How to Fuel an Ultra

sarah cuff Run Faster, Run Stronger, Running Performance 6 Comments

For any runner who aspires to run an ultra marathon (which is technically anything longer than the marathon distance) and for those of you who currently do tackle ultras, you’ll either have an idea or have personally experienced how daunting fuelling the longer distances properly can be.

I’ve now run a half dozen 50k’s and 50 milers along with 2 x 100 milers (Mountain Lakes 100 2016 and Rio del Lago 100 2017)… And have managed to run into fuelling challenges in most. Of course, the longer the race, the more likely I’ll have issues. It turns out running for more than 12-24 hours straight isn’t exactly easy on the digestive system! Many lessons have been learned and research compiled.

The leading reason behind runners not reaching the finish line in ultras (particularly 100 milers) is gastrointestinal distress. It’s no surprise then that I struggle with nausea and an aversion to nearly all food in every race lasting longer than 5-6 hours. And I’ve heard stories from other runners (and Ironman triathletes) of relentless vomiting and other forms of gastrointestinal distress that did them in. I know I’ve been lucky to never have fuelling issues take me out of a race.

Why Fueling Ultras is HARD

As if the actual act of simply running an ultra isn’t hard enough, fuelling them can be even harder (as demonstrated by the fact GI issues account for the highest percentage of DNF reasons in 100 milers). On a physiological level, it makes sense. Once we start running, the digestive system effectively shuts down and instead blood is directed to the working muscles (our legs). If we are running longer than 90 minutes (as one clearly must during ultras) our bodies require calories to refuel, primarily in the form of carbohydrates. But putting any kind of food or even liquid into the stomach means trying to turn on a digestive system that is more or less closed for business.

In fact, even many marathoners regularly find themselves running into problems with digestion during racing – but because they are out there a (relatively) short amount of time they can usually (but not always) make it to the finish line without being taken out by fueling issues. Of course the ultra runner, who might be running 5 to 12 hours – or even upwards of 24 hours or more – will find these issues simply intensify as the hours go by. Basically the longer the run, the greater the chance the runner will experience gastrointestinal distress – particularly if there is no fueling plan in place that’s been practiced.

In general carbohydrates take less work to digest than fats or proteins, but certain types of carbs appear to cause issues for some. For example, a very popular ingredient in many sports nutrition products called maltodextrin causes gastrointestinal issues in what I’ve found to be a surprisingly high percentage of runners.

On top of this we must rehydrate with the right amount of fluid and take in the correct proportions of electrolytes, specifically sodium (which is the main electrolyte we lose in sweat). But it’s a delicate balance – tipping to either end of too much or too little for both fluid and sodium can trigger GI issues.

While some runners seem have tough gastrointestinal systems able to process more calories and liquid during the stress of physical activity than others, the good news for everyone is that the gut is highly trainable. Unfortunately, just like training for a race takes planning and hard work, training your intestinal system to accept more fuel than what is currently comfortable can also be a tough go. But just as your run training pays off on race day, so too does gut training. And seeing how high the DNF rates are due to gastrointestinal issues, it’s obviously a very good idea to pay attention to both in the months leading up to your race.

What to Eat and Drink in an Ultra

Just to be clear, there’s no simple answer as to what is right for you to consume during an ultra. Different strategies and options work for different runners and while there is no straight up fail-safe calculation, there is a basic formula for all to follow. From there, finding the right calories and fluids to fulfill this formula will depend on trial and error.

Basic Formula to Determine Calorie, Sodium and Fluid Intake 

1. Calorie Intake: Sports scientists have determined that our gut can process 30 to 60 and up to 90 grams of carbohydrate per hour. We can digest and absorb up to 60g/hr using glucose (also called dextrose), sucrose (aka sugar which is glucose plus fructose in equal ratio – of interest, most fruit is also a roughly equal combo of glucose and fructose), glucose polymers (aka maltodextrin), or starch (such as rice or oats, which is broken down into glucose molecules upon digestion) – and up to 90g/hr if glucose is combined with fructose in a 2:1 ratio (note – Skratch formulates their exercise hydration in this special ratio for faster digestion / absorption).

However 60 to 90 grams of carb per hour tends to feel very high for many athletes – more than they can comfortably take in. Taking in more carbohydrates than the gut can handle causes nausea, bloating and other forms of gastrointestinal distress.

Of course, as noted above, the gut is trainable. That said, I prefer to both train the gut and also train my body to become more metabolically efficient (or at least fat-load pre-race) so as to reduce the amount of carbohydrate required.

The calculation for traditional athletes (who eat a high carb diet) is to aim to replace 30-40% of calories burned (via carbs). For fat-adapted athletes it would be approximately half that. These numbers are based on the approximate percentage of fat and carbohydrates utilized for energy during the run (we are always burning a mix of the two, at varying ratios given the intensity). Note – these calculations are imperfect, but they do provide a useful estimate in which to begin building your nutrition plan.

In order to come up with your personal carbohydrate requirement, you must first determine how many calories you burn per hour in your given activity (you might use an online calculator such as Once that is done, move to the step below to calculate how many of those calories are carbohydrate calories that need to be replaced.

  • Traditional (high carb) fueled athlete – multiply your hourly caloric expenditure  by 0.3 and 0.4 – then divide each number by 4 (this replaces calories in the form of carbs at a rate of 30-40%)
  • Fat-adapted (high fat) fueled athlete – multiply your hourly caloric expenditure by 0.15 and 0.20 – then divide each number by 4 (this replaces calories in the form of carbs at a rate of 15-20%)

For example, a 68kg (150 lb) female runner would burn 576 calories per hour running a 50-miler with a goal time of 10 hours (12:00 min per mile pace).

As a traditional athlete she would require 43 to 58 grams carb per hour (576 x 0.3 = 173 / 4 = 43 and 576 x 0.4 = 230 / 4 = 58).

As a fat-adapted athlete she would require 22 to 29 grams carb per hour (576 x 0.15 = 86 / 4 = 22 and 576 x 0.2 = 115 / 4 = 29).

She may end up consuming a greater amount of calories per hour overall, but that would be because she adds fat and/or protein to the mix.

At the very least these calculations give you approximately how many carbohydrates you need to be consuming per hour. And seeing as carbohydrates are the limiting fuel for all athletes (fat-adapted for not), that is the most important macronutrient to get the calculations right for.

2. Fluid Intake: the amount you need to take in depends on your sweat rate, which in turn depends on genetics (individual body chemistry – you know if you’re a heavy sweater!!), temperature, humidity, intensity (how hard you’re running) and duration (how long you’ll be out there running).

If you don’t take in enough fluid, eventually your gut will not be able to process food (unfortunately once you reach this point it takes a long time to rebound from it) and you’ll have difficulties regulating body temperature. If you take in too much fluid (particularly without enough sodium), you may end up with hyponatremia (low blood sodium levels leading to brain swelling) which can be detrimental and even life threatening. Both over and under hydrating can lead to nausea and GI distress.

A good general guide is to start with about 2 cups (500 ml / 16 ounces) of fluid per hour – and drink more or less depending on thirst (you’ll tend to need more in hot, humid situations and less in cool conditions). Using thirst as your guide as to how much to drink is a valuable asset and one athletes mustn’t be afraid to learn to rely on. Drinking ad libitum – in other words, drinking fluid whenever desired, or drinking to thirst, is the latest advice from the American College of Sports Medicine (taking the place of the outdated advice to ‘drink before you’re thirsty according to a schedule of 800-1200ml fluid per hour’). A 2011 study found that cyclists who drank according to thirst performed better than when they drank ‘below thirst’ or ‘above thirst’.

3. Sodium Intake: your sodium intake is dependent on your fluid intake. You want to aim for 300 to 400 mg of sodium per 2 cups (500 ml / 16 oz) of fluid consumed. This helps to prevent hyponatremia and deliver the electrolytes your body requires. Both too little and too much sodium can lead to nausea and GI distress.

Once you’ve done the calculations, it’s time to start figuring out what food and drink you might use to meet your needs. Here is a short list of items that can help you meet your targets:

  • Skratch Exercise Hydration (1 serving mixed up) – 2 cups fluid / 360 mg sodium / 21 grams carb
  • Plain water (500 ml) – 2 cups fluid / no sodium / no carb
  • Broth (such as full sodium regular veggie broth) (per 8oz serving) – 1 cup fluid / 540 mg sodium / 3 gram carb
  • Huma gels (orignal) – no fluid / 105-110 mg sodium / 21-25 grams carb
  • Huma gels (plus) – no fluid / 240-245 mg sodium / 21-25 grams carb
  • Power Cookie (per cookie) – no fluid / 50 mg sodium / 18 grams carb
  • AB & J Rice Cakes (per cake) – no fluid / 14 mg sodium / 50 grams carb
  • Tart Cherry Energy Bars (per bar) – no fluid / 108 mg sodium / 28 grams carb
  • Cherry Chia Energy Gel (per serving) – 0.3 cup fluid / 50 mg sodium / 22 grams carb
  • Citrus Chia Energy Gel (per serving) – 0.3 cup fluid / 50 mg sodium / 24 grams carb
  • Candied ginger (such as this Naked uncrystallized ginger which is my absolute favourite, probably because ginger helps alleviate nausea) (per 25 g) – no fluid / 5 mg sodium / 20 grams carb
  • Salt Stick Fast Chews (in case your food and sports drink just don’t cover off your sodium requirements) (per 2 tablets) – no fluid / 100 mg sodium / 2 gram carb
  • Skratch Energy Chews (per 1 pouch ) – no fluid / 160 mg sodium / 40 grams carb
  • Dried tart cherries (1/3 cup, Eden brand) – no fluid / 15 mg sodium / 36 grams carb
  • Oranges (2-3 wedges or ½ a medium orange) – 0.1 cup fluid (1 ounce) / no sodium / 8 grams carb
  • Watermelon (1 wedge/286g or ~1/16 of a large watermelon) – ~½ cup fluid / 3 mg sodium / 22 grams carb
  • Boiled baby potatoes (per 1 baby potato / 25g) – no fluid / no sodium / 4 grams carb
  • Potatoes are generally offered alongside salt at aid stations – FYI ¼ tsp of sea salt = 580 mg sodium
  • Banana chips (1/3 cup / 30g), All Good Provisions brand) – no fluid / no sodium / 13 grams carb (also, 11 grams fat)
  • Bacon (2 slices) – no fluid / 275 mg sodium / no carb (also, 6 grams fat)

Obviously this is a relatively short list as I’ve not included many sports nutrition products on the market or even real foods one might turn to. For example, aside from Skratch there are other properly formulated hypotonic sports drinks that contain at least 300mg sodium per 500 ml serving (such as Tailwind or Osmo) – it just so happens that I find Skratch to contain the most natural ingredients and taste the best, therefore it’s what I personally use and recommend. To make your own list, jot down the ones from above that interest you and then grab your favourites and figure out their fluid/sodium/carbs to add them to your plan.

Once you’ve chosen your mix, it’s time to start practicing! You must use every opportunity to practice, so at least once a week during your long run you are putting your strategies to the test. Even if you think a 2-hour easy run doesn’t require fueling, the fact is you need to use that opportunity to train your gut for race day (this statement is basically aimed directly at myself, ha).

What about using Fat as Fuel during races?

Using a LCHF (‘low carbohydrate high fat’ – generally between 100-200 grams carb daily and 50-70% calories as fat) or a more extreme ketogenic diet (consuming less than 30-50 gram of carb per day or up to 80 grams for very active athletes) on a daily basis can be very useful in becoming more metabolically efficient. It essentially trains the athlete to tap into a higher percentage of fat for fuel. However during intense activity (i.e. racing), carbohydrates are still burned (albeit at a lesser percentage) making them the limiting fuel (the anaerobic system uses only glucose, aka carbs, for fuel and we can only store about 90-120 minutes worth in our muscles). Therefore it is carbohydrates that primarily need to be replaced during races. However, for races longer than 8 to 12 hours, the more fat and protein may also need to come into the picture to provide satiation and more complete nourishment for the athlete.

For example, even fat-adapted and top performing ultra runner Zach Bitter takes in primarily only carbohydrates while racing (per his blogpost here), albeit at a much lower caloric rate than traditional sports nutrition because he has trained his body to burn a higher percentage of fat at higher intensities. Yes, even for fat-adapted athletes, carbohydrates will always be the limiting fuel at high intensities – while fat stores remain virtually unlimited. It is for this reason that technically the only fuel that needs to be replaced DURING racing is carbohydrates.

Carbohydrates, fluids and sodium – three pieces to a puzzle that’s admittedly not the easiest to put together. However armed with the info above you should at the very least find yourself further ahead than before. Personally, I’ve done my calculations and drawn up a new nutrition plan – and am now in the process of gut training, to be put to the test next month in a 100K race and also in May when I attempt my 3rd 100 miler! For all you ultra runners (and Ironman athletes or other ultra endurance athletes!!)… Take the time to calculate, plan and practice – and best of luck in reaching your finish line feeling good and strong.

To deliciously healthy food and stronger faster running… Cheers,


Sarah Cuff, R.H.N.
Holistic Sports Nutritionist
Founder Eat 2 Run Sports Nutrition
CSNN Sports Nutrition Instructor

Comments 6

  1. Congrats on your RDL finish! It was a muddy mess going up Goat Hill and alot of runners didn’t make the cut off. Love this article and I am in the process of ‘gut’ training as I have my first 100-miler coming up in February 2018 (Antelope Canyon). This information is very helpful for me to dial in exactly what the perfect mix will be for me. Good luck on your upcoming 100K and 100 miler races!

    1. Post

      Thanks so much Pam!! Yes, that Goat Hill was crazy, ha. Good luck to you as you prep and go through ‘gut training’ – it will pay off. You must be right in the thick of training, with your race at the end of February – such a good time to really ensure your nutrition plan is dialled in!! Would love to hear what mix you end up using and how it goes 🙂

  2. Very informative article. I am in the beginning stages of ultra running, and armed with this knowledge really puts into perspective the fundamental requirement to analyze fuel; instead of just my simple Gu set up.

    I’m curious to know if you have recommendations on vitamin and minerals during Ultras? Or if you have written about this before. I have a 50k in a few weeks, 50 Miler in June, and Kodiak 100 in August.

    Thank yo again for your research and help!

    Alex Ochoa

  3. Hi,

    Great article.

    I have a 50 miler (where you carry everything except water) coming up next year and have been following a low-carb approach to training, which has been working very well.

    I note that you write:

    “She may end up consuming a greater amount of calories per hour overall, but that would be because she adds fat and/or protein to the mix.”

    Getting fat into the fueling strategy doesn’t seem to hard, but what are the protein options for such long races?

    1. Post

      For protein, I think the best options are some type of ‘meat bar’ (such as by Epic brand) or beef jerky. I’ve carried both jerky sticks and meat bars in the past, and had them work really well. Bean burritos are another option. Not that protein is necessary though of course. In fact, I just finished my third 100-miler, and ate zero protein whatsoever (desert race so it was hot, I ate mostly huma gels and fruit from aid stations – but not much of anything really due to nausea, so it’s a very good thing I was fat-adapted!!). Whereas in past races, I really wanted protein. So typically I carry jerky on me for just in case I want it!

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