Marathon Carb Loading: The Complete Protocol

You have trained for months. You have racked up hundreds of kilometres, done your intervals, your long runs and tested the gels you will carry on race day. Your fitness is at its peak. But there is one factor that can ruin everything or turn your marathon into a memorable experience: what you eat in the three days before the start (World Athletics).

Carbohydrate loading before a marathon is neither a gimmick nor a trend. It is a nutritional strategy backed by decades of research that can boost your energy reserves by 30% to 50% above normal levels. That translates into more kilometres with fuel on board, a lower chance of hitting the dreaded wall and, in many cases, several minutes off your finishing time (Mayo Clinic) (WHO).

However, carb loading is one of the aspects that most runners get wrong. They eat too much fibre, try new foods, miscalculate the amounts or simply skip the protocol because they do not know how to do it. In this guide we explain step by step how to carry out an effective carbohydrate load, with exact amounts, a complete three-day menu and every mistake you should avoid. If you are interested in the full nutritional strategy for race day itself, including fueling during the 42 km, we also recommend our race day nutrition guide.

What is carb loading and why it works

To understand why carb loading works, you first need to understand how your body gets energy during a marathon. When you run, your body relies on two main fuel sources: fat and glycogen (the form in which your body stores carbohydrates in the muscles and liver). At marathon pace, your body depends primarily on glycogen because it is a much more efficient and faster fuel to use than fat.

The problem is that glycogen stores are limited. A runner on a normal diet stores between 300 and 400 grams of muscle glycogen, plus another 80-100 grams in the liver. That amount of glycogen provides energy for roughly 90 minutes of running at marathon intensity. Beyond that point, if you have not topped up with gels or food during the race, you start relying more and more on fat as fuel. And here is the catch: fat oxidation is a slower process that cannot sustain the pace you were holding. The result is what runners call "the wall": a sharp drop in performance, leaden legs, mental fog and the feeling that every kilometre lasts an eternity.

Carb loading exploits a physiological phenomenon called glycogen supercompensation. When you subject your body to a sustained high intake of carbohydrates over 2-3 days, your muscles respond by storing more glycogen than they normally would. Instead of the usual 300-400 grams, you can store between 500 and 600 grams, and even more in well-trained runners. That gives you an extra energy buffer that can make the difference between hitting the wall at km 30 and crossing the finish line with strength to spare.

The science behind this is solid. Studies from the 1960s through to recent research confirm that glycogen loading can improve performance in endurance events lasting over 90 minutes by 2% to 5%. In a 3:30 marathon, that can mean between 4 and 10 minutes of difference. It is not magic: it is applied biochemistry.

Of course, carb loading does not replace the need to take energy gels during the marathon. Even with full reserves, you will need to take in carbohydrates during the race. But starting with a full tank gives you an enormous advantage over someone who arrives at the start line with half-empty stores.

The classic vs the modern protocol

Carb loading has not always been done the way it is today. Understanding the evolution of the protocol will help you see why the modern method is more effective, more comfortable and carries fewer risks.

The classic 6-day protocol (Bergstrom, 1967)

The first glycogen loading protocol was developed by the Swedish physiologist Bergstrom in the 1960s. It consisted of two phases during the 6 days before competition:

1. Depletion phase (days 6 to 4): Intense exercise combined with a very low-carbohydrate diet (less than 10% of calories) for 3 days. The goal was to completely empty muscle glycogen stores.
2. Loading phase (days 3 to 1): A switch to a very high-carbohydrate diet (over 70% of calories) with reduced exercise. The body, starved of glycogen after the depletion phase, responded by storing more than usual.

This protocol worked, but it had serious drawbacks. The depletion phase caused extreme fatigue, irritability, poor concentration, increased injury risk and general malaise -- hardly ideal in the week of a marathon. Many runners arrived at the race mentally exhausted, even though their glycogen stores were full.

The modern 3-day protocol

Later research showed that the depletion phase is unnecessary. Studies by Sherman (1981) and Bussau (2002) proved that well-trained runners can reach maximum glycogen supercompensation levels simply by eating a high-carbohydrate diet (8-12 grams per kg of body weight per day) for 3 days while reducing training volume (tapering), without any prior depletion phase.

The modern protocol is much simpler and more tolerable:

• Days 6-4 before the marathon: Normal diet, training in taper mode (reduced volume). No drastic nutritional changes.
• Days 3-1 before the marathon: Increase carbohydrate intake to 8-12 g/kg/day. Reduce fibre, fat and protein proportionally. Continue tapering with very easy jogs.

The evidence shows that this 3-day protocol achieves results similar to or even better than the classic 6-day version, without the negative effects of the depletion phase. It is the approach currently recommended by most sports nutritionists and organisations such as the American College of Sports Medicine (ACSM) and World Athletics.

How many carbs to eat

The amount of carbohydrates you need during the load depends primarily on your body weight. The current scientific recommendation is to consume between 8 and 12 grams of carbohydrates per kilogram of body weight per day during the 3 days before the marathon. For most amateur runners, a range of 8-10 g/kg/day is sufficient. Elite runners or those seeking peak performance can aim closer to 12 g/kg/day.

Let us see what this means in real numbers:

Body weight	8 g/kg/day	10 g/kg/day	12 g/kg/day	Calories from carbs alone
55 kg	440 g	550 g	660 g	1,760 - 2,640 kcal
60 kg	480 g	600 g	720 g	1,920 - 2,880 kcal
65 kg	520 g	650 g	780 g	2,080 - 3,120 kcal
70 kg	560 g	700 g	840 g	2,240 - 3,360 kcal
75 kg	600 g	750 g	900 g	2,400 - 3,600 kcal
80 kg	640 g	800 g	960 g	2,560 - 3,840 kcal
85 kg	680 g	850 g	1,020 g	2,720 - 4,080 kcal

For a 70 kg runner aiming for 10 g/kg/day, that means consuming 700 grams of carbohydrates per day. In carbs alone that is 2,800 kcal (each gram of carbohydrate provides 4 kcal). Adding the minimum necessary fat and protein, total calorie intake can easily exceed 3,500 kcal per day. That sounds like a lot, and it genuinely is. That is why it is essential to choose foods that are high in carbs but low in volume and fibre, so it is physically possible to eat that much without feeling overly full (PubMed).

A very common mistake is thinking that eating a large plate of pasta the night before counts as a carb load. It does not work like that. A cooked portion of about 250 g of pasta provides roughly 75 g of carbohydrates. If you need 700 g of carbs per day, you would need the equivalent of nearly 10 plates of pasta in carbs alone. That is why the load must be carefully planned across 5-6 meals throughout the day.

To calculate your personal target, multiply your weight in kilos by 10 as a starting point (moderate protocol). If this is your first marathon, start at 8 g/kg and practise the load before a long run to see how it sits with you. You can use carb drink mixes like Maurten Drink Mix or SiS carb powder to hit your carb targets without having to eat solid food at every meal.

Which foods to choose

Not all carbohydrate-rich foods are equal for loading purposes. During the three days before the marathon, you need foods that tick these boxes: high carb content, low fibre, easy to digest and palatable enough to eat in large quantities without problems.

The best foods for carb loading

Food	Portion	Carbs (g)	Fibre (g)	Notes
Cooked white rice	300 g (1 large cup)	84	0.6	The king of carb loading. Low fibre, easy to digest
Cooked white pasta	300 g	90	2.4	The classic choice. Always white pasta, not wholemeal
White bread	100 g (4 slices)	50	2.7	Perfect for breakfast and between-meal snacks
Boiled potato	300 g (2 medium)	52	3.3	Very versatile. Fat-free mashed potato is ideal
Cooked sweet potato	300 g	60	4.5	Slightly more fibre than potato, but good profile
Instant oats	100 g (dry)	66	6	Fine/instant oats only, not steel-cut
Banana	1 large (120 g)	27	2.6	The perfect snack. Easy, tasty, quick
Honey	1 tablespoon (21 g)	17	0	Pure carbohydrate. Ideal for sweetening and adding extra carbs
Jam	1 tablespoon (20 g)	13	0.2	Similar to honey. No fibre, easy to add to toast
Fruit juice	250 ml	28	0.5	Liquid carbs. Does not fill the stomach
Sports drink powder	Per serving	30-40	0	Liquid way to add carbs without eating more solid food
Dates	3-4 pieces (80 g)	54	5.4	Fast natural sugars. Use in moderation due to fibre
Rice cakes	4 pieces (30 g)	24	0.3	Light snack, almost no fibre

Foods to avoid or limit

• Wholegrains, brown rice, wholemeal pasta: Too much fibre. Fibre slows digestion, causes bloating and can lead to gastrointestinal issues on race day. These 3 days are not the time to eat "healthy" in the wholegrain sense. Always choose refined versions.
• Legumes (lentils, chickpeas, beans): Although rich in carbs, they are high in fibre and cause gas. Avoid them in the 3 days before the race.
• Raw vegetables and large salads: High volume, lots of fibre, few net carbs. They will not help you hit your gram targets and take up stomach space.
• Fried food, creamy sauces, cream, hard cheese: Fat slows gastric emptying dramatically. If your pasta comes with a cream and cheese sauce, carb absorption is delayed and you may feel heavy.
• Spicy or heavily seasoned food: Can irritate the digestive system, which needs to be calm before the race.
• Foods you have never eaten before: The golden rule of marathon week also applies to loading: nothing new in the days beforehand.

Sample menu: 3 days before the marathon

This menu is designed for a 70 kg runner targeting a load of 10 g/kg/day = 700 g of carbohydrates per day. If you weigh more or less, adjust the amounts proportionally. The goal is to spread carbs across 5-6 meals to aid digestion.

Day 3 before the marathon (Thursday if racing Sunday)

Meal	Foods	Carbs (g)
Breakfast	100 g instant oats with 300 ml skimmed milk, 1 large banana, 2 tablespoons honey, 250 ml orange juice	155
Mid-morning	2 slices white toast with jam, 1 banana	80
Lunch	350 g cooked white pasta with tomato sauce (low fat), 1 slice white bread, 200 ml juice	155
Afternoon snack	4 rice cakes with honey, 1 banana, 500 ml sports drink	100
Dinner	300 g cooked white rice with grilled chicken (100 g), apple compote, 1 slice white bread	140
Before bed	Natural yogurt with honey and 4 dates	70
TOTAL		~700 g

Day 2 before the marathon (Friday)

Meal	Foods	Carbs (g)
Breakfast	4 slices white toast with jam and honey, 1 banana, 300 ml apple juice	145
Mid-morning	Smoothie: 250 ml skimmed milk + 1 banana + 2 tablespoons honey + 40 g instant oats	95
Lunch	300 g white rice with tomato sauce and 100 g chicken breast, 250 ml juice, 1 slice white bread	150
Afternoon snack	2 slices toast with jam, 500 ml sports drink, 1 banana	105
Dinner	350 g white pasta with tomato sauce, mashed potato (200 g), apple compote	155
Before bed	Rice cakes with honey, 200 ml juice	55
TOTAL		~705 g

Day 1 before the marathon (Saturday)

The day before the race is the most important and also requires the most care. Continue with the high-carb load but eat dinner earlier than usual (before 8:00 PM if possible) to allow time for full digestion.

Meal	Foods	Carbs (g)
Breakfast	100 g instant oats with skimmed milk, 2 tablespoons honey, 1 large banana, 250 ml juice	150
Mid-morning	2 slices white toast with jam, 500 ml sports drink	90
Lunch	350 g cooked white pasta with light tomato sauce, 1 slice white bread, apple compote, 200 ml juice	170
Afternoon snack	3 rice cakes with honey, 1 banana, 300 ml juice	85
Dinner (early)	300 g white rice with grilled chicken breast (100 g), 1 slice bread, mashed potato (150 g)	145
Before bed	Yogurt with honey, 3 dates	60
TOTAL		~700 g

Common carb loading mistakes

Carb loading seems simple in theory, but in practice there are many errors that can sabotage your marathon. These are the most frequent:

Eating too much fibre

This is the number one mistake. Many runners try to load with brown rice, wholemeal bread, high-fibre cereals, unpeeled fruit and big salads. The result: bloating, gas, heaviness and, in the worst case, gastrointestinal problems on race day. Fibre is excellent for everyday health, but in the 3 days before the marathon you need refined, easily digestible carbs. White pasta, white rice, white bread, honey, compote. It is the one time when eating refined is the best option.

Trying new foods

Marathon week is not the time to experiment. If you have never eaten couscous, if you want to try a new brand of energy bar or if someone has recommended a special dish for runners, save it for after the race. Eat only foods you already know your stomach tolerates well. The same applies to supplements, sports drinks and any new product.

Not eating enough carbohydrates

Many runners think they are carb loading because they ate a big plate of pasta the night before. But one plate of pasta provides about 75-90 g of carbs, and you need 560-840 g per day (for a 70 kg runner). You need carbohydrates at every meal and snack throughout the day, not just at dinner. If you do not calculate the amounts, you will almost certainly fall short. Use the food table and weigh your portions at least on the first day to calibrate.

Too much fat

Carbonara pasta, heavily buttered toast, chips, creamy sauces. Many runners add unnecessary fat while trying to load. Fat creates two problems: it slows digestion (leaving you full for hours and preventing you from eating more carbs) and it provides calories without contributing to glycogen stores. During the load, use light sauces (plain tomato, soy sauce), cook on the grill or in the oven, and limit oils, butter and cheese.

Getting dehydrated

Each gram of stored glycogen retains roughly 3 grams of water. If you store an extra 200 g of glycogen during the load, your body needs an additional 600 ml of water just for that storage. If you do not drink enough water during the 3 loading days, the supercompensation process is compromised. Moreover, dehydration before a marathon is one of the most dangerous risk factors for both performance and health (ACSM).

Loading without reducing training

The carb load must go hand in hand with tapering (reducing training volume). If you continue running long sessions or doing intense workouts while trying to load glycogen, you are burning what you are trying to store. The 3 loading days should coincide with very easy 20-30 minute jogs or complete rest. For more on general runner nutrition, see our runner nutrition guide.

Hydration during the load

Hydration during the carb load is just as important as the carbohydrates themselves. As we have already mentioned, glycogen is a molecule that is stored alongside water: for every gram of glycogen, your body retains roughly 3 grams of water. That means if your load is effective and you store an extra 200 grams of glycogen, your body needs to retain about 600 ml of additional water just for that process.

During the 3 loading days, your fluid intake should increase significantly:

• Total daily water: Aim for 3-4 litres per day, including water from meals and drinks. Sip steadily throughout the day rather than drinking large amounts at once.
• Sodium: Sodium is essential for fluid retention and electrolyte balance. Salt your meals normally (do not restrict salt these days) and consider adding an electrolyte tablet or half a teaspoon of salt to 500 ml of water once a day.
• Urine colour: Your urine should be pale yellow, almost clear. If it is dark yellow, you are not drinking enough.
• Sports drink powders: Using a sports drink powder like Maurten Drink Mix or SiS carb powder serves a double purpose: it hydrates you and adds extra liquid carbs.

A common mistake is drinking excessive water the night before the marathon. Over-hydration can cause hyponatraemia (low blood sodium levels) and will force you to get up several times during the night, ruining your rest. Drink regularly during the 3 days, but do not force it on the eve of the race.

Race day: pre-marathon breakfast

You have completed your 3-day load and it is marathon morning. The pre-race breakfast is the final piece of the nutritional puzzle. Its purpose is twofold: top up the liver glycogen stores (which will have partially depleted overnight during your fast) and provide one last easily accessible energy boost for the opening stages of the marathon.

When to eat breakfast

Breakfast should be 3 to 4 hours before the start. If your marathon starts at 9:00 AM, eat between 5:00 and 6:00 AM. Yes, it is early and means a very early alarm, but you need that window for digestion to complete and for the glucose to be available in your blood when you start running. Starting with digestion still under way is a sure recipe for gastrointestinal distress.

How much to eat

The goal is to consume between 100 and 150 grams of carbohydrates at the pre-marathon breakfast. That is roughly 400-600 kcal from carbs alone. The exact amount depends on your weight and what your stomach can handle at that hour: if you have never eaten breakfast that early, practise during your training weeks.

What to eat

As with the load, choose familiar foods that are low in fibre, low in fat and high in carbs. Here are three proven pre-marathon breakfast options:

What NOT to eat for your pre-marathon breakfast: eggs and bacon, wholemeal toast, high-fibre cereals, Greek yogurt with nuts, fruit with skin, coffee with whole milk. Anything that adds fat, excess protein or fibre will slow digestion and cause problems during the race.

Coffee is allowed (and recommended if you are a regular caffeine consumer), but take it black or with a splash of skimmed milk. The caffeine in coffee improves alertness and may have a small ergogenic effect. That said, if you do not normally drink coffee, marathon morning is not the time to start. To complete your in-race fueling strategy with gels, see our marathon energy gel guide.

Frequently asked questions

How many days before the marathon should I start carb loading?

The modern protocol recommends starting the carb load 3 days before the marathon. If you are racing on Sunday, that means Thursday through Saturday you should consume between 8 and 12 grams of carbohydrates per kg of body weight per day. The days before that (Monday to Wednesday), eat normally while reducing your training volume. You do not need the old depletion phase: science has shown that 3 days of loading are enough to maximise glycogen stores in trained runners.

Will I gain weight from carb loading?

It is completely normal to gain between 1 and 3 kg during the carb load, but it is not fat. Each gram of stored glycogen retains roughly 3 grams of water. If you store an extra 200 g of glycogen, your body retains 600 ml of additional water just for that process. Those extra kilos are literally your fuel for the 42 km and you will lose them during the race. If the scale goes up, do not worry: it is a positive sign that the load is working.

Can I carb load if I am coeliac or gluten intolerant?

Absolutely. There is a wide variety of carbohydrate-rich foods that contain no gluten. White rice is probably the best base for a gluten-free carb load: it is easy to digest, very low in fibre and provides plenty of carbs per serving. Other excellent options include potato, sweet potato, quinoa, certified gluten-free oats, fruits, honey, fruit juices and rice or corn pasta. The loading protocol works exactly the same with gluten-free foods.

Should I reduce fat and protein during the load?

Yes, proportionally. The goal is for carbohydrates to represent 70% to 80% of your total calorie intake during the 3 loading days. That means fat and protein should take a back seat, each accounting for 10-15%. You do not need to eliminate them completely: a bit of grilled chicken breast with your rice or a fat-free yogurt is fine. But creamy sauces, fried food, fatty cheeses and high-fat red meat should be avoided these 3 days because they take up caloric space you need for carbohydrates.

What happens if I do not carb load before the marathon?

Without the load, your muscle glycogen stores will be at normal levels: between 300 and 400 grams, enough for about 90 minutes of running at marathon intensity. Beyond that, the likelihood of hitting the wall increases dramatically, especially between km 28 and 35. You can partially compensate with a good energy gel strategy during the race, but starting with full reserves gives you an enormous safety margin. For any marathon that will take longer than 3 hours, carb loading is highly recommended. If you are interested in a complete training plan for the half marathon, check out our half marathon training guide.

Can I drink alcohol during the carb load?

No. Alcohol interferes with muscle glycogen storage because the liver prioritises metabolising alcohol over storing glycogen. It also dehydrates you, disrupts sleep quality, can cause intestinal inflammation and reduces muscle recovery capacity. Even moderate amounts (a beer, a glass of wine) can compromise the effectiveness of your load. In the 3 days before the marathon, eliminate alcohol completely. After you cross the finish line, celebrate however you like.

Does carb loading work the same for a half marathon?

For a half marathon, you do not need as intense a loading protocol as for a full marathon. Most runners finish a half marathon in 1:30-2:30, and normal glycogen stores (supplemented with 2-3 gels during the race) are usually enough. However, slightly increasing carbs in the 24-48 hours beforehand (6-8 g/kg/day instead of the 8-12 used for a marathon) can help you start with fuller reserves and feel more energetic in the closing kilometres. For complete preparation plans, see our half marathon training plan guide. And for race-day fueling, check our race day nutrition guide.

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