Fueling & Nutrition for the Marathon: Key Takeaways from a Registered Dietician
You know that moment around mile 18 when your legs suddenly feel like cement and every runner around you starts talking about "the wall"? That's not just mental toughness failing you — it's often a fueling problem, and it's almost entirely preventable.
I'm Leah Rosenfeld, a coach for runners worldwide, and I coach a group of rockstar runners throughout the summer and fall for all fall marathons. During our recent marathon fueling and nutrition Zoom call, our team learned from Maddie Alm (Harris), Registered Dietitian and Sports Nutrition Coach at Fueling Forward, who broke down exactly how to fuel and hydrate so you can run strong from the start line to the finish — not just survive the back half.
The content she shared was exceptional and grounded in the most current nutrition science, so consider this article a resource to come back to whenever you have questions. 📌
🥯 Pre-Run Fueling
Carbohydrates are your best friend. Performance nutrition is different from general health nutrition, so don't fear carbs before your run or throughout the day. Skipping pre-run fuel doesn't burn more fat — it just breaks down muscle tissue, especially in female athletes.
Stick to simple carbs (low in fat, fiber, and protein) to avoid GI issues, particularly if you're running less than 30 minutes after eating.
Quick rule of thumb: you digest approximately 1 gram of carb per minute — helpful for timing your snack.
Time Before Run What to Eat 1–2 hours out 45–60g carbs — two pieces of toast, a bagel with butter, or a toaster waffle with syrup Under 1 hour out Smaller & simpler — fruit snacks, a granola bar, a Honey Stinger waffle, or graham crackers
Try not to go below about 40g of carbs pre-run if you have time to digest it, and lean higher for longer or harder sessions.
Don't forget fluids either — 8–12 oz to start your day and before you head out, with a pinch of salt or an electrolyte tablet, which also helps your adrenals wake you up in the morning.
🥤 Post-Run Fueling
Protein alone isn't enough after a run. Pair it with carbs at roughly a 2:1–3:1 carb-to-protein ratio — around 20g of protein and 40–60g of carbs.
Good combos:
Fruit with a sports drink
A smoothie with Greek yogurt, fruit, and juice
If your appetite is low post-run, a cold liquid option works well — it also helps cool your core temperature, which brings your appetite back. And if you're lifting or doing yoga after a run, fuel in between the two — a simple chocolate milk works great here. 🥛
💧 Hydration
General daily fluid needs run about 2.5L for women and 3.5L for men, plus 8–16 oz per hour of training. Aim for at least 100 oz per day — coffee counts, but alcohol and energy drinks don't.
Quick hydration check:
You should pee every 1–3 hours
Urine should be pale yellow
It should take at least 10 seconds
Spread your fluids throughout the day rather than chugging at once, since it takes about 24 hours to fully rehydrate, and try to stop fluids about an hour before an evening run or bedtime.
🧂 Electrolytes, Training & Racing
Endurance athletes lose about 900mg of sodium per hour of training (this varies by individual), and most runners under-consume electrolytes.
When Sodium Target During a training block (daily) ~1,000mg/day During training & racing (hourly) 300–600mg/hour Fluids during training & racing 400–800mL/hour
Most handheld bottles hold 400–600mL.
Adding electrolytes — or even fruit for a "spa water" — can also encourage you to drink more overall.
⛽ Fueling During Runs
Glycogen depletion kicks in after about an hour, which is what eventually causes "bonking."
Research from the Seville Marathon found runners consuming 60–90g of carbs per hour were far more likely to break three hours — and most runners in that study weren't even close, averaging just 35g/hour.
Build toward that gradually: start around 20g/hour and increase by 10g every 1–2 weeks. Since most gels and chews contain about 20g of carbs each, hitting 60g/hour often means 2–3 gels, or a mix of gels, chews, and drink mix.
💡 Pro tip: Take your fuel by time (every 30 minutes), rather than by miles, since your mile pace can vary. Set a timer on your watch to beep every 30 minutes, and take a couple sips of water every mile — and you'll be set!
☕ Caffeine
Practice with caffeine well before race day — never try it for the first time on race day.
Starting dose: ~50mg/hour, working up to 100–200mg/hour
Daily cap: 400–500mg total (this includes everything — coffee, tea, pre-race caffeine, and gels combined)
A common strategy is one caffeinated gel at the start and another around the halfway point for a late-race boost.
Less is more here — too much caffeine can hurt performance and contribute to GI distress.
🍚 Carb Loading & Race Week
Two to three days before your race, aim for 10–12g of carbs per kilogram of body weight per day. Done properly, this can improve race performance by about 3% — roughly five minutes faster for a three-hour marathoner.
Easy ways to boost carbs:
Extra honey or syrup on oatmeal
Juice with meals (lemonade is a great option)
Bagels over bread
Don't force-feed yourself, and know that some weight gain or fluid retention beforehand is normal — and a good sign. It means your glycogen stores are full.
And remember: carbs are carbs. Gummy bears, soda, and candy all count toward your carb-load needs. 🐻 Microwaveable white rice packs are my (Leah's) personal favorite for carb loading — rice is easy on the stomach, and often contains a whopping 80–90g of carbs in one packet!
✅ The Bottom Line
No single gel, product, or performance diet is "best" — the one you can consistently enjoy and tolerate is the right one for you.
Practice your full fueling and hydration plan on at least two or three long runs before race day, and never try something new when it counts.
Trust your training, have fun, and celebrate making it to the start line — that's an accomplishment in itself. 🎉
🔍 A Gut Check After Reading
After reading this, it's worth pausing on something important:
If you're currently training and you feel healthy, have stable energy most days (it's normal to be tired for a day or two, but not completely wrecked), and generally feel like you're building momentum and fitness — then what you're doing is working. If that's you, use the information in this article to make small adjustments to your fueling and hydration, if any at all.
On the other hand, if you're feeling abnormally tired every day (unrelated to high temps/hot weather), keep noticing injuries or niggles pop up, feel like you're plateauing, or feel chronically revved up and stressed — it could be a sign you're underfueling.
Many runners hit this point at some stage in training. It's simply a tipping point where the accumulation of mileage and workouts, mixed with work, life, sleep, and the time of year, creates a situation where your body is asking for more energy to sustain this level of activity.
Rest days and recovery runs are important when this happens, but so is fueling your body to support your daily energy needs. Especially for marathoners and ultra runners — it's not a 5K you signed up for, it's a long race, and you can move through training feeling strong the entire way without bonking. You just need to fill the gas tank before the road trip. If this sounds like you, it may also be worth getting a blood test to better understand how to support your body, since it's working hard for you while training!
Either way, feeling pressured to overhaul your entire diet after reading this isn't the answer (it's common to feel that pressure after a nutrition talk). Instead, adjusting one small thing at meal time — like eating two slices of sourdough toast instead of one, or adding an electrolyte tablet to your water before a run — is a perfect place to start.
One step, one meal at a time. This is all a learning experience, and we're grateful to have experts like Maddie sift through the noise and give us what actually works. 🙏
Connect with Maddie:
🌐 fuelingforward.com
📸 Instagram: @fueling_forward
🎙️ The Fueling Forward Podcast
