Curious because I’m buying one too 🙂 Thanks!

    tom.hobbs on #60025

    Hello — I am confused about the definitions of AET. I seems it is defined on this site and in your book as the heart rate at which lactate increases by 1 m mole / liter or rises to about 2. It is also defined in this post and in your book at the crossover point between fat and carbohydrate oxidation. I recently had a treadmill lactate and oxidation test by the Human Performance Lab at Colorado State University, a well respected outfit. My AET by the first definition was far higher than my AET by the second definition. I would be grateful if you helped me to understand this discrepancy because it has substantial implications for my training. If I use the first definition, my AET is easily within 10% of my lactate threshold. If I use the second definition, my AET is probably 25% below my lactate threshold.

    Many thanks for your help

    Eddie on #60366

    Hi Tom,

    There are similar discussions in older posts. Check out, for example, the “Aerobic threshold definition,” “lactate test results and nose breathing” or “Analysis of gas exchange/lactate test for future training” threads, just to pick a few.

    From what I’ve seen, lab techs use a combination of lactate, VT, and/or RER to determine AeT (and zones). It seems that only the paragon, well-trained athlete’s AeT will typically overlap with fatMAX, 0.85 RER, ~2.0 mmol, etc.

    The definition of AeT varies as much as one’s own AeT day-to-day. And the 10% AnT rule is just a rule of thumb as well. As much as I want precision, none of this is an exact science, but I’m no expert. Personally, I go with an AeT definition that I can test consistently to validate and direct my training.

    Scott Semple on #60369

    Eddie makes an excellent point. A metric that you can test more often will be much more useful than something more precise but harder to measure. Frequent tests are ideal for guiding the training process as things change.

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