Rethinking roles of carbohydrates and fat for performance

By September 14, 2015 November 11th, 2016 Athletic Performance, Exercise, Fat-Burning Journal
Rethinking Carbohydrates

New study shows the importance of fat-burning during high-intensity training.

Through the years, it’s been difficult for many people to understand the role of fat as a fuel. We live in a sugary world where glucose is understood to be the only source of energy, as if fat were non-existent. We learn it in high-school biology class. It is written in exercise physiology textbooks, and of course, in sports magazines. We see it on websites and particularly in advertisements — sugar is our primary fuel.

Of course, it’s not true.

One could easily make the argument that fat is actually the body’s primary fuel since it provides long-term energy, and that glucose is secondary because it’s limited to a few minutes of fuel. The fact remains that we use both at all times. Whether we’re sleeping, sitting at our desks, warming up for a workout and even during a high-intensity interval session or race—we burn fat along with sugar (glucose).

A new study* adds a previously missing link to the fat-burning discussion. Researchers in Norway and New Zealand were able to measure fat-burning during high-intensity interval training. They showed for the first time that fat-burning, along with glucose, not only occurs during high-intensity efforts, but that the greater capacity to perform anaerobic work is mostly explained by higher fat-burning.

The notion that more energy from fat equals better performance has been a key part of my program of Maximum Aerobic Function (MAF) since the 1970s. I have shown improvements in MAF Tests, clinical research showing aerobic base building leads to very successful race performance, even before anaerobic training is added, and a diet higher in fat and lower in natural carbohydrates (void of junk food but adequate healthy protein) directly affects fat-burning. This also corresponds to a lower respiratory quotient — meaning those athletes who eat well and develop their aerobic systems burn more body fat both at rest and during sub-max testing. The new study adds a missing component in that they showed those burning more fat had a greater capacity for high-intensity training.

We can measure fat-burning in an exercise physiology lab quite accurately with a device called a gas analyzer. As part of a treadmill test, users breathe into a face mask with connected tubes enabling the measurement of oxygen and carbon dioxide from which a percentage of fat- and sugar-burning can be determined. Most athletes, including those who work out to keep in shape, don’t perform these tests due to cost and convenience, so testing like this is typically conducted during exercise research. Athletes who are tested must be evaluated with regularity so they know that fat-burning (and other parameters) are improving over time.

However, it’s not necessary, nor practical, for everyone to go through such a rigorous testing process. That’s because it’s usually somewhat obvious when the body is a poor fat burner — in most cases, the body stores too much fat. There are other indications too. Even if we can’t come up with a specific number for exactly how much fat and sugar we’re burning during a workout, like during a treadmill test, most of us can easily know if we’re burning a sufficient amount of fat for fuel or not. An inventory of clues — common signs and symptoms listed here with more details about them through this website — can help inform us of our fat-burning abilities. This important inventory of body function is similar to other questions clinicians often ask patients to quite accurately understand their health and fitness status (this also correlates well with other high-tech tests). Having one or two of the clues listed below may mean there’s room for improving your fat burning — this in addition to the first question of whether your body is storing too much body fat:

  • Consumption of refined carbohydrates.
  • Body fat above normal/healthy.
  • Frequent fatigue.
  • Blood-sugar irregularity (too low and or high).
  • Frequent hunger.
  • Risk for chronic disease such as diabetes (or pre-diabetes), high blood fats, hypertension, or
  • family history of these conditions.
  • Race performance plateau.
  • MAF Test or other sub-max performance evaluations not improving or worsening.

 With more fat-burning during competition, there is also less of a need to consume carbohydrate during long races. One could cut nutrient needs in half or greater just by relying more on fat stores. Having to eat less during competition takes a big burden off the gut, a significant source of stress for many athletes.

So, how can you improve fat-burning and reduce your reliance on sugar? The answer to this is somewhat individualized as metabolic, nutritional, hormonal and other bodily components play key roles. However, here are the two most common requirements I have seen in those who need to improve fat-burning:

  • Eliminate refined carbohydrates during meals and snacks (they’re fine during long training and racing but only when necessary). It is particularly important to avoid high carb intake before working out because this can impair fat-burning. Just this recommendation alone can quickly shift the body’s metabolism to increased fat burning — within 24 hours — especially if reduced carbohydrates are replaced by healthy dietary fats, which can also increase fat-burning.
  •  Train the slow-twitch aerobic muscle fibers, which contain mitochondria that burn fat. The ongoing “no-pain, no-gain” trend has taught us to work out hard, so we often don’t train those slow moving muscles. The real goal is to make the slow-twitch muscles move faster — this happens over time with the right training and food choices that improve fat-burning.

Burning both fat and sugar is how human energy systems evolved. Relying on sugar without adequate fat-burning often leads to compromised fitness and reduced health.

The world is getting fatter. That’s no surprise. However, what is surprising is that regularly working out no longer means being lean. Too many marathoners, triathletes, steadfast gym visitors, aerobic dancers, walkers, and other exercise enthusiasts are counting the wrong calories, gulping glucose drinks, eating low-fat, and training hard, only to find themselves in the growing ranks of the overfat.

For those with too much body fat, health issues, or whose fitness is waning, it’s time to rethink the carbohydrate myth and embrace fat for fuel.

*Ken J Hetlelid, Daniel J Plews, Eva Herold, Paul B Laursen, Stephen Seiler. Rethinking the role of fat oxidation: substrate utilisation during high-intensity interval training in well-trained and recreationally trained runners.” BMJ Open Sport Exerc Med, 2015.


  • Jon says:

    If I experienced rather severe “low carb flu” (fatigue, mental fog, lack of energy while running) throughout the 2 week test does that mean I just need to keep some carb in my diet, or should I continue restricting it in the hopes of “adapting” eventually?

    • Jon:

      Relying on carbohydrates to supply the basic energy needs of your body is not a solution. It means that your body essentially has a problem with providing fats for fuel. For most people, the two-week test is a long enough period that the body can shift from burning primarily carbs at rest or a light intensity to fats. But if you find that the two-week test period is too short for you or the symptoms are unmanageable, try ramping down your carbohydrate intake over the course of the two weeks. Start by added sugars, honey, and sweeteners, move onto high-glycemic fruits, then onto grains (even whole grains) and tubers, then onto low-glycemic fruits, and finally finish with legumes. Then stay at this level for say, one or two weeks, and slowly start adding things back in a reverse order. Whenever symptoms reappear, you know you’ve eaten too much of a particular food, and higher-glycemic foods will not benefit you.

      Let me know whether this makes sense to you and please don’t hesitate to ask clarifying questions.

  • James says:

    Hi Ivan
    I’m not CI – I’m naturally lean/typical ecto, waist is same as it was at 17 and I’m almost 27 now. However recently I have been trying to eat a higher proportion of fat and less carbs. I’ve been feeling really anxious for no particular reason – do you think this could be related?


    • James:

      Could be, but I have really no way of tying it to anything. Could you tell me more?

      • James says:

        Bit of work stress I guess that could be it ? Probably having more of an impact that I first assumed? Also I think I sometimes worry too much about whether a food is healthy or not and sometimes don’t eat enough?

        • James:

          Possibly. You’d be surprised how much aerobic function (which implies fat usage) is lost when stress levels increase. And yes—worrying too much about food can be a stressor itself. For example, I’d recommend that you don’t think about macronutrient ratios (fat, carbohydrate, protein) in terms of “healthy” and “unhealthy” but rather in terms of which energy system is going to activate thanks to them (fat-burning or sugar-burning).

          It’s not that the food itself is unhealthy, but rather that activating the sugar-burning energy system for too long is unhealthy. Conversely, not being able to activate sugar-burning channels because of chronic glycogen depletion can also be unhealthy. So, I’d recommend that you think like this: “will this food in this quantity given my current stress/exercise levels make my sugar-burning system go overactive?”

          For example, when you’re 30-40 minutes into exercise, even very high glycemic foods may not cause your sugar-burning system to become overactive. Rather, those sugars will mostly go towards enabling greater fat-burning. So whether a food is “healthy” or “unhealthy” or has a “good” or “bad” effect really is situational.

          (When you do have to think about foods in terms of “healthy” and “unhealthy” is in relation to chemicals, additives, etc.)

          • James says:

            Hi Ivan, thanks, I guess you pretty much answered the question in response to Aaron’s comment further up.
            Strange though – I often read about how stress (cortisol) causes weight gain, increased body fat, slower metabolic rate. It seems to have the opposite effect on me – I can eat so much when I am stressed and then feel hungry again soon after and pants begin to feel more loose etc. The opposite seems to occur when I’m relaxed i.e. I can eat a meal and not feel hungry for a long time.

          • James:

            True, cortisol does cause weight (fat) gain, but not when combined with testosterone and adrenaline. Together, these hormones increase the metabolic rate quite a bit. So you may have a particularly high tolerance to stress. (Most people these days have some level of adrenal insufficiency, which is why even a small rise in cortisol so often leads to weight gain: no adrenaline means no testosterone, means no increase in the metabolic rate).

            The reason you get hungry in this state is because one of the hormones that co-occur with cortisol and adrenaline is ghrelin, whose job it is to make you feel hungry.

            When you’re relaxed not only is your metabolic rate low, but other hormones are activated, particularly leptin, which increases fat-burning. In that case, your body doesn’t want to eat because it perceives that it’s well-fueled.

          • Matthew says:


            You mentioned above to James that eating a high-glycemic carbohydrate 30-40 minutes into a workout will “mostly go towards enabling greater fat-burning.” Why is this? I would think that it would contribute to less fat-burning.


          • Matthew:

            By that point, the aerobic system (particularly the leptin response) is going strongly enough that you really aren’t going to be able to reduce fat-burning by eating a lot of high glycemic carbohydrates. Since you are using fuel at massive rates, the insulin response will die down very very quickly (as soon as you finish utilizing the sugar) and it won’t really get in the way of fat-burning.

            The reason it contributes to greater fat-burning is because the body needs to burn a little bit of glucose in order to burn fats well. So any glucose you eat will just go to fueling that “starter motor” or topping off the tank that the body is drawing from.

            Conversely, when you eat glucose prior to exercise, it’s very easy to experience a runaway insulin response (a sugar rush) which suppresses the leptin response, dampening the response of the aerobic system.

          • Matthew says:

            Interesting. Would you have a recommendation for how much carbohydrate should be ingested at 30-40 minutes into a training session (for example, 1 gram/kg lean body mass…or something to that effect)?

            And should this be taken at every 30-40 minute interval after that?


          • Matthew:

            Kilian Jornet ingested 200 calories (most from carbohydrates) in his entire 7-hour circumnavigation of Denali.

            What I mean is that “you can ingest high-glycemic carbohydrates after 30 minutes of exercise without creating a strong insulin response” does NOT mean “you should ingest…”

            In other words, the more fat adapted you are, the less carbohydrate intake you need (because the more carbs you’re producing in your liver). Potentially, if you need it, you should ingest a small amount of carbohydrates every half-hour to an hour. But the reason I led with the example about Kilian Jornet is that there is no hard and fast rule about ingesting anything during races.

            To answer a hidden question: Would Kilian Jornet have run any faster had he ingested more carbs? Probably not—he’s fat adapted.

            Everybody digests differently and everybody absorbs differently, but when anyone needs carbs every half hour to maintain a certain level of exertion, they shouldn’t be running for more than half an hour at that speed—I can all but guarantee that they are powering a significant percentage of their run anaerobically. If you’re going to run a 10k, over 90% of your energy should come from the aerobic system.

  • Moses Wosk says:

    Thanks I figured it out…. your page has everything thanks for sharing this knowledge. I think its a key part of our evolution as humans!

  • Moses Wosk says:

    Hey I’m a rookie here and just listened to the audio book Natural Born Hero’s by Christopher McDougall. I am inspired and want to become an elite athlete for function more than competition.
    I currently am a Kinesiology student at California State University San Marcos and find this diet and overall method of fat burning more than fascinating. I wanted to ask if you guys have published any scientific research and if not, I myself want to put this to the test and maybe my college professors will let me use their equipment so I can potentially publish something if not just use it for my own gain. Obviously if I published anything I would reference Phill like crazy. Anyway that might not happen, but I really do want some scholarly article on this.

    • Moses:

      Thanks for reaching out to us. We have not published any research but are in the process of liaising with several labs in the hopes of doing so soon. If that’s Ok, I’ll e-mail you a list of sources that can be useful for your preliminary lit review. We’re always looking for people who can help buttress the MAF method with experimental (laboratory) evidence—most of it comes from clinical observation and clinical-based experiments.

      As far as the specifics, do you mean to research a LCHF diet or fat-burning in athletics (or both)? I ask so that I know what sources I can send you.

  • Aaron Hall says:

    As someone who has a very low body fat percentage, I find these guidelines tricky because they all seem to talk about reducing body fat by teaching your body to use fat as fuel. If my body fat is already low, and I eat a fair amount of carbs, then what does that mean for my body and a more fat based diet? Also, my main sport of bouldering is highly anaerobic. What does this mean for the type of fuel I need? Should I be eating carbs because of my metabolism and the type of exercise I do?

    • Aaron:

      Let’s consider the energy requirements of bouldering: Your muscles are generating high amounts of torque during each move (anaerobic). However, they need to recover very very quickly in order to be completely recharged and ready for the next move. This means that bouldering has a hugely aerobic component.

      For example, endurance runners (even ultrarunners) have to negotiate a similar situation: an ultrarunner is loading his or her leg muscles with up to 2 bodyweights. In order to stabilize the joints and stop the leg system from collapsing, the body has to use its fast-twitch (anaerobic muscle fibers) to respond instantly. The resulting lactate (which is a tiny amount) is processed by the slow-twitch (aerobic) muscle fibers located within that same muscle, before the step is over.

      The main message of the article is that having a powerful aerobic system means that you’re generating more of the power from any particular move from fats, and that performance in any sport that has an important aerobic component to it benefits from a powerful aerobic system.

      As a rock climber, you are likely already quite fat-adapted—in fact, that’s almost certainly one of the reasons that your body fat is quite low. This enables you to eat a higher amount of carbs, burn them aerobically, convert the rest into fat, and then use that fat at a high rate. It is almost impossible to be unhealthy with a powerful aerobic system. In other words, for that powerful aerobic system to exist, it needs a certain amount of fat stores in the tank—so it’s going to make sure that the tank is getting replenished.

      That could be one reason that your body is telling you to eat more carbs. In that case, it is healthy. The way to know for sure whether the amount of carbs you’re eating is good for you is to look for signs and symptoms of carbohydrate intolerance (CI). Take Survey 5 of this article and tally your results. If you have 1-0 “yes” answers, you’re at a low risk for CI (you’re in the clear), which means that it’s very likely a good idea to keep doing what you’re doing, nutritionwise.

      • Aaron Hall says:

        I find that when I try to eat a more ketogenic diet that I don’t feel as good or as full as I do when I include a fair amount of carbs. Does this mean that my body uses carbs well and I don’t need to worry about it, or am I doing it wrong? I am very intrigued by recent studies involving cancer and ketogenesis, not to mention the other supposed health benefits but I feel as though my body doesn’t react well to a low carbohydrate situation. You talked about how a powerful aerobic system makes it almost impossible to be unhealthy, what did you mean by this? And where does this fall on the issue of using carbs vs fat for fuel? Can you have a powerful aerobic system on a higher carbohydrate diet?

        • Aaron:

          Thanks for your great questions. This is a long answer, so I hope you don’t mind.

          Let me begin by saying that everything exists in context. I don’t like to talk about carbohydrates as “healthy” or “unhealthy”—instead I like to talk about them as “jet fuel” (as opposed to fats, which would be “gasoline”). Let me draw an analogy between people and airplanes. In this analogy, a sedentary person has an engine like a prop plane, and an endurance athlete has an engine like a fighter jet. If you feed jet fuel to a prop plane, you might be led to conclude that jet fuel is bad for airplanes, which isn’t really the case: It’s just fine if you are flying a fighter jet. However, feed fuel too quickly to a fighter jet and you can still flood its engine.

          Why draw this analogy?

          A great endurance athlete has the potential to burn through far more carbohydrates than a non-athlete, because they have many more mitochondria in their muscles, which are capable of oxidating (read: utilizing) glucose as well as fats. So, unlike the non-athlete, those carbs rarely sit around in the bloodstream of the endurance athlete—and consequently, the non-athlete develops carbohydrate intolerance but not the endurance athlete.

          This gets at your question of whether high-carb diets are healthy for endurance athletes. I’d re-phrase this and say that higher-carbohydrate diets are healthy as long as you are an endurance athlete. (The threshold of where it begins to be unhealthy, is I believe, dependent on how powerful (and how often used) someone’s aerobic system is—again, it’s contextual. The Tarahumara are a classic example of a tribe with an overwhelmingly high-carb diet, whose members generally have colossal aerobic systems.

          Why? The short answer is that long, slow runs allow them to develop their oxidative (aerobic) ability (which increases their ability to oxidate fats as well as sugars). The kicker is that these long, slow, 7 hour runs have to be primarily fueled by fats. This means that when carbohydrates do come along, they are quickly used for fuel, and the remainder is quickly converted to fats, which the Tarahumara can readily use due to their fat-burning ability.

          But what happens when you have a poor aerobic system? Since you aren’t good at oxidating fats, you need carbohyrdates to fuel yourself. This means that (1) you are constantly looking for carbohydrates to eat, (2) you tend to eat more carbohydrates than you need, (3) left-over carbs get converted to fats and (4) since you weren’t good at fueling with fat in the first place, you begin to gain weight. Let’s go back to the analogy I made earlier: when you don’t have a jet engine, you can’t use jet fuel.

          (I understand that fighter jets can’t use gasoline but endurance athletes can use fats for fuel, but you know, analogies are imperfect).

          Which brings me to why a powerful aerobic system—or more specifically, one that is functioning at a high level in real time—is almost always correlated with great health. Now, let me talk about what I don’t mean. I don’t mean that you can’t get a stomach bug, or the flu, or break a foot. A great aerobic system doesn’t make you indestructible, or impervious to external attack. But lets consider what a general state of “unhealthiness” means: it means that your body is constantly struggling—and constantly failing—to meet the demands that the environment places on it. In other words, it can’t meet it’s day-to-day upkeep. And if you look at any chronic disease or chronic injury, it’s not “special” or “different” from the non-chronic version of itself. In essence, the difference between normal and chronic diseases are that chronic diseases are those that the body can’t quite manage to heal from.

          Why can’t it heal from chronic issue? The reason the body can’t heal from a particular chronic issue is because that particular issue represents the “weakest link” in terms of health—and that weakest link cannot bear the tensile demands (read: stresses) placed upon the chain (read: body). But that’s not the big question. The BIG QUESTION is: how come the body never got around to bringing the weakest link (whatever it was) up to speed?

          The short answer is “economics”: it didn’t have enough energy to make the investment. That is what the aerobic system does—and why we should think about the aerobic system as a “fat-burning” engine in particular: because it takes two near-limitless resources (fats and oxygen) and combines them to produce a near-limitless flow of energy. If that system is sufficiently powerful, the body can almost always make the investments necessary to rid itself of chronic conditions.

          The reason, of course, I say “almost always” is because there is always an exception (genetics, thyroidectomy, etc).

          Let me address one last question: why your body doesn’t react well to a low-carbohydrate situation. There two most likely reasons have to do with what I already discussed above:

          1. You are burning energy at a high enough rate that it’s simpler (in terms of digestion, speed at which it becomes available, etc.) to get it from carbs. This is the case with a lot of elite endurance athletes, who need relatively high-carb diets in order to fuel their training and racing. (To go back to the analogy, they burn through jet fuel fast enough that eating mostly jet fuel is what makes sense).

          2. The anaerobic component of your sport requires a high degree of available muscle glycogen. Regardless of the fact that bouldering also has an aerobic component, you need to be well carb-fueled at all times, or your performance (on the wall or otherwise) suffers.

  • Matt says:

    I’ve done 3-hour training runs at the MAF heart rate without any fuel, and it’s been fine. However, I read that marathons should usually be done 10-15 bpm above the MAF heart rate. I assume that at the higher heart rate, my body will be using both fat and glycogen for fuel, right? If so, then I would probably want to replenish that glycogen during the run to make sure I don’t run out before the end of the race, since it’s at the end of the race that my heart rate is liable to be at its highest. How do I fuel with carbs that are easy for my body to change into glycogen during a race without spiking my insulin levels?

    • Matt:

      The ideal marathon is one where you burn through your muscle and liver glycogen at a rate that doesn’t deplete it before the end. A heart rate that is 10-15 BPM above MAF will do this, for most people. That said, it’s important to test it out before hand, since your mileage may vary. (Another measure you can test out is 10-15 seconds above MAF pace, which usually is 5-10 BPM above MAF).

      Once your aerobic system is at full burn (30 minutes into the race), fueling with candy canes and a coca cola chaser shouldn’t raise your insulin levels significantly (but would be extremely unhealthy for a variety of other reasons). The 2 energy gels per hour rule works well for a lot of people. But if you dialed in your heart rate correctly and manage the anaerobic component well, you shouldn’t need any gels.

      Of course, for any of this to work, you need to know that your aerobic system can carry you the entire distance.

  • Nat says:

    I see you have no recipes using Quinoa…
    What are you thoughts on coconut milk Quinoa porridge as a post workout (5km) breakfast meal? Would you accept this as low GI and high protein?

  • Rodrigo Leme says:

    Hi there! I wonder what an adequate pre-workout meal would be in my case. I normally train early in the morning and tend to wake up just 20-30 minutes before training. On the one hand, I’ve been told I should avoid training on an empty stomach. On the other hand, that I should avoid eating “complex” meals. I should just drink something with readily available sugar, 10 min before the workout. So I normally drink a glass of coconut water, or some sports drink, just 10 min before workout (for a 1-1.5 hour, strictly aerobic, workout). Would that fit into an ideal fat-burning workout routine?

    • Rodrigo:

      Generally speaking, not really. Easily available sugar has a huge impact on blood sugar, meaning that levels of insulin drastically increase. Insulin reduces the body’s fatburning ability. So you’re better off sticking to more complex carbs, throwing in some fat such as coconut oil, and a bit of protein to round out digestion. A great example of a pre-workout meal is Phil’s Shake

  • Gary says:

    Good article. So the take home is… As you condition your body to burn fat thru aerobic training and good nutrition, the body will contribute more fat for energy during periods of anaerobic training (HIT). And when that happens, performance is significantly better. Did I understand that correctly?
    Also, there’s a reference to the gas analyzer again. I emailed and researched about 6 months ago, only to fall short of what you are referring to…I think. Any good leads on the “gas analyzer” front? Or do you clinically stick to the “clues” listed? As always, thanks for your reply.

  • Lisa says:

    I think I get only use refined carbs as needed during a race or long training. And, it’s particularly important to avoid high carb intake before workout…and before race, I suppose? So…that leaves good fats and protein for pre-workout, pre-race meal? Isn’t that too much for digestive system? Or am I hoping to be SUCH a good fat burner….I only need a bit of carb 30-60 minutes into my workout or race to stoke the fat burning fire? What do you suggest pre long-workout/race? Thank you

    • Lisa:

      The problem isn’t eating too many carbs before a race. The problem is the impact that high-glycemic foods have on your insulin levels (more insulin means less fat-burning). So, if you drink a very low-glycemic fruit and veggie smoothie with 500 calories an hour before a race, your insulin levels won’t rise dramatically. So it’s perfectly fine to eat carbs along fats and protein as long as the emphasis remains on carbs and protein. Two great pre-workout/race foods are Phil’s bars and Phil’s shake.

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