Marathon Carb Loading Guide

A simple 3-day protocol to top up muscle glycogen before a marathon or any race longer than 90 minutes — without bloating, GI distress, or guesswork.

Why carb load

Trained muscles can store roughly 500–700 g of glycogen when deliberately loaded, compared with ~400 g on a normal mixed diet. That extra fuel is the difference between a strong final 10 km and a hard wall around 30 km.

How many carbs (g/kg of body weight)

• Race minus 3 days: 7 g/kg — start shifting the plate towards carbs.
• Race minus 2 days: 9 g/kg — the main loading day.
• Race minus 1 day: 10–12 g/kg — biggest carb day, lower fat & fibre.
• Race morning: 1–4 g/kg, 3 hours before the start.

Example: a 70 kg runner targets ~630 g of carbs the day before — about 2,500 kcal from carbohydrate alone.

What to eat

Favour easy-to-digest, low-fibre, familiar carbs:

• White rice, white pasta, white bread, bagels
• Potatoes (peeled), sweet potatoes in moderation
• Pancakes, oatmeal (small portion), low-fibre cereal
• Bananas, ripe fruit, fruit juice, sports drinks
• Honey, jam, maple syrup, dates
• Lean protein in small amounts: eggs, chicken breast, white fish

What to avoid the last 48 hours

• High-fibre foods: bran, legumes, raw cruciferous veg, whole grains
• Heavy fats: cream sauces, fried food, lots of cheese, nuts in volume
• Alcohol — impairs glycogen synthesis and hydration
• New foods, very spicy meals, or large restaurant portions late at night

A sample day-before plate

• Breakfast: bagel + jam + banana + orange juice (~140 g carbs)
• Snack: rice cakes with honey, sports drink (~60 g)
• Lunch: white pasta with tomato sauce, small chicken breast, white bread (~180 g)
• Snack: pancakes with maple syrup, banana (~100 g)
• Dinner (early, by 7 pm): white rice with mild chicken or fish, peeled potatoes (~150 g)
• Evening: toast with honey or a small bowl of low-fibre cereal (~50 g)

Hydration & sodium

Every gram of stored glycogen pulls ~3 g of water with it, so you'll feel slightly heavier — that's normal and a sign loading is working. Sip fluids steadily and add sodium (electrolyte drink, salted snacks) the day before, especially if the forecast is warm.

Common mistakes

• Eating more of everything. Loading is more carbs, not more fat or protein.
• Trying something new. Race week is not the time for a new pasta sauce.
• Late, heavy dinner. Finish the big plate by ~7 pm so you sleep well.
• Panicking about weight gain. 1–2 kg of water is the goal, not a problem.
• Skipping breakfast on race morning. Top up with 1–4 g/kg, 3 h before start.

Race-morning fueling

Eat a familiar, mostly-carb breakfast 3 hours before the gun: oatmeal with banana and honey, a bagel with jam, or toast with peanut butter (if tested in training). Sip 400–600 ml of fluid with electrolytes in the 2 hours before the start, then a final small gel or sports drink 15 minutes before.