Fueling for Race Day
Carb loading - more formally called glycogen supercompensation - is a nutrition strategy designed to maximize muscle glycogen stores before endurance events. When done correctly, it can store 20-40% more glycogen than normal, providing extra fuel for the final miles of a marathon or the last hour of an Ironman.
The concept is simple: fill your tank before a long drive. But the execution trips up many athletes. Eating too much, the wrong foods, or at the wrong time leads to GI distress, bloating, and poor race performance. This guide shows you how to carb load effectively.
Not Just "Eat More Pasta"
Effective carb loading is strategic, not just stuffing yourself with bread. It involves specific timing, the right types of carbohydrates, reduced fiber, and careful attention to portion sizes. A well-executed carb load feels comfortable, not bloated.
The Science of Glycogen Storage
Glycogen is your body's stored form of carbohydrate, primarily kept in muscles and liver. During endurance exercise, your body draws on these glycogen stores alongside fat for energy. When glycogen runs low, you "bonk" or "hit the wall" - a dramatic drop in performance as your body is forced to rely more heavily on fat oxidation.
Glycogen Numbers
Normal Muscle Glycogen
300-400g
(~1200-1600 calories)
After Carb Loading
400-550g
(~1600-2200 calories)
A marathon at 7:00 pace burns roughly 100 calories per mile, with about 60% coming from carbohydrates (more at faster paces). That's ~1600 calories of carbs for a ~26.2 mile race. Without carb loading and mid-race fueling, you're cutting it close. With proper loading, you have a buffer that prevents the wall.
When to Carb Load
Events That Benefit from Carb Loading
Full Carb Load Recommended
- Marathon (26.2 miles)
- Ultra-marathon
- Ironman / Half-Ironman
- Century rides (100+ miles)
- Any event over 90+ minutes at high intensity
Modified/Light Loading
- Half marathon - focus on night-before dinner
- Olympic triathlon - good dinner, proper breakfast
- 10K race - normal eating, quality breakfast
- Events 60-90 minutes
The 90-Minute Rule
Events under 90 minutes don't typically deplete glycogen enough to benefit from multi-day carb loading. Your normal glycogen stores are sufficient. Focus instead on a quality pre-race meal and adequate hydration.
How Much to Eat
Carb Loading Targets
Daily carbohydrate intake during loading phase:
8-12g per kg body weight
per day, for 2-3 days before the event
Examples by Body Weight
| Weight | Low End (8g/kg) | High End (12g/kg) |
|---|---|---|
| 55 kg (121 lbs) | 440g carbs/day | 660g carbs/day |
| 65 kg (143 lbs) | 520g carbs/day | 780g carbs/day |
| 75 kg (165 lbs) | 600g carbs/day | 900g carbs/day |
| 85 kg (187 lbs) | 680g carbs/day | 1020g carbs/day |
This is a LOT of carbohydrates - significantly more than most people normally eat. It requires eating frequently, choosing carb-dense foods, and temporarily reducing fat and protein intake to make room for carbs.
The Modern Carb Loading Protocol
Forget the old 7-day depletion-then-loading protocol. Modern research shows that 2-3 days of high carbohydrate intake combined with training taper effectively maximizes glycogen stores without the exhaustion and misery of the depletion phase.
Race Week Schedule (Sunday Marathon)
Best Foods for Carb Loading
Excellent Choices
- White rice
- White pasta
- White bread / bagels
- Pancakes / waffles
- Potatoes (without skin)
- Pretzels
- Fruit juice
- Honey
- Sports drinks
- Low-fiber cereals
Avoid During Loading
- High-fiber whole grains
- Beans and legumes
- Raw vegetables
- High-fat foods
- Spicy foods
- New or unfamiliar foods
- Alcohol
- Excessive dairy
- Gas-producing foods
- Cruciferous vegetables
The Low-Fiber Key
During carb loading, temporarily switch from "healthy" whole grains to refined carbohydrates. Less fiber means less bulk in your GI tract on race morning, faster digestion, and lower risk of mid-race bathroom stops. Resume your normal fiber intake after the race.
Sample Carb Loading Meal Plan
This sample plan targets approximately 600g of carbohydrates for a 70kg athlete. Adjust portions based on your body weight and targets.
Sample Loading Day
Breakfast
2 cups white rice with honey, 2 pieces white toast with jam, 12oz orange juice
~150g carbs
Mid-Morning Snack
Large bagel with honey, banana, sports drink
~100g carbs
Lunch
Large plate of pasta with marinara sauce, white bread, apple juice
~130g carbs
Afternoon Snack
Pretzels, dried fruit, another sports drink
~80g carbs
Dinner
Rice with lean protein, white bread, baked potato, fruit cup
~120g carbs
Evening Snack
Low-fiber cereal with milk, toast with jam
~50g carbs
Total: ~630g carbohydrates
Common Carb Loading Mistakes
1. Starting Too Late
Eating a big pasta dinner the night before isn't carb loading. You need 2-3 days of elevated intake to maximize glycogen storage. Plan ahead.
2. Eating Too Much Fat
Fettuccine Alfredo is mostly fat calories, not carb calories. Choose tomato-based sauces over cream sauces. Fat fills you up before you've eaten enough carbs.
3. Too Much Fiber
Whole grains, beans, and vegetables during loading creates GI bulk and discomfort on race morning. Switch to low-fiber refined carbs temporarily.
4. Trying New Foods
Race week is not the time for culinary experimentation. Stick to familiar foods you know your stomach tolerates well.
5. Skimping on the Night-Before Dinner
Worried about feeling heavy? A proper dinner the night before is crucial. Eat until satisfied. You'll burn it all the next day.
6. Ignoring Hydration
Glycogen storage requires water. Drink plenty of fluids with your carbs. Sports drinks do double duty - carbs and hydration.
Frequently Asked Questions
How many carbs should I eat when carb loading?
Aim for 8-12 grams of carbohydrates per kilogram of body weight per day for 1-3 days before your event. For a 70kg runner, that's 560-840g of carbs per day. This typically requires eating more frequently and choosing carb-dense foods.
When should I start carb loading?
Modern carb loading protocols start 2-3 days before the event. The old 6-7 day depletion-then-loading protocol is outdated and unnecessary. Most athletes benefit from 48-72 hours of increased carbohydrate intake while tapering training.
Will carb loading make me gain weight?
Yes, temporarily. Each gram of stored glycogen holds about 3 grams of water. Effective carb loading may add 2-4 pounds of water weight. This is normal and beneficial - it's extra fuel and hydration. The weight disappears during your event.
What are the best foods for carb loading?
Choose easily digestible, low-fiber carbohydrates: white rice, pasta, bread, bagels, potatoes (without skin), pancakes, pretzels, sports drinks, and fruit juice. Avoid high-fiber foods, excessive fat, and new foods.
Do I need to carb load for a half marathon?
For half marathons, a full carb loading protocol isn't necessary but eating well the day before helps. Events under 90 minutes rely primarily on existing glycogen stores. Focus on a good dinner and proper pre-race breakfast rather than multi-day loading.
Key Takeaways
- Start 2-3 days before - not just the night before
- Target 8-12g/kg body weight of carbs per day
- Choose low-fiber refined carbs - rice, pasta, white bread
- Reduce fat and fiber - make room for carbs
- Expect 2-4 lbs water weight gain - this is good!
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