Key Takeaways
- Carbohydrate loading helps maximise your muscle glycogen (carbohydrate) stores before race day
- Most runners should start carb loading 2–3 days before the marathon
- Aim for 7–10g of carbohydrate per kg of body weight per day
- Focus on low-fibre, high-carb foods to avoid digestive (gut) issues
- Carb loading is about adjusting your plate composition, not just eating more
- Practising your strategy in training is essential for success on race day
What Is Carbohydrate Loading?
Carbohydrate loading is a nutrition strategy used by endurance athletes to maximise the amount of glycogen stored in the muscles before a long event like a marathon.
Glycogen is your body’s stored form of carbohydrate and acts as a key fuel source during prolonged exercise. When you’re running for several hours, your body relies heavily on these stores to maintain pace and performance.
Under normal conditions, your glycogen stores are limited. That’s why many runners experience the dreaded “wall” around the 30–35km mark. Carbohydrate loading helps delay this by ensuring your energy reserves are as full as possible before you even cross the start line.
In simple terms:
You’re “topping up the tank” so you don’t run out of fuel halfway through the race.
Why Carbohydrate Loading Is Important for Marathon Runners
Running a marathon places a huge demand on your body. Even the most efficient runners will burn through a significant amount of glycogen during the race.
Without adequate fuel:
- Fatigue sets in earlier
- Pace drops
- Concentration suffers
- The likelihood of hitting the wall increases
Carbohydrate loading is one of the most effective ways to support endurance performance and delay fatigue.
For example, a runner who starts the race with fully topped-up glycogen stores is far more likely to:
- Maintain a steady pace
- Feel stronger in the later stages
- Recover better afterwards
It’s not about running faster in the first 10km—it’s about still having energy left at 35km when it really counts.
When Should You Start Carb Loading Before a Marathon?
One of the most common questions I get is: “When do I actually start carb loading?”
For most runners, the ideal window is 2 to 3 days before race day.
This timing works because:
- Your training load is reduced (taper phase)
- Your body is primed to store glycogen more efficiently
- You’re not burning through fuel at the same rate
During this period, the focus shifts from training hard to fueling smart.
However, a few common mistakes to avoid:
- Starting too early (you’ll just return to baseline)
- Leaving it until the night before (too late to fully load)
- Drastically overeating instead of adjusting your macronutrient balance
Carb loading isn’t about eating everything in sight—it’s about strategically increasing your carbohydrate intake while reducing fat and fibre.
How Much Carbohydrate Do You Need?
The amount of carbohydrate you need depends on your body weight.
As a general guideline, aim for:
- 7–10 grams of carbohydrate per kilogram of body weight per day
This might sound like a lot—and it is—but remember, this is only for a short period before race day.
Here’s a practical breakdown:
| Body Weight | Carbohydrate Intake Per Day |
|---|---|
| 60 kg | 420–600g |
| 70 kg | 490–700g |
| 80 kg | 560–800g |
| 90 kg | 630–900g |
To make this more manageable, think about increasing the carbohydrate portion of your meals and snacks, rather than just adding extra food. You’ll be reducing protein a bit too, as well as fat and fibre, so hopefully it won’t be too overwhelming!
For example:
- Add an extra serving of rice, pasta, or potatoes
- Include carb-based snacks like toast, bagels, or cereal
- Use drinks (like fruit juice or smoothies) to help meet targets
What to Eat During Carbohydrate Loading
When it comes to carbohydrate loading, the goal is simple: increase your carbohydrate intake without overcomplicating things or upsetting your stomach.
This is not the time to experiment with new foods. Stick with what you know works for you, and focus on easy-to-digest, familiar carbohydrate sources.
Practical options include:
- Porridge with maple syrup and banana
- White or wholemeal toast with jam or marmalade
- Pasta, rice, and potatoes (although we tend to eat more carbs when we eat pasta or rice than potatoes)
- Bagels, wraps, and sandwiches
- Low-fibre cereals (e.g. cornflakes, Rice Krispies)
- Fruit juices and smoothies
- Yoghurt with added fruit or granola
To increase your carbohydrate intake without feeling overly full, try:
- Adding an extra carb portion to each meal
- Including 2–3 carbohydrate-based snacks daily
- Drinking some of your carbs (smoothies, milk, juice)
A Simple Sample Day of Carb Loading
Here’s what a typical high-carb day might look like during the final days before your marathon:
Breakfast:
Porridge with milk, banana, honey + glass of orange juice
Mid-morning snack:
Bagel with jam + yoghurt
Lunch:
Chicken sandwich on wholemeal bread + crisps + fruit
Afternoon snack:
Cereal with milk or a smoothie
Dinner:
Pasta with tomato-based sauce + small portion of protein
Evening snack:
Toast with honey or a cereal bar
The key is spreading your intake across the day so it feels manageable.
What to Avoid When Carb Loading
While increasing carbohydrates is important, there are a few things worth limiting to avoid discomfort on race day.
Firstly, be mindful of fibre intake. While fibre is generally beneficial for health, too much in the days before a marathon can lead to:
- Bloating
- Gastrointestinal discomfort
- More frequent toilet trips (not ideal on race morning)
Try to reduce:
- Very high-fibre cereals
- Large portions of vegetables
- Beans and pulses
- Flaxseed/ linseed or chia seeds
- Berries
Secondly, avoid introducing new or unfamiliar foods. Race week is not the time to test a new recipe or product—you want everything to feel predictable.
Finally, keep an eye on fat intake. High-fat foods can:
- Displace carbohydrates in your diet
- Slow digestion
- Leave you feeling overly full
This doesn’t mean eliminating fats entirely—just avoid letting them dominate your meals.
Hydration and Electrolytes
Carbohydrate loading isn’t just about food—hydration plays a key role too.
For every gram of glycogen stored, your body also stores water. That means as your carbohydrate intake increases, your hydration needs also rise slightly.
In practical terms:
- Sip fluids regularly throughout the day
- Include water, milk, and possibly electrolyte drinks
- Monitor your urine colour (pale yellow is ideal)
In Ireland, we don’t always associate racing with heat, but even mild dehydration can impact performance.
You don’t need to overdo it—just stay consistently well-hydrated in the lead-up to race day.
Common Carb Loading Mistakes (and How to Avoid Them)
Carbohydrate loading is simple in theory, but it’s easy to get wrong in practice.
One common mistake is starting too late. Eating a large pasta dinner the night before is not enough to fully top up glycogen stores. Think of carb loading as a 2–3 day process, not a single meal.
Another issue is not eating enough carbohydrates. Many runners increase food intake slightly but don’t actually hit the recommended carbohydrate levels. Being intentional about your choices makes a big difference.
On the flip side, some runners go too far and overeat everything, leading to discomfort and sluggishness. Remember, the goal is to shift your macronutrient balance, not just increase calories dramatically.
Finally, skipping practice is a big one. Your race-day strategy should never be a surprise. Use your long training runs to test:
- Foods
- Timing
- Portion sizes
That way, you head into race week with confidence.
How Carb Loading Fits Into Your Marathon Nutrition Plan
Carbohydrate loading is just one piece of the puzzle when it comes to marathon nutrition. To get the most out of it, it needs to align with your overall fuelling strategy before and during the race.
If you’ve been practising your nutrition during training, carb loading becomes a natural extension of what you’re already doing.
For example, during your long runs, you should already be testing:
- What you eat before running
- How you fuel mid-run
- How your stomach responds under pressure
If you haven’t dialled this in yet, I’d recommend booking in with one of the dietitians in the clinic.
When all of these elements come together—training, daily nutrition, race fuelling, and carb loading—you give yourself the best possible chance of performing well on race day.
Final Thoughts from Orla
Carbohydrate loading doesn’t need to be complicated—but it does need to be intentional.
The runners who benefit most from it are the ones who:
- Plan ahead
- Practise during training
- Keep things simple and consistent
There’s no need for extreme approaches or “last-minute fixes”. Focus on familiar foods, give your body what it needs, and trust the work you’ve already put in.
And remember—what you do in the final few days won’t replace your training, but it can make a real difference to how you feel in the final kilometres.
Frequently Asked Questions About Carbohydrate Loading for a Marathon
How many days before a marathon should I start carb loading?
Most runners should begin carbohydrate loading 2–3 days before race day. This allows enough time to maximise glycogen stores without feeling overly full or uncomfortable.
How many carbs should I eat when carb loading?
Aim for 7–10 grams of carbohydrate per kilogram of body weight per day. This range supports optimal glycogen storage for endurance performance.
Do I need to carb load for a half marathon?
For most recreational runners, full carbohydrate loading isn’t necessary for a half marathon. However, increasing carbohydrate intake the day before can still be beneficial.
What foods are best for carb loading before a marathon?
Focus on low-fibre, high-carbohydrate foods such as pasta, rice, potatoes, bread, cereals, fruit, and yoghurt. Stick with familiar foods to avoid digestive issues.
Should I avoid fibre completely before a marathon?
You don’t need to eliminate fibre entirely, but it’s wise to reduce high-fibre foods in the 1–2 days before the race to minimise the risk of stomach discomfort.
Can you carb load the night before a marathon?
A single high-carb meal the night before is not enough. Effective carb loading happens over several days, not just one dinner.
Will carb loading make me gain weight?
It’s normal to see a slight increase on the scales due to glycogen and water storage. This is temporary and actually beneficial for performance.
Should I drink more water when carb loading?
Yes, hydration is important. As glycogen stores increase, your body stores more water, so aim to stay consistently hydrated in the days leading up to the race.
What are common carb loading mistakes?
Common mistakes include starting too late, not eating enough carbohydrates, overeating fats, and trying new foods too close to race day.
Should I practise carb loading before race day?
Absolutely. Testing your carb loading approach during training helps you understand what works best for your body and reduces the risk of surprises on race week.
