Interpreting lab data – CHO vs Fat

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  • #10107


    I’m trying to interpret my lab results into a % of fat vs CHO usage, but I only have g/min numbers as my test data. In order to take g/min and convert that into % of each contributing to energy usage, is it simply a straight gram-to-gram comparison? In other words, is the contribution of 1g of fat equal to the contribution of 1g of carbohydrates?

    If so, would my “crossover point” thus be roughly 130 bpm? That’s a bit of a bummer, as I’d thought I was focusing on zone 1 (sub 145 bpm) training 80% of the time. But I suppose the data don’t lie…

    If I wanted to move that crossover to a higher HR, is it mainly a matter of even stricter vigilance on my part to lock myself into sub 140 bpm training? Would training zone 3 and 5 as 20% of my weekly training time be detrimental to training a higher crossover point?

    I’ve read the TFTNA book multiples times and use it often to guide my training (thank you for this resource), as well as this website and other books. I create seasonal 24-31 week training plans for myself based off the methods outlined in the book. Since the test, I’ve begun fasted morning runs — so far been doing them 1-2 hours long but have felt perfectly fine doing so (I’d expected to feel worse based on the lab data). I’m also planning on shifting my diet to more fat-based as be part of the mix too. I adhere (evidently, less than I’d thought) to the 80/20 rule. Any other advise would be greatly appreciated.

    Thanks very much in advance.


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    Anonymous on #10121

    I’m not that familiar with RER and crossover analysis, but based on your test, you have everything you need to know to structure your training.

    With your AeT (~2 mM) at 164, you’ll want at least 80% (probably ~90%) of your training to be at or below that intensity.

    Your AeT is within 10% of AnT (164 / 175 = 94%), so you can safely start to add some intensity. However, I wouldn’t make 20% of your volume high intensity. That’d be a good way to blow up.

    From what I understand:

    When studies talk about an 80/20 breakdown between low- and high-intensity training, they’re usually using a “session goal approach”. A session goal approach categorizes training time by workout rather than by minutes spent at various intensities. So two out of ten sessions that include high intensity, regardless of how long the warm up and cool down are, would qualify as “20% high intensity”.

    Studies that use a “modified session goal approach” actually look at the time spent at each intensity regardless of the session goal. That’s a much more accurate way to evaluate the low/high relationship. When done like that, training logs of world-class athletes usually show that 90-95% of their actual volume is high intensity.

    The following is from The Training Characteristics of the World’s Most Successful Female Cross-Country Skier:

    “… total training time was distributed as 90.6% endurance-, 8.0% strength-, and 1.4% speed-training, with endurance-training time consisting of 92.3 ± 0.3% [low-intensity], 2.9 ± 0.5% [middle-intensity], and 4.8 ± 0.5% [high-intensity].”

    Anonymous on #10128

    Thanks for the reply Scott.

    I take it that you meant 90-95% of the athlete logs end up as low intensity (not high).

    Do you not focus training primarily on aiming to maximize fat metabolism then? Perhaps errroneously I was under the impression that this was the most important aspect of endurance training. Maybe I shouldn’t get too hung up on keeping my HR as low as possible (under 145)? Staying under 164 is certainly not an issue. It’s quite challenging for me to maintain it above that for any significant length of time, especially if I’m uphill training.

    Lastly, I was told at the lab that in order for me to be able to increase my LT I’d first need to raise my VO2max (zone 5 workouts) as that is what’s holding back improvement in my lower HR thresholds (the ceiling is too low, so to speak). Does that seem logical to you?


    Anonymous on #10129

    Yes, correct. Sorry for the confusion. 90-25% is low intensity.

    To be honest, I don’t pay any attention to raising VO2, and Zone 5 workouts should be used very sparingly in any case. In reading studies similar to the one that I linked to, when looking at multi-year graphs broken down by training intensity, and where the athlete’s VO2 has increased over the long-term (10+ years), the most notable difference year after year is the increase in total volume. And the vast majority of the increase in volume comes from the easiest end of the spectrum. The total volume of high-intensity training doesn’t increase that much.

    You can see that type of volume summary in the Marit Bjoergen study that I linked to above, as well as in a similar study about Bente Skari, another Norwegian world champ skier. Here’s Skari’s career volume breakdown:

    Anonymous on #10138

    That article was totally fascinating–and helpful. Probably most surprising to me was how little tapering she used, especially compared to for example, runners. But I suppose this was mainly due to a rather elongated competition period, where trail races and mountain objectives tend to be more single-event focused, or at least more spread out in the case of trail racing schedules, allowing repeated training cycles with pre-event tapers in each. Even still it did make me think about how I’ve been tapering, and perhaps experimenting with less.

    My only wish was that the authors had used the 5-zone intensity breakdown instead of the 3-zone.

    One of the biggest challenges I’ve faced in building out my training programs is how to distribute my HR zones. It’s pretty frustrating sometimes when presented with the plethora of calculation methods out there. You get numbers all over the place. I mean just look at that crazy excel sheet…

    Finally though, I think I’m getting a handle on it now. Here’s what I have in my head currently:

    LIT = under VT1 = under 2mmol
    MIT = between VT1 and VT2 = between 2-4mmol = hovering around LT
    HIT = over VT2 = over 4mmol


    Under VT1 = can carry on a conversation/recite the alphabet, etc.
    Between VT1 & VT2 = short sentences only.
    VT2 = speaking no longer possible, except maybe 1-2 words between breaths.

    Zone 1 = lower end of LIT. Zone 1 is where it’s at for 90% total endurance training time and helps with fat adaptation.
    Zone 2 = upper end of LIT. Less effective to train in than Z1 because of increased stress on the body for similar training effect benefit.
    Zone 3 = MIT. This is hovering around the lactate threshold and is a useful zone for working on building up a higher lactate buffer (pushing the red-line into a higher HR).
    Zone 4 = lower end of HIT. Depending on the source, this is a more effective HIT zone for the same logic that Zone 1 is the more effective LIT zone.
    Zone 5 = upper end of HIT. Depending on the source is either good for increasing VO2max, or should be avoided in favour of zone 4 as HIT workouts.

    Does that sound about right? Am I over thinking it–very likely.

    Anonymous on #10140

    I think we have too many Scotts on this post so to add to the confusion I’ll chime in here.

    Carbs contain 4kcal/gram and fat contains 9kcal/gram. You can convert the grams to kcals in your test and arrive at the % contribution from each.

    As Semple says: Your AeT and AnT are quite close together and to maximize aerobic endurance improvement you should begin to add some high intensity work 1-2 times/week. Start with Z3 at about 5% of weekly volume and see how you handle it. Do not drop volume when you add intensity. When you can handle 5% the start adding more Z3. I suggest you do not do much training in Z2 as you are too fit and this will be too hard for you. You’ll need to do most of your basic aerobic work in Z1.

    From your test you know the AeT/top of Z2 is 164 and the top of Z3/AnT is 175. The reason the 3 zone system is used is that in the real world in real training it is pretty hard to parse the zones within a few beats. They move from day to day due to recovery state. Some days, when you are a bit tired, 164 is going to ‘feel’ kind of effortful. Other days it is going to feel easy. Use 10% below AeT as the top of Z1. There is not metabolic event like AeT or AnT that separates Z1 from Z2 or Z4 from Z5. Thus the lack of standard definitions.

    These tests and the hard points they supply are guidelines but you still need to use your sensations and the feedback your body is giving. If 164 feels hard then its because you are not fully recovered and you need to run in Z1 or even slower till you feel recovered.

    For you; keep Z2 volume to less that 10% of weekly. Otherwise you will train yourself right into a hole. Read my discussion of Z2 in our book.I call it the black hole. It is this for those with high AeT relative to AnT.

    If you want to move that cross over point you will need to train below it. You can enhance that process but training fasted in the AM when doing Z1-2 work (not >Z2 though). You can restrict carbs in your diet. Both these will help you use more fat during all intensities.


    Anonymous on #10158

    How many Scotts does it take to… never mind.

    So, this is one of those eureka moments when I finally see everything coming together and making sense!

    I plotted the kcal data and got a cross-over point of 160 bpm. That’s more like it. Now I feel like I’m actually seeing real progress and results in the data. What a good feeling.

    Seeing 160 on the graph made me think that it’s close to my 164 AeT. Does it signify anything when those two match up? Is it physiologically possible to have a higher cross-over point than AeT? My gut tells me no, that shouldn’t be possible — one would need to first increase AeT before moving the cross-over even higher?

    I’ve updated my watch to 147 max Z1 threshold (164 – 10%). I feel like that’ll actually be fairly easy to keep under. I’m good about staying out of Z2 (the book’s warnings did not go unheeded, Scott J), I don’t think that’ll be too difficult outside of maybe while warming up to Z3 workouts, but I can’t see that getting close to 10% weekly volume. I’ve also added in some Z3 workouts to the schedule and am quite psyched to dive into that.

    Now I’m seeing why the fasted morning runs haven’t been painful — I always do them in Z1 — and in the 140s bpm range my body is almost using double the fat to carbs for fuel.

    Definitely I hear what you’re saying about listening to internal feedback and adjusting. That makes perfect sense that 164 would feel hard to maintain when needed recovery has been building up.

    Thank you both Scotts for taking the time to thoroughly answer my questions. I really appreciate it and feel well-armed to tackle the training. I should say it’s been going very well for me. I try to be diligent about it, and I can see significant improvements in my performance since I started using TFTNA a couple years ago. Through the book and the work on this website (I’m always amazed how much effort you all put into updating articles and responding to posts) you’ve really empowered people to get after it.


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