Fueling & Nutrition for the Marathon: Key Takeaways from a Registered Dietician
What a sports dietitian taught my marathon team about carbs, electrolytes, caffeine, and race week.
You know that moment around mile 18 when your legs turn to cement and everyone starts talking about "the wall"? It's usually not a toughness problem. It's a fueling problem, and it's almost entirely preventable.
I'm Leah Rosenfeld, a former pro athlete turned running coach, and I work with runners all over the world, from first-time marathoners to elites. My goal is simple: to help every runner train smarter, feel stronger, and have more fun along the way.
On a recent fueling call with my fall marathon team, Maddie Alm, Registered Dietitian and Sports Nutrition Coach at Fueling Forward, walked us through exactly how to fuel and hydrate so you feel strong from the first mile to the finish line. Here are the key takeaways. 📌
🥯 Pre-Run Fueling: Carbs Are Your Friend
Carbs are your body's main fuel when you run, so don't be afraid of them. Heading out on an empty stomach won't help you burn more fat. Instead, your body is more likely to break down muscle for energy. Women in particular should avoid skipping pre-run fuel.
Stick to simple carbs that are low in fat, fiber, and protein, and time your snack with this rule of thumb: you digest roughly 1g of carbs per minute.
Time before your run
Carb target
Easy options
1–2 hours
45–60g
Bagel with butter, two slices of toast, toaster waffle with syrup
Under 1 hour
Smaller snack
Fruit snacks, granola bar, Honey Stinger waffle, graham crackers
When you have time to digest, aim for at least 40g, and more before long or hard sessions. Drink 8–12 oz of fluid when you wake up and again before you head out, ideally with a pinch of salt or an electrolyte tablet.
🥤 Post-Run Fueling: Protein Needs a Partner
Aim for about 20g of protein plus 40–60g of carbs, roughly a 2:1 to 3:1 carb-to-protein ratio. A smoothie with Greek yogurt, fruit, and juice or a glass of chocolate milk both work well. If your appetite disappears after a hard effort, choose something cold and liquid; it cools you down and helps your appetite return. Lifting or doing yoga after your run? Refuel in between. 🥛
💧 Daily Hydration
Aim for about 2.5L per day for women or 3.5L for men, plus 8–16 oz for every hour of training. Coffee counts; alcohol and energy drinks don't.
You're well hydrated if you're peeing every 1–3 hours, your urine is pale yellow, and each trip lasts at least 10 seconds. Sip throughout the day rather than chugging, since full rehydration takes about 24 hours, and taper off about an hour before an evening run or bedtime.
🧂 Electrolytes: The Piece Most Runners Miss
Runners lose about 900mg of sodium per hour of training (it varies a lot by person), and most don't replace nearly enough.
When
Target
Every day during your training block
~1,000mg of extra sodium per day
During runs and races: sodium
300–600mg per hour
During runs and races: fluid
400–800mL per hour (most handheld bottles hold 400–600mL)
SaltStick is an easy option: each capsule has 215mg of sodium, and two chews have 100mg. Adding electrolytes or fruit to your water can also make it easier to drink more.
⛽ Fueling During Your Runs
Your glycogen stores start running low after about an hour, which is what leads to bonking. In a study of Seville Marathon runners, those taking in 60–90g of carbs per hour were far more likely to break three hours, but most runners averaged just 35g.
Train your gut like you train your legs. Start around 20g per hour and add 10g every 1–2 weeks, working toward 60–90g. Most gels and chews have 20–25g of carbs, so 60g an hour usually means 2–3 gels or a mix of gels, chews, and drink mix.
💡 Pro tip: Fuel by time, not miles. Your pace changes, but the clock doesn't. Set your watch to beep every 30 minutes, and take a few sips of water every mile.
Leah's picks: I rotate Maurten and Neversecond gels. Both are easy on my stomach, and Neversecond's C30 has 30g of carbs and 200mg of sodium, covering a big chunk of your hourly targets in one gel.
☕ Caffeine: Less Is More
Caffeine can give you a late-race boost, but never try it for the first time on race day. Start with about 50mg on a long run and increase gradually if your stomach handles it well.
Keep your total for the whole day under about 400mg, counting coffee, tea, pre-race caffeine, and gels. For example, one cup of coffee (~95mg) plus caffeinated gels near the start and at halfway (50–100mg each, depending on the brand) lands around 200–300mg. More won't make you faster, and too much can cause stomach issues and jitters.
🍚 Carb Loading & Race Week
Two to three days before your race, aim for 10–12g of carbs per kg of body weight per day. Done well, carb loading can improve performance by about 3%, or roughly five minutes for a three-hour marathoner. For a 60kg (132 lb) runner, that's 600–720g of carbs a day.
You don't need to force-feed yourself. Since you're tapering, most of the change comes from swapping, not adding: let carbs fill most of your plate, cut back a bit on fat, fiber, and protein, and add carb-rich snacks between meals. Easy options:
Bagels instead of bread, and extra honey or syrup on oatmeal
Juice or lemonade with meals
Dates, bananas, and sourdough toast
Candy, gummy bears, and soda, in moderation, for variety 🐻
Leah's favorite: microwaveable white rice packs, which are easy on the stomach and have 80–90g of carbs each.
Expect the scale to go up 2–4 pounds. Your body stores about 3g of water with every gram of glycogen, so it's a sign your tank is filling up, not fat gain.
✅ The Bottom Line
There's no single "best" gel, product, or diet. The best one is the one your stomach handles and you'll actually stick with.
Everyday eating and performance fueling aren't always the same. Whole foods with fiber, protein, and healthy fats are a great foundation, but marathon training calls for more carbs overall, especially quick, easy-to-digest carbs around your runs.
Practice your full plan on at least two or three long runs so race day feels familiar, and save anything new for after the finish line. If you're constantly tired, picking up nagging injuries, or stalling in training, you may not be eating enough. Start with one small change, like an extra slice of toast at breakfast or a chocolate milk after your long run.
Then trust your training, have fun, and celebrate making it to the start line. That alone is an accomplishment. 🎉
If this helped, check out my other blog posts, Youtube page, and follow along on Instagram at @leahdanarosenfeld for weekly advice and encouragement.
Connect with Maddie
🌐 Website: fuelingforward.com
📸 Instagram: @fueling_forward
🎙️ Podcast: The Fueling Forward Podcast
This article is for general education and isn't a substitute for individualized advice from a registered dietitian or doctor.
