Do You Need Carbs When Training for a Marathon?

December 20, 2025 •

3 min read

Research shows endurance athletes who restrict carbohydrates often experience diminished performance, slower recovery, and decreased focus. So, do you need carbs when training for a marathon? The short answer is yes, they are a crucial fuel source for long-distance runners.

Quick Summary

Carbohydrates are the primary fuel for endurance running, providing energy to sustain intensity and prevent glycogen depletion. Strategic carb intake is essential for optimizing performance and recovery during marathon training.

Key Points

Glycogen is Your Fuel: Carbs are stored as glycogen, the primary energy source that powers your endurance workouts and race day performance.
Timing is Crucial: Strategic carb intake before, during, and after your runs maximizes performance, aids recovery, and prevents energy crashes.
Practice Your Fueling Plan: Always test new gels, chews, or fuel timings during training, not on race day, to ensure gastrointestinal tolerance.
Carb-Loading is Essential: For events over 90 minutes, a 2-3 day carb load before the race is necessary to fully maximize glycogen stores.
Match Carbs to Training: Adjust your daily carbohydrate intake to match your training load, eating more on intense or long run days.
Low-Carb is Risky: Relying on very low-carb diets during marathon training can compromise high-intensity performance and slow recovery.

The Science of Carbs and Marathon Performance

Carbohydrates are the body's primary and most efficient energy source for distance runners. They are converted to glucose and stored in muscles and the liver as glycogen. Glycogen powers endurance workouts and race performance. While fat is also used, it's less efficient, especially at higher intensities. Depleted glycogen stores lead to the dreaded "wall," causing fatigue as the body relies on less efficient fat metabolism. The total carbs burned in a marathon exceed storage capacity, highlighting the need for fueling before, during, and after.

Fueling During Marathon Training

Carbohydrate intake during training should align with workout volume and intensity, a strategy called carbohydrate periodization. Higher intake is needed on heavy training days to replenish stores and support recovery, while lower amounts suffice on easy or rest days.

Before the Run

The timing and duration of your run dictate pre-run fueling. A carb-rich meal 3-4 hours before long runs (over 90 minutes) is recommended, while a smaller, easily digestible snack 30-60 minutes before is suitable for shorter or early morning runs.

During the Run

For runs over 90 minutes, consuming 30-60 grams of carbohydrates per hour is vital, potentially higher for elite athletes. Fuel sources include gels, sports drinks, chews, bananas, and dried fruit. Practice during training is key to avoid stomach issues on race day. Combining glucose and fructose may improve absorption.

Post-Run Recovery

Consuming carbs and protein within 30-60 minutes after long or intense runs aids recovery. Carbs replenish glycogen, and protein repairs muscles. A 3:1 carb-to-protein ratio is common.

The Marathon Taper and Carb Loading

The tapering period before a marathon is ideal for carb loading, a strategy to maximize glycogen stores. This involves increasing carb intake to 7-12g per kg of body weight daily for 2-3 days, choosing lower-fiber carbs, and staying well-hydrated.

Low-Carb Running: The Risks and Reality

While some explore "training low," sufficient carbohydrates are crucial for optimal marathon performance. Very low-carb diets like keto are generally not recommended as they can hinder high-intensity efforts, increase perceived exertion, and slow recovery. The benefits of fat adaptation from low-carb training are often outweighed by the reduced capacity for high-intensity work.

Comparison of Carb Strategies for Marathon Training


Phase	Goal	Carb Type Focus	Timing	Example Fuel Source
Daily Training	Replenish glycogen, prepare for next session	Nutrient-dense complex carbs, fiber	Scaled to training load	Whole grains, fruits, starchy vegetables
Long Run (>90 min)	Maintain energy levels, prevent hitting the wall	Simple, quick-digesting carbs	30-60g/hr during run	Energy gels, sports drinks, chews
Post-Run Recovery	Replenish glycogen, repair muscle	High GI carbs and protein	Within 30-60 min post-run	Smoothie, chocolate milk, bagel with peanut butter
Carb Loading (Pre-race)	Maximize glycogen stores for race day	High carb, lower fiber	2-3 days before race	White rice, pasta, pancakes

Conclusion

Carbohydrates are essential for marathon training, supporting recovery, adaptation, and race-day performance. A periodized and personalized fueling plan that includes daily nutrition, in-run fueling, and pre-race carb loading is key to having the energy needed to train effectively and finish strong.

Science In Sport Marathon Fuelling Guide

Frequently Asked Questions

If you don't consume enough carbohydrates, your body will deplete its glycogen stores during the race. This leads to severe fatigue, a significant drop in performance, and the experience known as 'hitting the wall'.

Daily carbohydrate needs should be scaled to your training intensity. Recommendations range from 6 to 10 grams per kilogram of body weight, with higher amounts on heavy training days to ensure adequate fuel and recovery.

For shorter runs, a small, easily digestible carb source like a banana or sports drink 30-60 minutes prior is sufficient. For long runs, it's best to have a carb-rich meal 3-4 hours before, along with a carb-rich dinner the night before.

A combination of complex carbs (whole grains, oats) for sustained energy in daily meals, and simple, quick-digesting carbs (gels, sports drinks, bananas) for use during and immediately after runs, is most effective.

During your long training runs, experiment with different products (gels, chews, drinks) and timings. This helps you determine what works best for your digestive system and performance on race day.

Carb-loading maximizes your glycogen stores but won't directly increase your speed. Instead, it provides a larger energy reserve, allowing you to sustain your optimal pace for a longer period and delay fatigue.

Chronic low-carb availability can compromise high-intensity performance, slow recovery, and impair training adaptations for endurance athletes, as carbs are the most efficient fuel source.

Consume a combination of carbohydrates and protein (e.g., in a 3:1 ratio) within 30-60 minutes of finishing. This helps to kick-start glycogen replenishment and muscle repair.