Fueling smart: cracking the carb code

If you’re an ultra endurance athlete, you already know the long miles don’t just test your legs — they test your gut, your energy, and your mind. Whether you’re heading out for a two-hour training run or grinding through back-to-back long days on the trails, what you put in your bottles and pockets can make the difference between powering through or bonking hard. One of the biggest game changers? How you fuel with carbohydrates.

Start with 90 Grams

For runs longer than about an hour — especially anything between 90 minutes and 3 hours — your body’s stored muscle glycogen simply won’t last if you’re pushing the pace. Studies show that taking in around 90 grams of carbohydrates per hour helps keep energy levels steady, muscles fueled, and your brain clear when fatigue sets in.

Most runners underfuel — sipping on a gel here, a sports drink there — falling well short of what they could actually use. While 60 grams per hour is good, 90 grams can really maximize your performance, if your gut is trained for it.

Next: Use a 2:1 Glucose: Fructose Mix

It’s not just about how much — it’s about the type of carbs. Your gut can handle more when you combine two kinds of sugar instead of just relying on one. A good rule of thumb is to aim for about twice as much glucose-type carb as fructose.

Why? Different sugars take different pathways through your gut. Combining them helps you absorb more total fuel, which means less chance of stomach issues and more energy when you need it most. This is why companies like Tailwind and Skratch produce products that have "multiple transportable" and "easily digestible" carbs--this is what they are talking about. They usually are working to create a convenient powder where you can get your 90 grams of carbs in a 2:1 easily digestible glucose to fructose ratio. But what if you don't have those on hand, or you get palatte fatigue after a few, or you just don't like them? This is when we start looking at real food.

How does your aid station food measure up?

Coke, M & M's, and grapes aren't bad for an aid station pick-me-up--especially if that's all that looks good to you in the moment! However, these are higher in fructose than glucose, so if you camp out on them all day, you won't be optimizing your energy, and you might even get some stomach upset. You can instead pair them with more glucose-heavy options, such as rice cakes and white bread. Or have something with a more naturally balanced 2:1 ratio, such as cantaloupe.

How to Hit That 90-Gram Goal

Whether you prefer store-bought drink mixes or real foods, aim for about 20–30 grams every 20 minutes on longer runs. Mix it up so your gut stays happy. Try different combinations in training, not just on race day — you need to train your stomach like you train your legs!

My (current) go-to DIY Fuel Solution

With blueberries readily available this summer, I started mixing a pitcher of homemade lemonade and adding in some blended up blueberries. I switched out the water for coconut water, and added some salt, both for additional electrolytes. The table sugar in lemonade is a 1:1 glucose to fructose ratio, so we had to tweak a bit. Rice crackers are easily digestible, and are 100% glucose when their starches break down. They also have salt and make me feel like I've got a little something in my stomach. Here's the breakdown:

✔️ DIY Lemonade: Use just about any homemade lemonade recipe with sugar, coconut water instead of the water, and a bit of salt. Blend in a cup of blueberries if you like. It's got:

• 1:1 glucose to fructose ratio

• About 65 grams of carbs, depending on the size of your handheld

✔️ A serving of Rice Crackers: These add quick, easy-to-digest carbs. It's got:

• Almost 100% glucose

• About 25 grams of carbs

✔️ Plain Water: Bring extra plain water so you stay hydrated and help your gut process all that fuel smoothly.

Assuming you don't really need much on the first hour of your run, this can take you through the second hour.

Bottom line: Ultra endurance athletes who fuel with around 90 grams of carbs per hour in a balanced 2:1 glucose to fructose mix stay energized longer and finish feeling stronger. Small tweaks like this can transform your training — and your gut will thank you come race day.

Happy running

Lauren Callahan is a Certified Sports Nutrition Coach, Ultra endurance athlete, and doctoral student.

Ultra Nutrition provides ultra compassionate coaching for ultra endurance athletes. Ready to improve your performance, energy, mood, and hormones? Book nutrition coaching here.