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Fueling and Nutrition for Marathons, Ultras, and FKTs

This is the first of a 4 part series on nutrition for endurance athletes. Subscribe for updates as we drop other articles:

  1. Fueling and Nutrition for Marathons, Ultras and FKTs (congrats, you’re here!)
  2. Race Strategy Nutrition for Ultramarathons
  3. Common Foods for Marathons, Ultramarathons and FKTs
  4. Ask Me Anything: Food and nutrition Q&A for runners

Nutrition is an essential part of life and a topic of constant conversation among athletes. There is an entire industry devoted to sports-specific nutrition that is so bloated and convoluted that if often leaves folks more confused and frustrated than when they began their research.

While it certainly doesn’t need to be as complicated as the marketing companies would have you believe, there are several elements of nutrition that runners of all distances should know in order to properly fuel for the desired event.


Nutrition Basics

Photo by Juan José Valencia Antía on Unsplash

Here we’ll review the basics of nutrition and what your food is comprised of. This becomes more important as we look at what makes good race and endurance fuel – and it starts with calories, macronutrients, and micronutrients.


A calorie is a unit of energy. The name itself is misleading, though, as the term actually refers to a kilocalorie, aka “kcal” (1,000 calories = 1kcal), which is a measurement of how much energy it takes to raise the temperature of one gram of water by one degree Celsius.

The amount of calories in a given food item is measured by placing it inside a “bomb calorimeter”, which zaps it with energy until it combusts. The amount of energy it takes to get to this point is the amount of calories the food is said to contain. Nutrition labels typically list the energy in calories for clarity, though in reality its referring to kilocalories. A calorie is comprised of both macronutrients and micronutrients, which play essential roles in fueling and nourishing your body.


Macronutrients are the major providers of energy (calories) in food and consist of carbohydrates, protein, and fat.

Carbohydrates are one of your body’s main fuel sources. Once consumed carbohydrates are broken down into glucose, which is transported through your bloodstream to supply energy to your muscles, tissue, brain, and central nervous system. A simple sugar can be used by muscles with 15 minutes of entering you mouth. One gram of carbohydrate contains four calories.

Protein is a major building block of your cells. It is used to used to repair and create tissue structures that form your organs, muscles, hair, skin, bones, and more. It is is comprised of a string of amino acids, some of which are created in the body and others which must come from food. Animal sources of protein offer what is known as a “complete protein” because all the necessary amino acids are present. Other sources of protein such as fruits, vegetables, and nuts, lack one or more of the essential amino acids and must be consumed with others in order to provide the necessary amino acids (source). One gram of protein contains four calories.

Fats are often disparaged as unhealthy, although they play a crucial role in our health. Not only does it provide a crucial energy source, but it also assists in the absorption of vitamins and nutrients from your food. There are many kinds of of fats, but a good rule of thumb is consume unsaturated fats (oils, fish, nuts)  and avoid saturated fats (trans fats, hydrogenated oils, lard) when possible. One gram of fat contains nine calories.


Micronutrients consist of vitamins and minerals that support metabolism, neurological functions, immune system support, and overall health. Though they do not provide caloric energy, they are nevertheless essential. The the most common vitamins and minerals that assist in keeping us healthy are Sodium, Vitamin B6, Vitamin C, Vitamin E, Magnesium, Calcium, and Iron, most of which can be obtained through a regular balanced diet. That being said, some athletes require vitamin supplementation in order to achieve regular levels due to deficiencies which can be extremely detrimental to performance. Sodium, potassium, magnesium and calcium are most important for fluid balance and should have special considerations during a race.

Fueling for Long-Distance Running

Nutrition is more strategic in longer races with less resupply options and farther distances. Photo from Kayvan Mazhar on Unsplash.

Your body loses carbohydrates and minerals through exercise, so the purpose of fueling for long-distance events is simple: get sugar into your bloodstream that will provide energy to your muscles to continue moving. After 1.5-2.5 hours of exercise your body runs out of glucose stores, the primary energy source for muscles, and in order to perform at your best you need to consume enough carbohydrates to cross the finish line. Fueling issues (such as low energy, poor mood, etc.) can mean the end of your event if not handled properly, but can often be fixed in a relatively short amount of time simply by eating.

While at rest, your body mainly uses fat for its energy source. As your activity and its intensity level increases, your body changes the composition of fuel to utilize a higher percentage of carbohydrates. This is because carbohydrates can be broken down into usable energy faster than other sources, and when your body is demanding more fuel than it can produce (e.g. during a fast marathon effort) then it prioritizes carbohydrates because of its fast-acting properties. However, our bodies can only utilize existing carbohydrate stores for 1.5-2.5 hours at a moderate-high intensity level. The reason we need to supplement our nutrition intake with carbohydrates is because we can run out of our usable energy in a relatively short period of time and will experience a significant drop in performance when it happens.

Whether you’re running a traditional road marathon, a hilly 50-miler, or a multi-day FKT, adequate fueling will make or break your performance. In all of these situations the key to success is determining how much, what kind, and when to consume your fuel.

How Much and When

Determining how many calories you should take in during an event is a function of how much energy your body burns and how many calories your gut can process without significant gut issues. The typical formula used for energy consumption while running is one calorie per kilogram per kilometer on flat terrain with an additional 1:10 ratio supplement to account for vertical gain. So for someone like myself who weighs 82Kg/180lbs running a 21km/13mi route with 1,500m/4,920ft of elevation, I would burn approximately 2,952 calories:

  • 82kg x 21km = 1,722 calories (calories needed on flat terrain)
  • 1,500m x 10 = 15km, and 15km x 82kg = 1,230 calories (additional calories to account for vertical gain using 1:10 ratio)
  • Total: 1,722 + 1,230 = 2,952.

Although this formula is not 100% accurate, it’s a great starting point in determining your energy needs. The amount of food your body can process is usually expressed in calories per hour. The typical recommendation is to take in 150-400 calories per hour while moving. If I was able to finish the 21km route mentioned above in 3 hours, that would call for nearly 1,000 calories per hour which would be way too much for my gut to handle. The strategy here and in longer events is not to replenish calories as fast as you are burning them, but rather to take in however many calories your gut can handle when your body’s demand for blood flow is at its highest (when moving) and supplement with higher caloric intakes when you stop at aid stations, to sleep for the night, or significantly slow your pace. That gives your gut the opportunity to process more calories and keep you from falling behind on your nutrition while not overburdening your system while running.

For most runners 60g of carbohydrates per hour works well as it equates to 240 calories, though research shows that some can take up to 120g per hour with training and practice in longer/slower efforts.

Sometimes when you are having a tough race, it is best to sit down and take care of yourself.

What Kind

The age-old question of what to take in during a race or long event has an unsatisfactory answer: it depends. Factors such as how long your race is, the climate, what intensity you will be operating at, and how well trained your gut is all make a difference in what kind of fuel will work best. For fastpacking adventures, the weight of your nutrition also plays a role since you may be carrying everything with you for multiple days.

For marathons and below at higher intensities:

Easily-digestible carbohydrates such as gels and drink mixes (Maurten, Tailwind, etc.). Nothing too complex or heavy, as your body’s demand for blood flow will be high and it will be difficult to process. Intake needs are less because the event is shorter and you can run yourself into a deficit without significant consequence as your next meal is only a few hours away.

For ultramarathons and multi-day fastpacking/FKTs:

For longer distance events when your overall pace is slower and demand for blood flow is less, you can (and should) incorporate more wholesome food into your nutrition strategy which will not only be more satisfying but also help sustain your energy and mood over an extended period of time. What’s more, the finish line is much further away than typical races so you can’t afford to make continually poor nutrition choices and expect to finish in good shape.

It is recommended that you take in carbohydrates that equal 30 to 40% of your energy expenditure, which was calculated above for me at about 1,000 calories per hour for the specified event. Therefore, using that example, I should take in 300-400 calories per hour or  75-100g of carbohydrate (because there are 4 calories per gram of carbohydrate as mentioned above). This estimate may be too high for some runners, so it’s imperative that you experiment with different levels of intake in training to see what works for you as taking in more carbohydrate than you can process will lead to GI issues.

Though carbohydrate intake is necessary for fueling, it’s also important to eat foods with fat and protein as well. For multi-day events, these foods are taking place of your regular meals and therefore should be wholesome.

Rules of thumb for race fueling:

Being self-relient and packing foods you know work for you is always a winning strategy.
  • Eat carbs, this is the most effective nutrient to be converted to energy. Studies show runners can typically digest 60g of carbs (240 calories) in an hour, with potential for up to 90g.
  • Consume calories at consistently, every 30-60 minutes. This will allow your body to constantly replenish the glucose your body is losing.
  • Nothing new on race day.
  • If running for 5+ hours, aim to consume ~15% of your calories from protein.

Does the kind of food I eat affect my performance?

Yes, there is an ideal 2:1 ratio of sucrose to glucose which is what sports drinks are engineered around. For faster races, like a marathon, your body is under more acute stress. The high intensity will make non-sports performance foods difficult to tolerate.

At a lower intensity pace like in an ultramarathon, your stomach will have more flexibility of what it is able to handle. As the event gets longer (6+ hours), fats and proteins can be considered.

How should I fuel before and after an event?

Prior: Have a small amount of carbs prior to your training session. I typically like to have a bowl of oats with blueberries, a bagel with peanut butter and jelly, or even a bowl whole grain cereal with almond milk. Avoid dairy, heavy foods and excessive fats.

Post: Within 15 minutes of exercising, consume ~200 calories of carbs (half a gatorade or a ProBar bite). Within 4-6 hours, consume a combined ~600 calories of carbs and ~200 calories of protein. Some foods, like processed foods, can trigger inflammation. You will recover more quickly by sticking with clean and non-inflammatory foods.

Final Thoughts

Fueling for long-distance events and multi-day adventures is a common source of confusion among athletes, but what it comes down to is ensuring you’re taking in calories on a consistent basis. You won’t (and shouldn’t) be able to replace all calories expended during the event while moving, but instead should aim to take in 200-400 calories per hour, mostly carbohydrate, and more when your body’s demand for blood flow is lower.

There is more to the topic of race nutrition such as when to incorporate salt and electrolytes, how to hydrate, and what foods you may want to test for performance–but those will be covered in future articles. For now, the important takeaways are learning to calculate your expenditure and understanding the roles that macro and micronutrients play in performance.

Looking for more? This is a 4 part series on nutrition for endurance athletes:

  1. Fueling and Nutrition for Marathons, Ultras and FKTs (congrats, you’re here!)
  2. Race Strategy Nutrition for Ultramarathons
  3. Common Foods for Marathons, Ultramarathons and FKTs
  4. Ask Me Anything: Food and nutrition Q&A for runners

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