Long run fueling: how to tell if your plan actually worked
You took your gels on schedule and finished your long run. But did your fueling plan really work? Your watch already has the answer, you just need to know where to look.
The ELYO team
The sports coach that reads your workouts
The short answer
Your fueling worked if your pace holds until the end of the run with a fairly steady heart rate, and you don't get a sudden energy crash near the end. A pace that drops while your heart rate climbs sharply can point to a lack of fuel, or of water in hot weather. For efforts of 1 to 2.5 hours, the American College of Sports Medicine recommends about 30 to 60 g of carbohydrate per hour.
Key takeaways
- For efforts of 1 to 2.5 hours, guidelines suggest about 30 to 60 g of carbohydrate per hour, and up to 90 g for longer efforts.
- A pace that holds to the end at a steady heart rate is the best sign your fuel kept up.
- Heart rate rises a little late in a long run even when everything goes well; what matters is how much and when.
- Fueling is a skill you train: the long run is where you test it, not race day.
The long run is the workout where fueling makes or breaks you. Under an hour, your body's own stores are almost always enough. Past 90 minutes, what you eat and drink on the move starts to decide how your run ends.
The good news: you don't need a lab to know whether your plan held up. Your pace, your heart rate and how you felt tell the story, as long as you read them together.
What your body needs on a long run
During a long effort, your muscles mostly burn carbohydrate and fat. The carbohydrate stored in your muscles and liver (glycogen) doesn't last forever: on a long run at a solid pace, it can run out. That's the famous "wall", when your legs simply stop responding.
The major sports science bodies give clear guidelines:
- Efforts of 1 to 2.5 hours: about 30 to 60 g of carbohydrate per hour.
- Efforts over 2.5 hours: up to 90 g per hour, using a mix of glucose and fructose that your gut absorbs better.
- Water: drink enough to keep weight loss during the effort to about 2%, without drinking more than you lose.
These guidelines are shared on both sides of the Atlantic: you'll find them in the 2016 North American joint position statement and in the work of European researcher Asker Jeukendrup, both listed in the sources.
A typical gel provides about 20 to 25 g of carbohydrate. One gel every 40 minutes works out to roughly 30 to 37 g per hour: the low end of the range, and often enough for a 2 to 2.5 hour run.
Clue one: your pace over the last kilometers
Look at your pace kilometer by kilometer (or mile by mile), especially over the final third of the run. If you ran at a steady pace and your last kilometers look like your first ones, your fuel kept up.
A pace that suddenly falls apart near the end, with no reason from the course (a hill, wind, surges), is a classic sign of empty tanks. A pace that slips gradually can come from something else: a tiring week, heat, or starting too fast.
Clue two: your heart rate, and especially when it climbs
On a long effort at a constant pace, your heart rate creeps up a little over time, even when everything is going well. Researchers call this cardiac drift: heat and fluid loss make your heart beat faster for the same effort.
So the question isn't whether your heart rate goes up, but by how much and from when.
Compare the two halves of your run
A check many coaches use: compare pace against heart rate for the first half and the second half. If, at the same pace, your heart rate only rose by a few beats, your endurance and your fueling held up. If it jumps clearly after a specific kilometer, note that kilometer: that's often where something gave out.
Clue three: how you felt, noted right away
Data doesn't tell you everything. Right after your run, write one sentence about how you felt: hungry, a sudden energy crash, an uneasy stomach, nothing special. That's what lets you tell a lack of fuel (hunger, empty legs) apart from a stomach that can't yet handle your gel schedule (bloating, nausea).
The two are fixed differently: the first by eating more or earlier, the second by training your gut to take in carbohydrate while you run. Because fueling can be trained, just like your legs: a gut that's used to it absorbs better and complains less.
What you can do on your next long run
- Write your plan before you go: at which minute you take each gel, and how much you drink.
- Take your first gel before you feel hungry, often around 40 to 45 minutes. If you wait until you're hungry, you're already behind.
- After the run, compare pace and heart rate for the first and second halves, and write down how you felt in one sentence.
- Change only one thing at a time from one run to the next (timing, amount or product) so you know what works.
- Test in training what you'll use on race day, never the other way around.
This is the kind of reading ELYO does after your runs: it compares your pace and heart rate across the workout, and asks whether you stuck to your fueling plan so it can connect the two.
Frequently asked questions
How many gels should I take on a 2 hour long run?
With a gel of about 25 g of carbohydrate, three gels over two hours give you about 37 g per hour, within the recommended 30 to 60 g range. Only two gels would leave you around 25 g per hour, below the range. Start at the low end if your stomach isn't used to it yet.
Why does my heart rate go up at the end of a run when my pace stays the same?
That's cardiac drift: heat and fluid loss make your heart beat faster for the same effort. A small rise is normal. A sharp, sudden rise can be a sign you're short on water or fuel.
Do I need to eat during a run shorter than an hour?
Usually not: your stores are enough for an effort under an hour. For a very hard session, rinsing your mouth with a sports drink may help, but it isn't essential.
How do I know if it was my stomach or my fuel that let me down?
Hunger and empty legs late in the run point to a lack of fuel. Bloating, stomach cramps or nausea point more to a gut that isn't used to it yet. If stomach problems keep coming back, talk to a doctor or a sports dietitian.
Sources
- Thomas D. T., Erdman K. A., Burke L. M. (2016). Nutrition and Athletic Performance, position commune de l'American College of Sports Medicine, de l'Academy of Nutrition and Dietetics et des Dietitians of Canada. Medicine & Science in Sports & Exercise. DOI
- Jeukendrup A. (2014). A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine. DOI
- Jeukendrup A. E. (2017). Training the gut for athletes. Sports Medicine. DOI
- Coyle E. F., González-Alonso J. (2001). Cardiovascular drift during prolonged exercise: new perspectives. Exercise and Sport Sciences Reviews. DOI
Written with the help of AI.
