Why Some Runners Eat Only Carbs Before a Long Run

You eat only carbs before a long run because they top off your muscle and liver glycogen, your fastest energy source. At marathon pace, up to 90% of your fuel comes from carbs, especially during efforts over 90 minutes. Aim for 1–2 grams per kilogram of body weight 1–2 hours before-like 70–140g for a 70 kg runner-using easy-to-digest options like white toast, banana, or a gel. This delays fatigue, spares muscle, and keeps your pace strong; see how your body responds with different carb types and timing to find your ideal prep.

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Notable Insights

• Carbs are the primary fuel source during intense endurance exercise, especially at marathon pace.
• Glycogen stores from carbohydrates provide quick energy and delay fatigue during long runs.
• Eating carbs before a run ensures muscles are primed with readily available energy.
• Runners aim to maximize glycogen levels to avoid “hitting the wall” after 90–120 minutes.
• Easily digested carbs minimize stomach issues while efficiently topping off energy stores.

Why Do Carbs Help You Run Longer?

When you’re prepping for a long run, loading up on carbs isn’t just habit-it’s science. Carbohydrates are your body’s go-to fuel source, stored as glycogen in your muscles and liver. These glycogen stores provide a dense, quickly accessible form of energy, delivering about 4 calories per gram. During a long run, your body uses glycogen first because it supports faster energy production than converting fat. At marathon pace, up to 90% of your energy comes from carbs. They deliver sustained energy, especially when you’re relying on digestible carbohydrates from gels or sports drinks. Once glycogen drops low-usually after 90–120 minutes-you risk hitting the wall. But consistently fueling with 30–60 grams of carbs per hour helps maintain blood glucose and delays fatigue, keeping your stride strong mile after mile.

How Many Carbs Do You Need Before a Run?

How much fuel do you really need before pounding the pavement? For peak performance, aim for 1–2 grams of carbohydrates per kilogram of body weight 1–2 hours before a run. That’s 70–140 grams for a 70 kg runner-think a large banana with white toast and honey. This range supports fueling strategies that preserve muscle glycogen and improve endurance. Long-distance runners often stick to easy-to-digest carbs like pretzels, sports drinks, or white bread to avoid discomfort. Avoid eating more than 2 grams per kilogram of body weight-higher amounts don’t boost performance and can upset your stomach. And if you’re eating less than an hour out, smaller portions work better. While complex carbohydrates are great for daily meals, refined, low-fiber options are smarter when you eat before a long run.

What & When to Eat for Pre-Run Energy?

Why does timing matter just as much as what’s on your plate before a run? Because your body needs time to convert carbohydrates into usable energy without causing digestive issues. For a pre-run meal 2–4 hours out, aim for 85–95% carbs-think whole grains and a moderate calorie intake to fully top off glycogen stores. If you’re eating 1–2 hours before, go for 1–2 grams of carbs per kg of body weight from easily digested, carb-rich sources. Refined carbohydrates like white bread or bananas work well, as they quickly boost blood glucose. Need a quick fix 30–60 minutes out? Try 20–30g of simple carbs-half a banana, a gel, or sports drinks-to maintain energy. These strategies keep your glycogen steady and your pace strong.

Who Should Skip High-Carb Pre-Run Meals?

Could your high-carb pre-run meal be doing more harm than good? Runners with gastrointestinal sensitivities, insulin resistance, or prediabetes-especially if you’re 30+-may see blood sugar spikes despite being training for a marathon. If you’re a low-mileage runner logging under 90-minute efforts, you likely don’t need heavy carbohydrates to top off energy stores. Heavy sweater or not, your exercise and how much glycogen you burn matters. Some athletes, like those on low-carb or keto diets, rely more on fats than carbohydrates and fats for fuel. If metabolic testing shows you’re a high fat oxidizer or low glycogen storer, skip the carb load. A registered dietitian can help-this dietitian can help tailor nutrition around medical conditions and real performance. Consult a registered dietitian to align fueling with your goals.

On a final note

You’ll run stronger when you fuel right, and carbs are your go-to for long runs-they fill your glycogen stores, delay fatigue, and keep energy steady. Aim for 1–4 grams of carbs per pound of body weight 1–4 hours pre-run, using easy-to-digest options like toast with honey, a banana, or Clif Bloks. Test what works in training; some thrive on higher carbs, others feel bloated. If you’re doing shorter or low-intensity runs, you might skip the heavy carb load. Always pair with hydration-sip 16–20 oz of water, and consider adding Nuun or Skratch Labs mix for electrolytes. Listen to your body, stick to proven foods, and avoid new snacks on race day.