Carb Loading: What Actually Works (and What Makes It Worse)

Carb loading before a marathon isn't about eating a massive pasta dinner the night before. Done wrong, it causes GI distress on race day. Done right, it can add miles to your glycogen stores.

By Alex · 7 min read

The night before my first marathon, I ate a plate of pasta so large the waiter raised an eyebrow. I woke up at 2am feeling like I'd swallowed a brick.

I'd been told to carb load. What nobody told me was how to actually do it.

Carb loading is real, it's effective, and it measurably improves performance in races over 90 minutes. It's also one of the most misunderstood things in recreational running. Here's what the science says and what I actually do now.

Why Carb Loading Works

Your muscles store glycogen — chains of glucose that your body burns as fuel during sustained effort. The problem is capacity: a well-trained runner can store roughly 90-120 minutes of glycogen at marathon race pace. After that, the body dips into fat for fuel, which burns slower. This is "the wall."

Carb loading extends your glycogen stores by training the muscles to hold more than their baseline capacity — provided you do it correctly. Studies consistently show it delays fatigue and improves time-trial performance by 2-3% in endurance events over 90 minutes.

A 2-3% improvement at a 4:00 marathon pace is about 5 minutes. Worth doing right.

The 3-Day Protocol (What Actually Works)

The most evidence-backed approach is a 3-day carb loading protocol starting 3 days before your race:

Days -3 and -2 (Thursday and Friday before a Sunday race):

  • Increase carbohydrate intake to 8-10g per kg of body weight per day
  • Reduce intensity and volume significantly (taper is already happening)
  • Prioritize easy-to-digest carbs: white rice, white pasta, bread, bananas, sports drinks

Day -1 (Saturday — the day before):

  • Same high-carb intake, but moderate the total volume
  • Avoid anything that causes you GI distress (no new foods, no high-fiber vegetables, no legumes)
  • Eat your largest meal at lunch, not dinner
  • Keep dinner smaller than you think you need

That last point is the one most runners get wrong. You don't load on race eve — you load Thursday and Friday. By Saturday evening, your muscles are already full. The dinner the night before is maintenance, not the main event.

How Much Is 8-10g per kg?

For a 70kg runner, that's 560-700g of carbohydrate per day. To put that in perspective:

  • 1 cup of cooked white rice ≈ 45g carbs
  • 1 large banana ≈ 27g carbs
  • 1 bagel ≈ 55g carbs
  • 1 cup of pasta ≈ 40g carbs
  • 1 sports drink (500ml) ≈ 30g carbs

To hit 600g, you're eating carbs at every meal, including snacks. It's a deliberate, structured process — not an excuse to eat a giant dinner.

What to Avoid

High-fiber foods. Broccoli, beans, brown rice, whole grain pasta — these cause gas and GI distress. Save them for after the race.

High-fat foods. Pizza, fried foods, heavy sauces — they slow digestion and take up space that carbs should occupy.

New foods. This is not the week to try the local speciality at the race expo dinner. Your gut knows what it knows. Stick to foods you've eaten before.

Alcohol. It disrupts sleep, dehydrates you, and impairs glycogen synthesis. Even one drink the night before a marathon is measurably bad.

What About Shorter Races?

For races under 90 minutes — 5K, 10K, most half marathons for recreational runners — carb loading provides minimal benefit. You won't run out of glycogen at those distances.

Instead, focus on race-morning nutrition: a carbohydrate-rich breakfast 2-3 hours before the start (oatmeal, toast with peanut butter, banana), and top-up carbs in the 30 minutes before the gun.

For a half marathon, whether carb loading helps depends on your finishing time. If you're running under 1:30, you probably don't need it. If you're 1:45 or slower, a light carb load over 1-2 days is worth doing.

The Race Morning Piece

Carb loading fills your glycogen tank. Race morning keeps it topped up and gives you energy for the starting hours.

My current protocol for a marathon:

  • Wake up 3.5 hours before start
  • Eat 80-100g of carbs (oatmeal with a banana, or a bagel with jam)
  • Drink 500ml of water
  • 60 minutes before: a small snack (half a banana, or a gel)
  • 15 minutes before: a gel or a few sports chews

This is just one approach. The most important rule: don't try anything new on race day. Practice your race morning nutrition during your long training runs.

The Bigger Principle

Carb loading doesn't replace in-race fueling. Even with perfect pre-race glycogen stores, you'll need carbs every 45-60 minutes during a marathon. The Race Fuel Planner calculates exactly how much you need based on your weight, pace, and race distance.

Load correctly before, fuel consistently during. That's how you finish a marathon feeling strong instead of crawling through the last 10K.