Fat Loss

Problems w/ Low Carb Diets & Exercise: Carb Cycling Explained

by Mike Mutzel


New research suggests you should be personalizing carbohydrate intake based upon your metabolic health history and fitness goals. Let's dive into the details…




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Links to studies and videos mentioned:


Podlogar, T. & Wallis, G. A. New Horizons in Carbohydrate Research and Application for Endurance Athletes. Sports Med 1–19 (2022) doi:10.1007/s40279-022-01757-1.

0:33 The Zero Carb camp says that carbs are not essential and do not benefit sports performance.

1:09 Energy In/Energy Out energy balance model says that carbs do not matter if you are in a calorie deficit.

2:04 Carbohydrate intake should match carbohydrate demand of training or competition.

3:19 Zero carb or fasted exercise may limit your exercise intensity or volume, limiting adaptations and hypertrophy.

5:09 Carbohydrates are the primary fuel that is oxidized during prolonged or intense exercise sessions.

5:39 Untrained athletes may get increased benefit from training in a low glycogen state.

6:34 Intra-workout carbohydrates, ingested or swished, can maximize high volume or high intensity exercise adaptations.

10:34 Energy deficit has suppressive effects on hormone production.

13:14 The more trained you are, the more fat you oxidize.

14:19 Prioritize exercise intensity over sticking to dogma.

14:49 Standard recommendations for pre-workout, intra-workout, and post-workout carb intake are high and not personalized.

17:19 Sauna therapy can enhance your exercise capacity and ability to assimilate nutrition.

19:09 Low carb athletes can replenish carbohydrates, even though they are not eating them.

19:49 The amount of muscle you have impacts how many carbs you should be consuming.

    • Hi there Elise!

      I had an issue with the software I use to organize studies (and format the references). The post has been updated!


  1. Hi Mike. Can you comment on the use of exogenous ketones in the context of a “fasted workout” and how they could help with fat oxidation and keto adaptation for both trained and untrained athletes?

  2. Hi Mike. I think the point that you are making that you should not compromise the intensity of your workouts due to being in a low-carb state is valid… that is NOT a wise trade-off long-term. HOWEVER, I don’t ever experience that… my body has been in a low-carb fat-burning state for >22 years… I never run out of energy because I never run out of fat. I regularly do 3 hours of HIIT in MMA (e.g., an hour of Brazilian Jiujitsu (with a few competitive 5 minute rolling/sparring rounds at the end), followed by an hour of Krav Maga (often with defense against multiple attackers at the end), and then an hour of Muay Thai sparring (a dozen or so 3 minute competitive sparring rounds))… always in a low carb state… often in a fasted state. Some of the carb eaters often aren’t making it through one hour before their energy is depleted… only a small percentage do multiple hours like I do. In our 6-7 hour Krav Maga belt tests, the carbers have to bring lots of carbs to eat. I just drink lots of zero-calorie fluids… I don’t need to eat… and I keep up with them just fine, even though I am >50 years old (and they are 20’s-30’s usually). We’re not world-class athletes… but some of those I am sparring with do fight amateur or even semi-professional bouts. So, I think these are genuinely high-intensity workouts (fit-looking visitors and newbies often can’t keep up in single-hour classes).

    So, my questions for you is on some of the biochemistry that you are alluding to, but not discussing explicitly:

    (1) You mention that carb-intake is not needed for a less-than-90-minute work-out, but is needed if you go longer. What is that 90 minutes based on? Is that saying our muscles can only store 90 high-intensity minutes worth of ready-to-burn energy? And then also assuming our bodies can’t be simultaneously converting fat to replenish that consumed energy? Or is that based on surveys of some athletes’ personal experiences? Or? Is that assuming constant-level workouts (marathons) or HIIT or?

    (2) How fast can our bodies convert stored fat into glucose / glycogen / whatever that our muscles can then readily use? That seems critical to the whole discussion in this video! If my body can do that as quickly as needed, then why would I ever need a dietary carb, assuming I never work out so long that I burn ALL my fat stores away? If it can’t convert stored fat to usable forms of energy as quick as I can burn it in a high-intensity workout, then sure there’s going to be some point where I run out of usable energy, even though I still have stored fat (I’ve never experienced that, but theoretically that is clearly possible)… but then that leads to the next question…

    (3) It seems there should be a mathematical model that we can build here… and I have the tools to build such… happy to do so… but I need the facts:
    (3a) In your 3 levels of exercise in your chart at the end, how fast are the muscles burning energy (kcal/min perhaps per kg of muscle)?
    (3b) And how much readily available energy is stored in those muscles in the rested state prior to starting the workout (kcal*min / kg of muscle)?
    (3c) And how much does that vary with carb-loading pre-workout? Or is that just putting carbs in your digestive system and blood for quicker replenishing to your muscles?
    (3d) And then how fast can the body replenish those stores converting stored fat to available energy in the muscles (kcal / min)?
    I realize those are going to be specific to the individual and their metabolic health… but can you give me reasonable ranges of those numbers for healthy athletic people? Or are you aware of a link to any existing mathematical models of that energy burning / storing / replenishing process? It seems like such a math model would help clear all this up. I’m sure it’s more complicated than just those 4 numbers I ask for above… but I gotta start somewhere… if somebody else hasn’t already built it.

    Thanks for any info!!
    — Brian

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