Reader question: How would I fuel during a long run?

A couple of weeks ago, SteveL asked me in the comments:

“How would you fuel during a long run?”

Allow me to be a bit tongue-in-cheek here. If SteveL means what we usually do by “long run”—that is, a training run and not a race—my answer is, “with body fat and oxygen.” In other words, not at all.

The physiological details of this are best left to another post, but the short answer is this: the goal of a long run is not just to run for a long time, but to develop the system that helps us run long. Crucially, that system is known as the aerobic system, which you can think of as the system that burns fats in presence of oxygen.

Here’s the critical detail: the fat you eat doesn’t reach your bloodstream for a few hours. So, unless your long run is very long, any fats you ingest during the run aren’t really going to go towards fueling that run.

Let’s discuss the more conventional fuels people use on their long run (sugar-laden fuels such as a 3-6% carb solution or gels). Great for races, but I’ll get to that in a bit. If you ingest them during a training run, you’re enabling your body to lean on its sugar-burning energy system (which it uses for short-duration, high-intensity bouts) for a long time.  This means 3 things:

  1. First and foremost, it enables your long run to be faster than is healthy: you’re liable to do what looks like a long run but is actually a bunch of short, medium/high-intensity training runs that in aggregate masquerade as a long run.
  2. Because you’re fueled with sugar lets your aerobic (fat-burning) system off the hook , which is the system that is supposed to power your long run.
  3. You’re using the short-duration, high-intensity (sugar-burning) system for a what should theoretically be a long-duration, low-intensity activity (which you’re effectively turning into a long-duration, medium/high intensity activity). You’ll wear your body down disproportionately.

To recap: no fuel for long training runs. Fats won’t help, and sugar is counterproductive.

(While the body does burn a mix of sugar and fat at all times, the longer the duration, the more fat should be in the mixture. Because the rate of fat-burning peaks at around 50-55% maximum work rate for most people, very long training sessions shouldn’t exceed this low intensity.)

Fueling during a race

Fueling during a race is different. You’re not trying to train anything here. You’re trying to get every bit of power you can from the machinery you’ve been developing in training, with the provisos that you (1) finish the race and (2) don’t blow the engine.

This means that you want to make sure you’re well-fueled (and you stay well-fueled) during a race. For anything that’s marathon length or below, fats still won’t help. For the vast majority of us, it’s still too short of a race. So, such races are the ones you want to approach with the run-of-the-mill advice on race fueling: your carb solutions and gels work great here.

There’s one consideration: don’t start fueling until you’re 20-30 minutes into the race. When your body isn’t already warmed up, it’s very easy for a shot of sugar to kick up your insulin levels, which reduces your fat-burning ability. But once you’re warmed up and burning fats at a high level, sugar has a much smaller effect.

 Fueling during an ultramarathon

Here’s where it gets tricky. There’s two sets of priorities to discuss: the physiological needs of the body and the practicality of fueling on the run.

The physiological needs:

  • Hydration (water plus electrolytes)
  • Nutrition (the right combination of macronutrients)
  • Digestion (continued function of digestive system throughout the run).

The practicality of fueling on the run:

  • Combining hydration with nutrition.
  • Creating a food that fits easily through the valve of a handheld water bottle.

Here’s a drink recipe which (for me) meets all these criteria:

Basic Ultramarathon fuel 



  1. Add 1 cup of water into blender.
  2. Add all ingredients (heavier ingredients first).
  3. Blend on low until well-chopped.
  4. Add the rest of the water.
  5. Blend until smooth.
This is what I use…

Nutrition Facts_1

Nutritional Rationale


For a recipe such as this, I usually drink one serving (about 42 oz) over a period of 2-3 hours. This generally takes care of both my fueling and hydration concerns.

This suggestion is a TEMPLATE for people to try out during training runs. It’s important to adapt this or any recipe, workout, training plan, or racing strategy to your personal needs.


The reason I like including sizeable portions of all 3 macronutrients (carbs, fats, and proteins), is to incentivize the body to maintain the digestive system activated in a low-key but comprehensive way. This applies for protein in particular: while protein will not go towards fueling the body during a race, I put a small amount of it in order to create a more balanced digestion process.

The same goes with fiber (occurring mostly in the spinach, blueberries, and chia seeds). Ultramarathoners are prone to cramps, indigestion, and other digestive issues during the race. By putting a small amount of natural fiber in here (not so much that it slows down digestion), we can help “smooth out” digestion during the race.


As you’ll notice, one serving of this drink has a staggering amount of potassium (almost 1.3g) and a respectable amount of sodium (just over 0.13g). The reason I like this 10:1 ratio of potassium to sodium is because a lack of potassium is linked to muscle cramps, and reduced nervous system function. (This can lead to lower coordination and reaction time, which can cause an injury).

Generally speaking, more potassium is better (up to a point, of course). 130 mg of sodium every 2-3 of hours is quite enough to keep a well-adapted athlete going during a long race.

Carbs and fat

The sugar calories are straightforward: these will go towards topping off your glycogen tank, in order to stave off fatigue and help your aerobic engine continue to burn fats.

Now we get to the tricky bit. Supposing that you drink 1 serving of the recipe in 2 hours, you’re getting around 160 calories of carbs and 155 calories of fats an hour. You might think that’s not really a lot of carbs. However, that’s the reason a majority of the fats in the drink come from medium chain triglycerides (MCTs), sourced from coconut oil.

Why MCTs?

MCTs are relatively easy to digest relative to other fats, and they also become available for fat-burning very quickly upon hitting the bloodstream, helping to increase fat-burning and accelerating the metabolism. In other words, “reducing” the possible sugar content of this drink by balancing it out with coconut oil is an excellent strategy for endurance races.



18 thoughts on “Reader question: How would I fuel during a long run?”

  1. Hi Ivan,

    Great article and some good insights. Just had a quick question about this comment:

    “f you ingest them during a training run, you’re enabling your body to use its sugar-burning energy system (which it uses for short-duration, high-intensity bouts) for a long time.”

    It is possible to ingest sugar and burn sugar during a long run aerobically. E.g. train at a very low HR and not burn fat but rather sugar through pyruvate > acetyl CoA > Krebs etc. Not recommending or saying best use of fuel source but this could easily last you 2 hours (a long run).

    Is this causing wear on the body as you highlight?

    I am in no way advocating doing this, just trying to understand physiologically why burning sugar at low intensities causes issues / wear.



    1. Grant:

      Thanks for commenting.

      Perhaps I need to make the article clearer. At low intensities you do indeed burn sugar. But the lower the intensity, the greater the percentage of fat-burning. If at a low intensity (40-50% VO2Max) you are burning primarily sugar, then I’d expect to find signs and symptoms or risk factors of metabolic syndrome if I go looking for them.

      The muscle fibers that burn sugar fatigue quite quickly, which means that even if you can burn sugar for longer periods of time (and, as you point out, you can) you’re exhausting them (and damaging them) disproportionately.

      So, my ballpark estimate when thinking about what duration I want to run when sugar is my primary (propulsive) fuel, if you will, is 1 “tank” of glucose/glycogen. In other words, around 2 hours for a tempo run, 45 minutes for an anaerobic threshold run, etc.

      But beyond that—in other words, insofar as you prolong your athletic output (and to the degree that you do) your inputs of sugar should have the purpose of helping you maintain a high level of fatburning, rather than being utilized as the primary source of athletic fuel.

      UPDATE: I made a few changes to the article. Would you say it explains it better?


      1. Thanks Ivan – yes I think that explains the points better. I think you answered the question in my head from this “the muscle fibres that burn sugar fatigue quite quickly”. I was not aware that type 1 slow twitch muscle fibres could not utilise sugar but only fat. Hence if you were running at low intensities off sugar you are running / using your type 2 fast twitch fibres and not your fat burning slow twitch muscle fibres.

        I was more thinking that by eating a high sugar snack just prior to a workout you would inhibit fat burning and hence switch on sugar burning which would mean you have shut down your slow twitch fibres and turned on your fast twitch. Yes metabolic syndrome would be of concern if primary fuel was sugar at low intensity, but you have almost just gone ahead and done something similar by ingesting sugar right before a low intensity run, right?

        Last question then, when you mention entering the race ‘well-fueled’ this would mean you have consumed a breakfast of high fat (assuming the race is 2 hours + post breakfast to get fats into blood stream) and low carb to ensure you don’t spike insulin. I am going to assume that liver and muscle glycogen are already at normal levels due to glycogen synthesis (which appears to be the same as high carb athletes even though ingesting significantly lower amounts of carbs – glycerol and lactate contributing to the production of glycogen

        Hence it would appear that there would be zero need to ingest any more carbs leading up to a race than you would normally consume to remain ketogenic / optimally burning fat. As you said, 20-30 minutes in once the fat burning engine is warmed up, then yes carbs / sugar have less of an effect and can be utilised quickly.


      2. Grant:

        In fact, Type I fibers do use sugar. To be overly specific, they use Acetyl-CoA, but they work better for longer periods when they are getting their Acetyl-CoA from fats. This is not because Acetyl-CoA is different between fats and sugars, but because of the body’s internal logic: The main reason the body would want to use more sugar is because it needs to increase its athletic output.

        This is the key: the more Acetyl-CoA you get from sugar, the more likely it is that you have a higher athletic output, which means there’s a greater likelihood that you are also using some of that sugar anaerobically. (I’m sure there are exceptions, but this is the general rule for 99% of cases). And since you break down fats “directly” into Acetyl-CoA (meaning that you bypass glycolysis), you can’t burn fats anaerobically. Furthermore, lactate (produced from anaerobic function) inhibits fat-burning. So, the higher your rate of fat-burning, the higher your aerobic output. This is why Phil Maffetone talks about fat-burning and aerobic function nigh-interchangeably, even though fat-burning isn’t the whole of aerobic function.

        And yes to your question about using sugar at a low intensity. But that’s essentially a bad habit you don’t want to train your body into. It’s basically the exercise version of eating lots of sugary snacks all day.

        It’s hard to be prescriptive in terms of pre-race nutrition, but by and large I agree with you completely. I just make sure that my glycogen levels are normal (like you say), but I don’t do anything that even smells like carbo-loading.

        For me, the most important thing is to make pre-race nutrition, (by which I mean pre-race dinner) the “least special” of all meals, so that I don’t prime my body to freak out about race day—more stress=less aerobic function. And for me, what works best in terms of macronutrients is to eat a small meal of 60% low-GI carbs (low GI doesn’t spike insulin for a trained athlete) such as veggie chili with cheese and the works, with a side of corn tortillas. So, a decent array of carbohydrates and fats, and a bit of protein (my big protein meal is lunch).

        But again, that’s my staple light dinner to replenish glycogen spent during the day.

        And depending on the time of race, I’d rather make myself a medium-carb, high-fat smoothie similar to the above, except perhaps with the addition of coconut cream and avocado. So almost a breakfast shake, if you will.

        Does this jive with your questions?

        Liked by 1 person

      3. Thanks Ivan. Yes that does answer my questions and the explanation of Acetyl CoA being the same but the bodies internal logics preferring it coming from fat works well and I can obviosuly link that to all the benefits of fat burning / glycogen preservation etc.

        Interesting to see some studies on low GI carbs and how they affect RQ / blood sugar levels / insulin secretion in trained athletes post ingestion. I assume that pre night dinner / breakfast shake would mean you enter the race outside of ketosis but fat burning is still very high (and ketone production would most likely ramp up during the race anyway). I am still playing around with a level of carbs which produces optimal performance, weight management / loss, optimal fat burning / RQ etc. Hence for a period of time (maybe 8 weeks) I am staying in nutritional ketosis to ensure my body fully makes the switch / adaptations to burning fats and ketones. Then I would like to see how much of those benefits and improvements in fat burning can be maintained by remaining low carb, high fat, but not necessarily in nutritional ketosis. So far I saw better training during a period of low carb high fat (but outside nutritional ketosis or at least more cyclic) vs my current experiment of constant nutritional ketosis. However I am only 3 and a bit weeks into that experiment and could very well still be making adaptations.

        Love your work.

        * side note do you do running / gait analysis remotely (e.g. I am in AUS) and very interested in addressing a heel whip I have one my R side. Or a blog article could help 😉

        Liked by 2 people

      4. Yes. I don’t worry too much about being in ketosis going into a race—it’s more important for me to be “keto-capable” and be well-fueled with glycogen.

        Very cool. Do you keep a journal on your experiment? I’d love to see your results as they go along (performance, mood, etc.)

        About the abductory twist (heel whip), why don’t you send me a message on my Facebook site ( with your name and e-mail and I’ll send you some questions and more info.

        I work with a bunch of people from Australia so there may be someone in our network who knows a gait specialist there. I’m glad to help, but if I can refer you out to someone local, it’ll be better for you. Clinical issues like this, while simple, can be quite time-consuming to tackle without a face-to-face. Even with gait analysis, it takes a lot longer (more correspondence, essentially) to go down the checklist of what the cause could be.


  2. Hi Ivan, that was a good read. I was wondering if the same applies to doing shorter, higher intensity training runs? Do you need to have some carbs when trying to do tempo and marathon pace runs or even 800 repeats?


    1. Brett:

      Good question. I’d like to devote an article to this, but let me answer briefly.

      You only need to take carbs in if you’ve exhausted your glycogen stores from exercise. When your glycogen stores are still reasonably full, there’s no real need to take carbs.

      Because of the way the anaerobic system works and how it stresses the body, I don’t think it wise or effective to train anaerobically for more than 1 “fuel tank.” In other words, when my muscle glycogen runs out, I cool down and the workout is over.

      The ins and outs of this are really interesting and have a lot of applications in training program design and periodization. A couple of people are also suggesting I write some more about the anaerobic system, so you’ll see a post about this soon!


  3. Hola Ivan. (perdón que te escriba en español).
    Crees que el magnesio es importante para evitar los calambres, porque desde que estoy llevando una dieta baja en carbos y entrenando Maf hr ya me han dado dos calambres y son los únicos calambres en toda mi vida. Sé que tienen muchos motivos como una mala hidratación o exceso de entrenamiento o esfuerzo, etc.
    Igualmente todo esto me llamó la atención porque justo lo empezaron a discutir en el grupo de facebook Maffetone Method.
    -Pero la cuestión es, si te parece que el magnesio es importante para consumir en un entrenamiento o carrera: ¿cómo le puedo agregar magnesio a tu fantástico batido de forma natural y en una proporción justa?
    Tu batido es lo único que tomo para 2 o 3 hs de carrera y me sienta muy bien en el estomago, no consumo geles ni gatorade.
    Desde ya muchas gracias César.


    1. Perdón Ivan, debo aclararte que los calambres me dieron en los días de carrera intensa de 3 hs por la montaña, no corriendo en Maf HR, y el batido lo tomo solo esos días. Gracias otra vez.


      1. Hola Iván, me contestó a mí mismo para ahorrarte tiempo ya qué a la vuelta de tus vacaciones tendrás muchas preguntas acumuladas.
        -Primero, cuando dejé de comer harinas refinadas y azúcar incorporé productos que no comía a diario, cómo frutos secos, huevo, chocolate amargo, verduras de hojas verdes y chía, es decir que mi ingesta diaria de magnesio y demás electrolitos es mayor que antes.
        -Segundo, incremente el entrenamiento y la intensidad esto puede ser razón para los calambres.
        -Tercero, no encontré ninguna referencia seria que diga que el magnesio es mas importantes que el resto de los electrolitos.
        -Cuarto, tu batido tiene bastante magnesio, entre 200 y 250 mg (cálculo rápido).
        Igual espero algún comentario tuyo, alguna idea de por qué me pasa y como evitarlo en carreras largas o muy largas de mas de 3 hs. Muchas gracias César.


      2. Cesar:

        Gracias por su comentario. El electrolito del que hablo yo no es magnesio, sino potasio. En efecto, el funcionamiento del sistema nervioso está predicado en un balance delicado entre el sodio y el potasio (y los calambres se dan principalmente cuando una falta de potasio precipita cambios negativos en este balance.)

        Es muy raro que una falta de sodio—pero no de potasio—cree tal imbalance.

        Cuando uno incrementa el entrenamiento, pueden pasar varias cosas:

        1) Imbalances musculares que no afectaban al cuerpo con un menor entrenamiento lo pueden comenzar a afectar. Esencialmente un imbalance es cuando un músculo trabaja demasiado y su opuesto trabaja menos de lo que debe. El músculo sobretrabajado está recibiendo muchos más impulsos nerviosos, significando que los canales nerviosos que llegan a este músculo están utilizando mucho más potasio que los de aquel que trabaja menos. Por lo tanto, el calambre se localiza en este músculo sobretrabajado.

        2) Calambres generalizados (en 2 o más extremidades) típicamente significa que los mecanismos regulatorios de potasio en el cuerpo no son en tiempo presente suficientemente fuertes como para soportar el incremento en entrenamiento. En este caso, bebidas que incorporan mucho potasio, fácilmente digeribles son muy útiles.


      3. Hola Ivan, yo sé que tu te refieres al potasio y lo explicas muy bien.
        Mi duda o confusión se produjo debido a un comentario que alguien dijo en el grupo de FB Maffetone Method.
        igual creo que estoy consumiendo diariamente una buena cantidad de todos los electrolitos, así que voy a equilibrar la intensidad de mis entrenamientos.
        Te agradezco nuevamente la respuesta y la aclaración.


  4. Gracias, disfruta de tu salida de “mochilero”, esperaré con ansias tu respuesta, pero mientras yo voy a ir experimentando y luego también te contaré como ha ido. Abrazos César.

    Liked by 1 person

  5. Hi Ivan,

    I’m running a relay race in a mountainous area soon where I have 25 miles split over 4 segments that will be run about 5 – 6 hours apart. I have been following MAF training and low carb/high fat diet for the last two months or so and feel that I am pretty well fat adapted. During the relay, do you think something like your ultramarathon fuel would be good to take immediately after each segment to help get ready for the next?

    I love your site and all the information you provide!


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