Part of what makes a marathon so challenging is the chance of bonking. Your nutrition before and during the race can mitigate this risk and help you run strong and steady through the end. Carb loading is traditionally viewed as part of a marathon nutrition strategy. But does it effectively work? Learn how to carb load before a marathon – including some of the best foods for carb loading and a handy chart to help you calculate your carb goals.
Quick Tips for Effective Carb Loading
- Determine how many carbs you need each day.
- The recommended amount for an effective carb load is 8-10 grams of carbohydrate per kilogram of bodyweight per day of your carb load. One kilogram is 2.2 pounds. To calculate your daily carbohydrate needs for the carb load, multiply 8 x (your bodyweight in kg). For example, a 150 pound runner (68 kg) would calculate 8 x 68 = 544 grams of carbohydrate per day.
- A carb load should last 2-3 days before a marathon.
- Start increasing your carbohydrate intake three days before the race. If your marathon is on a Sunday, aim to start eating your goal carbohydrate intake on Thursday, Friday, and Saturday. The sample runner above would eat 544 grams of carbs each day for those three days.
- Spread the carbohydrates out throughout meals and snacks.
- Carb loading involves eating a lot. Spread those carbohydrates out equally throughout the day, aiming for three meals and two to three snacks.
- Opt for simple carbohydrates.
- Increase fluid intake.
- Glucose requires water to store as glycogen. Additionally, this will help ensure that you are hydrated going into the marathon.
- Liquid calories count.
- If you are struggling to consume a large amount of food, try liquid calories. Sports drinks and juices provide carbohydrates.
- Avoid high-fat foods.
What is Carb Loading?
The human body can only store enough glycogen for two hours to two and a half hours of running. The duration is even less when you are running at a higher intensity. (So if you are racing a marathon at a faster pace than you run in training, you are at a higher intensity.) One of the adaptations of endurance training is that your body learns to store more glycogen in the muscles. As you train for the marathon, you increase your body’s ability to store more glycogen. However, you need to adjust your diet properly to maximize that glycogen storage.
Carb loading does exactly that. Carb loading is also called “glycogen loading” or glycogen supercompensation.” It is a deliberate increase in dietary consumption of carbohydrates to increase glycogen stores before a race. For endurance events lasting longer than 90 minutes, carb loading ensures optimal glycogen stores. Typically, it is practiced by marathoners to avoid hitting the wall.
Why Should You Carb Load Before a Marathon?
Glycogen depletion is the reason that many runners hit the dreaded wall. Glycogen is the form that carbohydrates take when they are stored in the muscles for energy. Without enough carbohydrates available via glucose or glycogen, your body resorts to fatty acid oxidation to produce ATP (energy). However, this is a slower process that occurs at lower intensities. Thus, your pace slows down. Your body will also begin to catabolize the muscles in order to synthesize protein into glucose for energy.
Slowing down due to glycogen depletion is also one of your body’s protective mechanisms. If your muscle glycogen is fully depleted, your muscles would be damaged beyond repair. Additionally, hypoglycemia (too low of blood sugar) can cause neural damage. Thus, you slow down to protect your muscles and your central nervous system from damage. Not even the toughest-willed marathoner can overcome that level of fatigue.
You prevent glycogen depletion and optimize your performance by consuming carbohydrates (gels, chews, etc.) during the race. However, this is only part of the puzzle. Carb loading reduces the risk of glycogen depletion. A combination of proper carb loading and mid-race fueling form a cohesive fueling strategy – one that allows your training to express to its full potential on race day.
However, carb loading is not always how the media portrays it or some runners practice it. It certainly isn’t cramming down a heaping plate of fettuccine Alfredo before your race. Carb loading is the practice of eating more carbohydrates in the two to three days before your marathon in order to stock your muscles with their preferred energy source for the race.
What Research Says about Carb Loading
A majority of elite runners do increase the carbohydrate intake in the two to three days before a marathon. Most coaches advocate for a carb load. Why? Because it is effective in reducing your chance of hitting the wall.
Some runners look to fat adaptation as an alternative. However, no research to date indicates that fat adaptation improves performance in the marathon distance. Yes, you would have energy to run the marathon distance. Your speed would be compromised, though.
A study from the International Journal of Sports Medicine examined how carb loading affected runners at the 2009 marathon. This was not a controlled trial in a lab. Instead, this study looked at runners in the real world, thus offering some of the most salient conclusions on carb loading. Of the 250 marathons studied, the researchers found that those who ate more than 7 g of carbs per kg of bodyweight in the day before ran about 13.4% faster than their counterparts who did not carb load. 13.4% faster is a significant amount of time in the marathon!
Metabolic adaptations of endurance training make the carb load effective. During marathon training, your long runs burn up a significant amount of your glycogen (stored carbohydrates). As a result of this, your body adapts to increase its ability to store glycogen. By the time race day arrives, your body is able to store more glycogen than previously. Carb loading and the marathon taper combined allow you to take full advantage of this training adaptation on race day.
So in short, carb loading works. Yes, some runners may not need it – there are always variances in individual diet and physiology – but for a majority of runners, a smart carb loading strategy will help you avoid the bonk and optimize both performance and experience in your next marathon.
How to Carb Load Before a Marathon
To effectively carb load, you eat more carbohydrates in the two to three days before your race. How much is more? For the simplest calculation, carb loading is eating approximately 8-10 grams of carbohydrate per kg (2.2 pounds) of body weight per day, for 2-3 days before your race.
This table shows how many grams of carbs you should eat per day before your marathon, based on bodyweight. These carbohydrates should be spread out throughout multiple meals and snacks each day. Additionally, you should increase your fluid intake, as water is part of glycogen.
If you are unsure about a very high carb load total (>800 grams of carbs per day), you may be able to spread the carb load over longer periods. For example, instead of 900 grams of carbs over two days, you could try a four day carb load with 450 grams of carbs per day. (If in doubt, consult a registered dietitian.)
| Weight (lbs) | Weight (kg) | Grams of Carbs Per Day | |
| 99 | 45 | 360 | 450 |
| 110 | 50 | 400 | 500 |
| 121 | 55 | 440 | 550 |
| 132 | 60 | 480 | 600 |
| 143 | 65 | 520 | 650 |
| 154 | 70 | 560 | 700 |
| 165 | 75 | 600 | 750 |
| 176 | 80 | 640 | 800 |
| 187 | 85 | 680 | 850 |
| 198 | 90 | 720 | 900 |
| 209 | 95 | 760 | 950 |
| 220 | 100 | 800 | 1000 |
| 231 | 105 | 840 | 1050 |
| 242 | 110 | 880 | 1100 |
| 253 | 115 | 920 | 1150 |
| 264 | 120 | 960 | 1200 |
You do not have to download a macronutrient counting app and carefully count carbs. You can approach carb loading intuitively, by deliberately increasing your portions of carbohydrates and opting for higher carbohydrate foods. Adding in sports drinks or juice can help sneak in extra carbohydrates.
A change in diet can add unwanted effects, including sluggishness and gastrointestinal upset. Stick to foods that are familiar to you.
You may opt to reduce fibrous foods and eat easily digestible carbs the day before the race. Be careful not to eat too much fiber as you carb load! Too much fiber on the day before the race could lead to runner’s trots.
The Best Foods for Carb Loading
Bland, simple carbohydrates are your best choice. Stick with foods that are most familiar to you. If you have allergies, avoid any possible triggers.
- Breads and bagels
- Pretzels (hard and soft)
- White rice
- Potatoes (baked, boiled, roasted)
- Bananas
- Pasta and rice noodles
- Rice balls
- Graham crackers
- Fig bars
- Tortillas
- Oats
- Ginger ale
- Sport drink (Skratch hydration, etc)
Do You Need to Carb Load Before a Half Marathon?
If carb loading helps for a marathon, should you also carb load before a half marathon? The answer depends on your finish time. Broadly speaking, a 1-2 carb load could benefit those running a two hour half marathon or longer.
Six Common Carb Loading Mistakes to Avoid
How you carb load matters. Some common errors – including taking carb loading to an excess – may negate the performance benefits of carb loading and leave you feeling bloated and sluggish.
1. Over-eating
You are still fueling your body for performance. Yes, you want to eat more carbs than normal – but not so many that you feel bloated, sick, and sluggish. You are still fueling your body for performance. Save rich, high-fat treats for after the race. Your GI tract will thank you!
2. Eating only carbohydrates
You may be eating more carbs, but you shouldn’t eat only carbs. If you tend to feel sluggish or tired during a carb load, you may want to make sure you are adding protein or fat to your carbs. You may eat slightly less than normal, but they will still be present in your diet.
3. Deliberate depletion
The old-school approach of deliberately depleting carbohydrate stores before a carb load through means of a long workout and low carb diet is not necessary. This approach was based on studies in the 1960s done on sedentary individuals. Runners, on the other hand, partially deplete their carb stores repeatedly through training. The body adapts to increase its ability to store glycogen. Additionally, a sudden switch to a low-carb diet can render you moody and tired shortly before the race – not exactly how you want to feel.
4. Not drinking enough water
Your body requires water to aid in the storage of glycogen. Without drinking enough fluids, carb loading may not be as effective. Also, you may be slightly dehydrated going into your marathon. Neither are ideal!
5. Worrying about weight
Don’t step on the scale. You may gain some temporary water weight from carb loading. (As noted above, storing extra glycogen requires storing extra water in the muscles.) The benefits of carb-loading are well worth it! Just don’t psyche yourself out about your weight.
6. Not testing your carb load in training
You likely have heard the adage, nothing new on race day – and that includes a carb load! For athletes I coach, I encourage a 1-day trial of carb loading before their longest long run of marathon training. This trial carb load ensures they know what foods to eat and understand how they feel. If they do not respond well, we can troubleshoot before the race.
Carb loading is only part of the picture for achieving a marathon PR or avoiding the wall. Your mid-race nutrition, proper nutrition during training, and a sound training plan all will affect the outcome of your race. Carb loading alone won’t compensate for training errors. But for runners who have trained well, carb loading helps you reach the potential of your training on race day.
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