Marathon Carb Loading Guide
How to load glycogen in the days before your marathon — four evidence-based protocols, a food database, sample meal plans, and the practical tips that make it work. No depletion phase required.
Quick summary
- →Start carb loading 2-5 days before the race depending on your protocol.
- →Target 7-12 g of carbs per kg of body weight on heavy loading days.
- →No depletion phase needed — modern research has retired this approach.
- →White rice, pasta, bagels, and potatoes are your best friends this week.
- →Liquid carbs (juice, sports drinks, oat milk) help hit targets without the volume.
- →Expect 2-4 lbs of water weight gain — that means it's working.
- →Eat 5-6 smaller meals instead of 3 large ones.
- →Race morning: 2 g/kg pre-race meal, 3-4 hours before the gun.
How much do you need to eat?
That's roughly .
Why Carb Loading Works
Your muscles store glycogen — their primary fuel at marathon intensity. On a normal diet, a trained runner stores about 300-400g of muscle glycogen, enough for roughly 90-120 minutes of running. A marathon takes most people 3-5 hours. The math doesn't work without loading.
Carb loading — systematically eating more carbohydrate than normal in the days before the race — can push muscle glycogen stores to 500-600g. Combined with in-race fueling, that's the difference between hitting the wall at mile 18 and maintaining pace through 26.2.
The depletion myth
The original carb loading protocol from the 1960s (Bergström & Hultman, 1967) included a 3-day depletion phase — exhaustive exercise followed by a low-carb diet — before the loading phase. Modern research has conclusively shown this is unnecessary. Sherman et al. (1981) demonstrated that a high-carb diet alone, without depletion, achieves near-maximal glycogen stores. The depletion phase increases injury risk, causes fatigue and mood disruption, and suppresses the immune system — exactly what you don't want in race week. Every protocol in this guide is loading-only.
Four Loading Protocols
All four protocols are evidence-based and effective. The right one depends on your experience with loading, how your gut handles high carb volumes, and how much time you want to dedicate. When in doubt, start with the Modern 3-Day.
3 loading days + race morning · Best for: Most runners (recommended)
The most widely recommended protocol in current sports nutrition literature. Start at ~7 g/kg three days out to begin elevating glycogen stores without GI stress, then jump to 10 g/kg for two full days of supercompensation. The taper from training is essential — reduced activity lets muscles absorb and store the extra carbohydrate.
Source: Burke & Hawley (2011)
2 loading days + race morning · Best for: Experienced loaders, time-constrained
Well-trained athletes can achieve full glycogen supercompensation in just 24-48 hours at 10-12 g/kg without a prior depletion phase. This is the most aggressive evidence-based protocol — it minimizes the number of days you need to eat uncomfortably large carb volumes, but requires very high daily totals. Not recommended for first-time loaders.
Source: Bussau et al. (2002)
4 loading days + race morning · Best for: First-timers, GI-sensitive runners
Spreads loading over a longer window with a gradual ramp from 5 to 8 g/kg, never exceeding 8 g/kg on any single day. Total glycogen stored is slightly lower than the 3-day protocol, but GI comfort is significantly better. Ideal for runners who've never carb-loaded before or anyone prone to bloating.
Source: Thomas, Erdman & Burke (2016)
5 loading days + race morning · Best for: Most conservative, longest ramp
The longest modern protocol. Starts near normal carb intake (~5 g/kg) and incrementally raises targets by 1-2 g/kg per day, peaking at 10 g/kg the day before the race. The gradual ramp gives the gut time to adjust to higher carb volumes and is the most forgiving protocol for runners who struggle with large meals.
Source: Hawley et al. (1997)
Calculate Your Carb Targets
The g/kg targets above are abstract until you convert them to actual grams for your body. Enter your weight to see what each strategy demands on peak loading days.
Carb Loading Calculator
Enter your body weight to see how many grams of carbs you need on peak loading days for each strategy. Hover over bars to see per-day targets.
0g of carbs looks like
Based on Modern 3-Day peak (10 g/kg). Standard diet staples shown.
Want the full day-by-day breakdown? A racecast.io premium dossier generates an interactive loading plan for your specific race — with gram targets for every day, meal combos scaled to your weight, dietary preference filters (vegan, GF), and macro context. Pick a strategy, tap a day, see exactly what to eat.
Find your race →High-Carb Food Database
These are the foods that make carb loading practical. All values are per standard serving. Focus on white/refined versions during loading — they're easier to digest in volume.
Grains & Starches
Fruits
Drinks & Liquids
Snacks
All items above are vegetarian. Items without wheat (rice, potatoes, quinoa, fruits, drinks) are also gluten-free and vegan-friendly.
Sample Loading Days
Here are two realistic meal plans for a heavy loading day. Quantities shown are for a ~76 kg (168 lb) runner at 10 g/kg — scale up or down proportionally for your weight.
~760g carbs (for 76 kg runner at 10 g/kg)
~740g carbs (for 76 kg runner at 10 g/kg)
What About Protein and Fat?
During loading, your macro split shifts heavily toward carbs: roughly 70% carbs, 15% protein, 15% fat. That's not a permanent diet — it's a 2-5 day strategy to maximize glycogen.
You still need protein (1.2-1.6 g/kg) for muscle maintenance, but don't let it crowd out carbohydrate. Keep fat low because it's calorie-dense (9 cal/g vs 4 cal/g for carbs) and slows gastric emptying — the opposite of what you want when eating 700g+ of carbs.
Approximate daily calories during heavy loading
At 10 g/kg. Based on 4 cal/g carbohydrate. Protein/fat estimated at ~15% each.
Practical Tips
Reduce fiber & fat
Swap whole grains for white versions. Less fiber = more room for carbs without feeling stuffed. Cut back on high-fat foods — they slow gastric emptying and take up caloric space you need for carbohydrate.
Liquid carbs count
Sports drinks, juice, smoothies, and oat milk help hit targets without the volume of solid food. If you're struggling to eat enough, drink more of your carbs.
White > whole grain
White bread, white rice, and white pasta are easier to digest and pack more carbs per volume during loading. Save the whole grains for after the race.
Water weight is good
Expect to gain 2-4 lbs during loading. Every gram of glycogen binds ~3g of water. That weight gain means your fuel tank is full — don't panic and cut carbs.
Spread it out across 5-6 meals
Eating 700-900g of carbs in three meals is miserable. Graze throughout the day — 5-6 smaller meals plus snacks makes the volume manageable.
Stick to foods you know
Race week is not the time to try that new pasta place. Stick to foods you've eaten in training. Your gut should already know how to handle everything on your plate.
Managing GI Stress During Loading
GI distress during carb loading is common and preventable. Research from the 2023 Boston Marathon found that intestinal injury markers varied 33x between runners — meaning gut tolerance is highly individual. The good news: most GI problems during loading come from the same few mistakes.
Switch to low-fiber, refined carbs
Fiber is the #1 cause of bloating during loading. White rice, white pasta, white bread, and peeled potatoes are your best friends. Save the whole grains, legumes, and raw vegetables for after the race. Aim for under 20g of fiber on heavy loading days.
Lean on liquid carbs
Sports drinks, juice, smoothies, and oat milk deliver carbs without the volume or GI stress of solid food. If you're struggling to hit 700g+ in solid food, shifting 30-40% of your carbs to liquids makes the targets manageable — and your gut will thank you.
Consider low-FODMAP in the final days
Research shows 70% of runners benefit from reducing high-FODMAP foods (onions, garlic, wheat, certain fruits, dairy) in the 3-6 days before racing. This reduces fermentable carbohydrates that cause gas and bloating. Low-FODMAP doesn't mean low-carb — white rice, potatoes, bananas, and oat milk are all low-FODMAP and high-carb.
Your gut training history matters
If you've consistently practiced fueling at 60-90g/hour during long training runs, your gut has adapted — SGLT1 transporter density increases with repeated exposure. You can handle more aggressive loading. If you haven't practiced race-day fueling, start with the Conservative 4-Day protocol and emphasize liquid carbs.
Heat changes everything
Racing in warm conditions (70°F+) significantly increases GI distress risk. Heat redirects blood flow from the gut to the skin for cooling, slowing digestion and absorption. If your race forecast is warm, shift even more aggressively toward liquid carbs in the final 24 hours, minimize fiber, and prioritize hydration during loading — not just on race day.
The Weight Gain Question
You will gain 2-4 lbs during carb loading. This is not fat. This is your fuel tank filling up.
Every gram of glycogen binds approximately 3 grams of water in muscle tissue. A proper loading protocol that adds 400-600g of glycogen above baseline will add 1.5-3 lbs of glycogen + water weight. This is the entire point of loading — those are the carbohydrates that will fuel miles 18-26.
During loading
+2 to 4 lbs
Glycogen + bound water. Scale goes up. This means it's working.
During the race
Burns off by mile 18-20
Glycogen is consumed as fuel. The water is released and excreted as sweat.
Don't cut back
The most common loading mistake is panicking at the scale and reducing carb intake on the final day. Research from the 2023 Boston Marathon study found that runners with lower pre-race caloric intake actually had worse GI outcomes — not better. Trust the process. The weight comes off during the race.
Race morning: the final top-off
Every protocol ends with a 2 g/kg pre-race meal 3-4 hours before the gun. This tops off liver glycogen (which depletes overnight) without overfilling the stomach. Stick to familiar, low-fiber foods — a bagel with honey, white rice, a banana, and sports drink is a classic combination. For a complete race-morning timing guide, see the Marathon Fueling Guide.
Get a plan built for your race
Race weather, your body, and your gut change everything
This guide gives you the science. A racecast.io premium dossier builds you a personalized loading plan for your specific race — adjusted for race-day temperature, your GI risk profile, body composition, and experience level. Day-by-day timeline with weather-adjusted coaching, fiber warnings, scaled meal combos by dietary preference, and an expected weight gain prediction. The plan a sports dietitian would build, automated for your race.
Find your race →Research Sources
Burke & Hawley (2011) — Carbohydrates for training and competition. Journal of Sports Sciences 29(S1):S17–S27.
Primary reference for modern 3-day loading protocol and g/kg targets.
Bussau et al. (2002) — Carbohydrate loading in human muscle: an improved 1 day protocol. European Journal of Applied Physiology 87(3):290–295.
Demonstrated full supercompensation in 24-48 hours at 10-12 g/kg without depletion.
Thomas, Erdman & Burke (2016) — ACSM Joint Position Statement: Nutrition and Athletic Performance. Medicine & Science in Sports & Exercise 48:543–568.
Current ACSM position on pre-competition glycogen loading targets.
Hawley et al. (1997) — Carbohydrate-loading and exercise performance: an update. Sports Medicine 24(2):73–81.
Foundation for gradual loading protocols. Early evidence that depletion is unnecessary.
Sherman et al. (1981) — Effect of exercise-diet manipulation on muscle glycogen and its subsequent utilization during performance. International Journal of Sports Medicine 2(2):114–118.
Showed high-carb diet alone (without depletion) achieves near-maximal glycogen stores.
Costa et al. (2017) — Gut-training: the impact of two weeks repetitive gut-challenge during exercise. European Journal of Applied Physiology 117(12):2483–2494.
2-week gut training reduced GI symptoms 60-63% and improved performance 4.3-5.2%.
Gaskell et al. (2025) — Minimizing gastrointestinal symptoms during endurance exercise. Journal of the International Society of Sports Nutrition (2025).
Low-FODMAP pre-race reduces symptoms in 70% of runners. Liquid carbs better tolerated than solids in heat.
Jeukendrup (2014) — A step towards personalized sports nutrition. Sports Medicine 44(Suppl 1):S25–S33.
Foundation for individualized carb targets. Gut capacity is trainable; one-size-fits-all plans are suboptimal.