What should I eat? Tips for endurance fueling

John and an unknown mountain biker fuel up during the 2009 Vermont 50-mile mountain bike and ultra run.

Over the past few weeks, I’ve received several emails and tweets asking in one way or another, “What should I eat?”

Indeed, nutrition before, during and after racing is a key pre-occupation for many runners and triathletes. I suppose that is so because nutrition is so darn tricky. What works for one person may or may not work for another.

But, don’t throw up your lunch just yet! There are some general principles you can apply to help you find your race-day fueling sweet spot. I’ll share some of them here. Note: These tips are geared toward race-day fueling for distance events: marathons, ultramarathons, and half and full ironmans. Shorter events require a different approach, but that’s a post for another time.

It is also important to note that proper race fueling  requires good nutrition throughout training and in the days leading to the race. However, pre-race nutrition and “every day nutrition” will have to wait for another post.

What’s inside of us already

First things first: Your body does NOT contain enough internal fuel (glycogen or glucose) to carry you through a marathon or beyond. At 80-90% of max intensity, we have about 2 hours worth of fuel stored in our muscles (glyocogen) and blood (glucose). But, for most athletes, performance will begin to decline before you get to that two hour mark—unless you restock.

And so was born the gel and sports drink industry.

So, it should be easy right? Just eat a gel, grab some aid station cookies and pretzels, drink some fluids and be on our merry way. Ha! Ha! Not quite so fast, you epicurean daredevil, you!

If you’ve been at this long enough, you’ve likely had at least one nutritional mishap–more likely, you’ve had several. If you’ve ever felt apathetic, anxious, extremely fatigued and so on, you’ve “under” fueled.  If you’ve ever experienced stomach cramps, bloating, or vomiting, you may have “over” fueled.

The trouble is that the human GI system is quite variable in terms of the amount and type of fuel it can handle. Some of us treat aid stations like an all you can eat buffet, eating whatever we like with little consequence. While others must carefully learn through trial and error what their stomach will and will not accept during racing. In addition to the individual variability, intensity, duration, climate, and race conditions can also switch up nutritional needs.

So, what to do? You have to train your digestive system just like you train your cardiovascular and neuromuscular systems. The stomach is very jealous of the attention that the heart gets. It just wants a little bit of love and attention – is that so wrong?

How much and what type of calories depends upon the duration and intensity of your race. Marathons and half ironmans are typically run at a higher intensity than ultra marathons or full ironmans. Therefore, how much you can eat, as well as the type of foods the body will be able to digest easily, will differ to some extent. No matter what the distance: endurance athletes need carbohydrates on race day. So, let’s start there.


In general, carbohydrates are the primary energy source for endurance athletes.  Carbohydrates are such an excellent source for athletes because they can be easily broken down and used by our bodies – without the need for much oxygen, which is good because we are using oxygen for other important stuff in the middle of a race ;).

The body stores carbohydrates in the form of glycogen (muscles) and glucose (blood). Glycogen is the main source for endurance energy, while glucose is most useful for helping to keep our mental facilities sharp and for supplying quick bursts of energy (like when you want to pass that person in right front of you).

Athletes have a limited amount of stored carbohydrate energy, as I mentioned earlier. This is why proper recovery nutrition is so important: it ensures that you restock the used stores after each workout. This is also why many tout the benefits of pre-race carb loading: it ensures that those glycogen stores are filled to the top. And finally, this is why we must consume gels, chomps, sports drink and the like throughout our endurance endeavors. We will run out of energy if we don’t.

When some runners talk about hitting the wall, what they might really be referring to is hitting the end of their glycogen stores.

Knowing how much to restock depends, at least in part, on how much carbohydrate you can and need to digest, given the intensity and duration of your specific race. Studies have found that, on average, we can process about 1 gram of carbohydrate per minute. So, this means the average mid-size athlete can digest around 60 grams of carbs per hour, working at about 80% of max intensity. To put this number in perspective, sports gels contain about 30 grams of carbohydrates.  Research has found that a minimum of 10 grams per hour is necessary to stave off the bonk.

The consumption of more carbohydrate and calories than the stomach can process will lead to GI issues because the stomach will not be able to process the calories and empty the stomach. The outcome? Bloating, cramps, vomiting and so on.

Additionally, 1 gram per minute is not a “magic” number for everyone. It’s an average, which means most people will need slightly less or slightly more. If you are a smaller female, it is likely you will need less; for a larger male, it is likely you will need more.

So, you must practice in training.

Our friend and nutritionist Chris Draper recommends adding and subtracting calorie amounts in small increments. I’ll use myself as an example. I am a 127 pound, 5’3” female. I consume around 40-45 grams of carbohydrate (roughly 150-180 calories) per hour while cycling, and about 30 grams of carb (100 calories) per hour while running.

I found this “magic” number through testing in my training. I used to consume only 100 calories/hour while cycling. That led to bonking.  I tried 200 calories an hour, which led to bloating and even cramps in one case. I fiddled around with my calorie counts in small increments (adding and/or subtracting 25-50 calories per hour) until I found this magic number for fueling. I will go through the same process again next year, as it is likely my body physiology will change. (I’d also like to be 7 pounds lighter, but that’s another story!)

The takeaway: practice your nutrition!

To aid with gastric emptying, it is important to drink fluids with your calories. This helps the stomach process the fuel, get the energy where it needs to go, and empty the stomach more quickly. The research has found that differing solutions of carbohydrate to water can affect performance, with a concentration of somewhere between 7-12% being “optimal.” (Most commercial sports drinks are in this range.) Again, however, the research has found that there are individual variations, underscoring how important it is to practice your race day fueling during every workout.

Have I mentioned yet that you should practice your race day nutrition while training? You really, really should.

To further aid with digestion, eat regular, small amounts of calories. As I’ve written in previous posts, I will consume some type of calories every 15 minutes. So, if my total calories for the hour is 150, then I will break this up into 4 feedings. I set my watch to beep every 15 minutes so I don’t forget.

On the run, I’m sure to consume calories every 2 miles/15-20 minutes. I will sometimes use miles on the run since aid stations are typically spaced by about a mile in most marathons. For ultras, I carry a hand held water bottle with sports drink. I sip that regularly, and then eat solid food at the aid stations. But, again, not huge amounts – small, regular feedings have been found to be most effective.

The higher the intensity, the more important portion control becomes if you want to keep your stomach from turning into the big nasty.

Different types of carbohydrates

During racing, you want to consume simple carbohydrates. Said simply: sugars. There are different types of sugars some of which digest more quickly than others. Slower digesting sugars, such as maltodextrin, have been found to cause cramping at higher intensities for some people. Yet, at lower intensities, this may not be as much of a problem. Furthermore, many runners and triathletes report stomach issues with high fructose concentrations. For example, I tend to have problems with Gatorade, which has high fructose concentration. (Gatorade is the DEVIL!)

I much prefer power bar products, which have a mix of different sugars, which gives the body three different pathways to digest the energy. I also love Cliff Shot Bloks, which is yet another type of sugar, thus complimenting the power bar gels and drinks. As an added bonus, the small size of the bloks makes it very easy to portion your calories.

While carbohydrates clearly reign supreme for nutrition, fueling and recovery, they are not the only macronutrient that benefits endurance athletes.

Fat is your fuel

Unlike muscular glycogen, our bodies can store enough fat to fuel up to 2000 hours of activity. The trouble is, this energy is not readily usable because fat does process as easily or quickly as carbohydrates. And, in order to burn fat, our bodies need both carbohydrates and oxygen to keep the fire burning.

Even so, for endurance events run at about 75% of our max, our body does rely upon its fat stores. Some studies have found that at moderate intensities, we can get up to 50% of our energy from our existing fat stores. It’s better than liposuction!

The higher the intensity, however, the less ability the body has to metabolize fat, mostly because there is oxygen deprivation. At anaerobic intensity, the body is burning virtually no fat, and all carbohydrate. But, if you are anaerobic in an endurance event, you have bigger trouble than whether or not your body is burning fat. 😉

Given this chemisty, the endurance athlete will use a mix of both carbohydrate and fat as fuel during racing. Over time, endurance training teaches the body learn to burn fat for fuel more efficiently. This is another good reason why you should practice carbohydrate intake during training. In addition to learning what types and amounts of calories you can consume, it will be an integral part of teaching your body to become a fat-burning machine.

While training or racing, there is no need to consume fat calories. You simply consume an appropriate amount of carbohydrates, and the body will take care of eating into your fat stores.

Goodbye, inner thighs–Hello, six-pack!

Power in Protein

Of late, attention has been focused on the performance benefits of combining carbohydrate and protein during training and racing. The question is: does this really make a difference over products that only contain carbohydrate?

The research does not seem to think so, and in fact, the consumption of protein while racing (or training) may actually impede the body’s ability to burn fat for energy. This inhibition of fat burning is especially problematic for long distance events.  On the other hand, for longer distance, lower intensity events, and for cycling, the research has found that small amounts of protein may help the body maintain its glycogen stores for a longer period of time.

It takes the body a very long time to digest protein, and it may not even be able to do so effectively at race intensities. So, if we are going to take the advice of the scientists, it looks like protein should not be a primary race fuel for most distances. For recovery, however, the consumption of protein following exercise can be quite beneficial to help muscles repair and strengthen following training and racing adaptations.

You are probably getting the sense at this point that race fueling is anything but an exact science. This is why so many of us have been doubled over, fighting nausea and cramping, or feeling light headed and overly fatigued. However, I hope these general principles help you as you prepare your race fueling strategy.

We spend too much time training our muscles and our hearts to let our stomachs get in the way.


  1. Thanks for answering my question! I went for a 45 mile training ride on Saturday and only had about 300 calories. I definitely think that I need more! I will start increasing it and figure out what works for me. Thanks for the help!

  2. Since I am new to long distance triathlons, I am curious as to why you would eat fewer calories while running (100) as you do while biking (150-180). Don’t we burn less calories while biking? I find that I’m not hungry while biking like I am while running and i was under the impression that it takes less energy to bike for an hour than run, but I could be wrong! Or do you eat more on the bike so that you are properly fueled for the run? If you are doing a training bike ride, would you still eat the same amount of calories?

    1. @Nicole – great questions! Nutrition is one of the hardest parts to figure out. So, it’s good that you are reading about it and asking questions. You eat more on the bike to fuel the run. It’s very very hard to eat the amount of calories you actually need while running–especially after swimming and biking.

      I practice my nutrition in almost every training session so that my stomach and digestive system is prepared to handle the caloric intake. The only exception is for training sessions that are less than 60 minutes. In this case, I will consume only sports drink. Practicing your nutrition is CRUCIAL for the long bike and the long run. You don’t want to try anything new on race day – and this is particularly important when it comes to nutrition. Your body should be so accustomed to the fuel – both type and amount – that there is NO REASON for GI distress. yes, there can be individual factors on race day that lead to tummy issues, but you greatly decrease the chance of a problem if your body is 100% used to what you consume on race day.

      Proper fueling during training sessions is also important for your recovery following those workouts. If we starve our muscles during training, they will not recover as quickly. We also risk the body using muscular protein for fuel, instead of glycogen, which can (over the long term) result in strength loss.

      Okay, I read WAY TOO much about this stuff. Hope these answers help. Let me know if you have more questions 🙂

  3. oooh this was so timely, thank you! i’m working on tweaking my nutrition and it is a frustrating mess! i used to only take in solids on the bike, which was great for long slow rides but it turned into a mess at race intensity. now i’m trying liquids only (EFS, liquid shot, etc) and my stomach is UN HAPPY. hrrmph.

    1. @Katie – yay! I thought it might be timely. Have you tried the power bar products–gels, power bars & Ironman perform? They will be on the course at CDA, so you may want to see if you can tolerate them. That way, you won’t need to carry as much with you on race day. For myself, these are the best combo. I LOVE ironman perform. Power bars are okay. I usually only eat one of them during the first two hours on the bike. And, the gels are my favorite gels – very liquidy and easy to pop in your mouth.

What do you think? Share your thoughts and experience!