Race Day Fuel Ready: Carb Loading Explained

What Is Carb Loading?

If you’re a runner, you’ve likely heard about carb loading. Maybe you’ve even tried it and wondered if it really helped. Carb loading, short for glycogen supercompensation, is the process of maximizing your body’s stored carbohydrates, known as glycogen, before an endurance event.

Carbohydrates from foods like bread, pasta, rice, and cereal are stored in your muscles and liver as glycogen. During long runs or races, your body relies heavily on these glycogen stores to maintain energy levels and delay fatigue. While fat also contributes to fuel, glycogen is your body’s most efficient energy source for endurance performance.

Why Carb Loading Matters for Runners

Carb loading is most beneficial for endurance events lasting longer than 90 minutes, such as half marathons and marathons. Research shows proper carb loading can:

• Delay the onset of muscle fatigue

• Improve performance by 2–3%

• Support longer durations of high-intensity effort

  
  

A well-planned carb load can increase glycogen stores by up to 1.6 times their normal level. However, this strategy only works if you’re already eating enough carbohydrates throughout your training. You can’t make up for weeks of under-fueling by trying to cram in enough carbs a few days before the race.

Daily Carbohydrate Needs During Training

To prepare your gut for effective carb loading, aim to meet your carbohydrate needs consistently throughout your training cycle. Here’s a general guide based on activity level:

Training Duration	Grams of Carbohydrate per kg Body Weight	Example for 150 lb (68 kg) Athlete
30 minutes/day	3–5 g/kg	205–340 g carbs/day
60 minutes/day	5–7 g/kg	340–475 g carbs/day
1–3 hours/day	6–10 g/kg	410–680 g carbs/day

These amounts help ensure your body is consistently stocked with glycogen, improving recovery and performance.

How Many Carbs to Eat for a Carb Load

Before a marathon, plan for 8–12 g of carbohydrate per kg of body weight each day for the 2–3 days leading up to the race. Before a half marathon, use the same range for 1–2 days.

For a 150 lb (68 kg) athlete, that’s about 545–820 grams of carbohydrate per day.

Start practicing carb loading during long-run weekends or any tune-up races throughout your training cycle. Going from a moderate-carb diet to a full load without practice can cause GI issues like bloating, discomfort and/or diarrhea (and as a runner, we say ‘no thank you’ to that!)

How to Carb Load Without Feeling Miserable

To hit your carb goals comfortably and effectively:

• Choose low-fiber carbs such as white rice, pasta, and white bread.

• Reduce fruits and vegetables as they take up space without contributing many carbs.

• Use simple sugars from honey, maple syrup, juice, or sports drinks to boost intake easily.

• Cut back on protein and fat so carbs can take center stage.

• Eat smaller, frequent meals (about 5–6 times per day) to avoid feeling overly full.

In short, carb loading isn’t about eating a giant pasta dinner the night before your race. It’s a strategic, multi-day approach that works best when you’ve consistently fueled your training with adequate carbohydrates. Done correctly, it can help you start your race feeling energized, confident, and ready to perform your best.