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A complete marathon fuel & hydration guide

Icon of calendar09/02/2024

When preparing for a marathon, many people tend to focus solely on their training regime and overlook the importance of nutrition - often only considering it in the lead-up to the race itself. However, this approach can be a costly mistake and may ultimately determine whether or not you achieve your personal best or even complete the race at all. Common issues such as lacking energy, gastrointestinal problems, and dehydration can be easily prevented with the right nutrition. In this guide, we'll cover topics like fueling, hydration, carb-loading, and more to help you prepare for your next marathon. 

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The function and importance of carbohydrates and fats

During a marathon, the energy that fuels every movement and breath we take comes from the foods we consume. However, not all energy sources are equal. Let's explore the science behind the two main fuels our body uses for training and racing: carbohydrates and fats.

Carbohydrates: The body's preferred fuel
When we eat carbohydrates, they are broken down into simpler sugars such as glucose. This glucose provides us with immediate energy by entering our bloodstream, or it is stored in our liver and muscles as glycogen. During high-intensity activities, our body prioritizes the use of these glycogen stores. Why? Because glycogen can be rapidly broken down anaerobically to produce ATP (adenosine triphosphate), which meets the immediate energy demands of strenuous activities. However, our glycogen reserves are limited. Once they are depleted, our performance will decrease, leading to the undesirable phenomenon "hitting the wall" or "bonking". To prevent glycogen depletion, you can employ strategies such as carbohydrate loading in the days leading up to a race and consuming easily digestible carbohydrates during the event itself.

Fats: a slow energy source
Fats are stored as triglycerides in adipose tissues and can provide a significant amount of energy, which is highly beneficial during prolonged physical activities. When the body runs out of carbohydrate stores, it starts using fats to produce ATP. Although the process of breaking down fats to produce energy is slower than glycogen breakdown, it releases more energy per gram of fat, thus ensuring a sustained energy output. The challenge lies in training the body to efficiently tap into these fat reserves, especially during the later stages of long-duration events. Fat metabolism becomes increasingly significant at moderate intensities, where aerobic conditions allow for the sustained oxidation of fat, a process that becomes even more crucial as glycogen stores wane.

How heart rate dictates energy/fuel use
The body's choice of fuel is closely linked to heart rate, which acts as a proxy for exercise intensity. As heart rate and exercise intensity increase, the body gradually shifts to relying more on carbohydrates. This change occurs because the body needs rapid energy production that fats cannot supply quickly enough. When exercise intensity reaches a point where the demand for ATP outstrips the oxygen available, usually above 70-80% of the maximum heart rate, the body increasingly depends on anaerobic metabolism, which primarily uses carbohydrates. 
Glycogen, which is stored in the muscles and liver, becomes the primary source of energy because it can be broken down into glucose without the need for oxygen, providing quick bursts of energy. However, since glycogen stores are limited, a higher reliance on this energy pathway can lead to quicker depletion, resulting in fatigue and decreased performance - often experienced by athletes as 'hitting the wall'. 
This relationship between heart rate and energy source is why endurance athletes often train at different heart rate zones to improve their body's ability to oxidize fats and save glycogen, delaying or even preventing the onset of fatigue. Such training can help in 'teaching' the body to become more efficient at fat metabolism, even at slightly higher intensities, which is beneficial for long-duration sports where preserving carbohydrates as long as possible is advantageous. How your heart rate behaves throughout a marathon depends on several factors, such as dehydration and loss of blood plasma volume, increased body temperature, the build-up of metabolic byproducts, and fuel shifts. This gradual increase in heart rate over time, known as 'cardiac drift,' can subtly shift fuel usage from fats back towards carbohydrates, influencing nutrition and hydration strategies. So, as your heart rate increases, your preferred fuel source shifts as well. 
Understanding the interplay between carbohydrates and fats during a marathon is crucial, as it informs our nutrition strategy, ensuring we have a steady energy supply and can adapt as the race progresses and our body's demands shift.

The function and importance of hydration & sodium

Hydration, while crucial, is frequently overlooked in fueling strategies for endurance sports. It's vital not only for maintaining fluid balance but also for facilitating nutrient transport, including carbohydrates, and supporting key physiological processes. It is not just about regulating your fluid balance, but it also plays an important role in the transportation and absorption of carbohydrates, gastric emptying, heart rate, and metabolism. Simply focusing on carbohydrates is not enough. Therefore, we want to highlight the significance of hydration and sodium. 
Approximately 60% of the human body is water, with its distribution varying across different tissues. Next we'll explore how water and electrolytes, vital for numerous bodily functions, interact to maintain performance.
Minerals are naturally occurring inorganic solids present in small amounts in the diet. When dissolved in water, they either have a positive or negative charge, depending on the type of mineral, and are called electrolytes. Electrolytes can be acids, bases, or salts and occur in the body in extracellular or intracellular fluids. Electrolytes have multiple functions in the body due to their electrical charge, and an imbalance in electrolyte concentration disturbs homeostasis. 
One of the primary functions of electrolytes is to regulate fluid levels. In the body, most membranes are semipermeable, meaning water can diffuse across the membrane, but the molecules dissolved in water cannot without the use of transporters. When a solution is divided into two compartments with a semipermeable membrane, such as the intracellular and extracellular spaces, only water can move over the membrane to establish an equilibrium between the two compartments. Concentration gradients and hydrostatic pressure both influence the water balance. Hydrostatic pressure is created by the difference in water volume percentages between cells and blood plasma. 
Besides regulating fluid levels, electrolytes are essential in transmitting nerve signals, muscle contraction, regulating blood pH, and macronutrient absorption (such as carbohydrates) in the small intestine.

Water is vital for various physiological functions, including nutrient transport, body temperature regulation, and cellular structure. When you exercise intensely, your body generates heat that must be dissipated to maintain balance. The primary way your body loses heat is through sweating, which is affected by environmental factors such as humidity and wind. Inadequate heat loss can result in hyperthermia or heatstroke. Sweat not only cools your body but also causes fluid and electrolyte loss, which can affect your performance. Dehydration can reduce skin blood flow and sweating rate, making it difficult to cool your body. Even a 2% decrease in body weight due to fluid loss can lead to symptoms such as decreased alertness and performance. Electrolyte loss, especially sodium and chloride, can disrupt cellular function and cause muscle cramps. 
Your body doesn't naturally compensate for fluid and electrolyte loss during sweating, so you need to replenish them before, during, and after your run. You can estimate your sweat rates by measuring body weight changes before and after exercise and taking into account fluid intake and urine output. 
Drinks for hydration are classified as hypertonic (high sugar and salt concentration), isotonic (similar to blood), or hypotonic (lower than blood). Isotonic drinks are absorbed faster. Overconsumption of hypotonic drinks, in particular, can result in hyponatremia, a dilution of blood sodium, with symptoms that mimic dehydration. To restore electrolyte balance and prevent hyponatremia, it is recommended to consume isotonic or hypertonic drinks. Pre-exercise drinks with added sodium can increase plasma volume and performance, particularly in moderately trained individuals. During exercise, drinks should contain around 20-30 mmol/L sodium to replace losses and maintain taste. Post-exercise drinks should be 150% of total fluid loss to compensate for urinary losses, with a similar sodium content to during-exercise drinks to facilitate rapid fluid and electrolyte balance restoration.

Sodium is a positively charged ion that is mainly present in the extracellular fluid. It plays an important role in regulating extracellular fluid volume and blood pressure, which are essential for the transport of substrates and the excitability of nerve and muscle cells. Electrolytes move from inside to outside the cell and vice versa, which is necessary to transfer the signal over a nerve. Nerve conduction is crucial not only in the brain but also for muscle cells to contract and relax.
Additionally, sodium helps maintain cellular homeostasis (maintaining internal cell balance - important because it ensures cells can perform essential functions such as nutrient absorption, energy production, waste elimination, and more) and water balance. A high sodium concentration increases water absorption in the gut and reabsorption in the kidney. There is a potential link between sodium loss and muscle cramps, so it is important for athletes suffering from cramps to consume enough salts. For physical activity shorter than two hours, it is recommended to consume an isotonic drink (0.5-0.7 g/L Na+). Training or racing for more than two hours should include sodium (0.7-1 g/L of water) in their drink to increase palatability, promote fluid retention, and prevent hyponatremia. For ultra-endurance exercise, it is recommended that the sodium concentration of beverages is around 1.7 to 2.9 g/L. 

In short
To achieve endurance during a marathon, it's important to maintain a proper balance of hydration, sodium, and carbohydrates. This trio forms the foundation of your fueling strategy. Hydration is important for nutrient transport and thermal regulation, but it's closely linked to sodium as well. Sodium helps with fluid balance, nerve function, and muscle contraction, making it crucial for optimal absorption of water and carbohydrates. Carbohydrates are a well-established energy source, but efficient uptake is dependent on proper balance of fluids and electrolytes. Drinking isotonic solutions can help with carbohydrate absorption, providing a steady stream of energy while also optimising fluid uptake and reducing gastrointestinal discomfort. This balanced approach ensures that each component (water, sodium, and carbohydrates) is utilized effectively, boosting endurance, maintaining electrolyte balance, and preventing dehydration and hyponatremia. You should tailor your intake strategy to maintain this delicate balance, especially during long events where the risk of imbalance is higher.

Hitting the wall & “bonking”

Endurance athletes are well acquainted with the concept of "bonking" or "hitting the wall." It refers to a moment during prolonged, high-intensity exercise when the body's quick-access energy currency, known as glycogen reserves, run very low. These reserves typically fuel two-ish hours of  activity, and the intensity of the exertion and pre-existing glycogen levels determine the exact duration. Somewhere between the 25 and 32k mark is often when this energy crisis occurs, and it's a critical juncture where your body's demand for rapid energy conversion outpaces its dwindling glycogen supply. 
To avoid this decline and extend performance boundaries, strategic carbohydrate consumption before and during your training and/or race is essential. Unlike fats, which the body has in ample supply, carbohydrates are a limited resource. Therefore, they require careful management for endurance pursuits. As a result, nutritional strategies for endurance sports primarily focus on maximising carbohydrate availability. This ensures that these vital stores are replenished and readily accessible when starting the race. This preparation enables you to maintain your desired pace deeper into the race, pushing back the physiological limits and averting the onset of fatigue.


Carb loading is a strategy used to optimise your glycogen reserves before a race. The method involves a high intake of carbohydrates (typically 10 to 12 grams per kilogram of body weight) during the 3 to 4 days before the race. For instance, an athlete weighing 70 kilograms would need to consume 700 to 840 grams of carbohydrates daily. 
To supplement this, you should reduce training intensity to allow the muscles to saturate with glycogen without complex loading protocols. Carb-loading may increase overall body weight due to additional glycogen (three parts water for every part of glycogen) but the benefits of enhanced energy stores and hydration outweigh any potential drawbacks. 
You should consume high-carb foods such as rice, pasta, cereals, and sweet spreads while avoiding high-fat and high-fibre options. The goal is to maintain a tasty and familiar diet that encourages the consumption of the necessary volume of food without leading to overindulgence. Incorporating carbohydrate-dense liquids like high-energy drinks or gels can further contribute to achieving carb-loading goals without contributing to fullness. Fibre is a non-digestible carbohydrate that could add unnecessary weight and affect race-day agility. Therefore, you should go for low-fibre options during this time. 

Pre-race breakfast

On the big day, breakfast plays a crucial role in providing sustained energy levels. The timing of your breakfast is essential to balance with adequate rest, especially when the marathon has a varied start time. When you wake up on race day, your body's liver glycogen, which fuels vital functions overnight, is quite low and requires replenishing to prevent hypoglycemia. 
To fuel your body in the morning, consume between 100 to 200 grams of carbohydrates within the 3 to 4-hour window before the race. Instead of having a single large meal, divide your carbohydrate intake across several smaller snacks or carbohydrate-rich drinks. This strategy caters to those who may find it challenging to eat a full meal due to pre-race nerves or digestive concerns. 
When choosing your pre-race food, go for items that are low in fat, fiber, and protein, as these can slow down the digestive process and cause gastrointestinal distress during the race. Dairy is often well-tolerated, but those sensitive to lactose may need to limit their intake on race morning. Your breakfast should consist of carbohydrate-rich foods like cereals, white bread, jam, honey, pancakes, and fruit juices. 

Pre-race caffeine intake

Caffeine is known for its ability to boost mental focus and reduce the feeling of exhaustion during intense physical activity. It works by blocking adenosine, a molecule that signals fatigue, leading to an increase in neuron activity and a sense of alertness. 
Athletes often use caffeine to enhance their performance, and it's recommended to take a moderate dose of 2 to 3 mg per kilogram of body weight, around 30 to 60 minutes before the activity. For example, a 70 kg person should consume around 140 to 210 mg of caffeine. 
Caffeine has several benefits, including mobilising fatty acids, enhancing muscle contraction, and triggering the release of endorphins that help reduce pain and potentially prolong endurance. 
While caffeine can cause dehydration when taken in large quantities (this is often exaggerated), maintaining proper hydration is still crucial, especially if you choose a caffeinated beverage that contains diuretics. Keeping a balance between fluid intake and caffeine consumption can help sustain energy levels without leading to dehydration. 
In summary, caffeine is a useful tool for athletes when used strategically alongside proper hydration, carbohydrate intake, and attention to individual tolerances. As each runner crosses the finish line, the elation felt may be a testament to the strategic interplay of caffeine, discipline, and the sheer will to outpace fatigue. 
We recommend taking 01 Before 30 minutes before the race to optimise its effects.

Race fuel & hydration

Now, the million dollar question: What to use during the race itself...? Well, as said, it's important to consume enough carbohydrates per hour during a race as they provide several advantages such as more energy, improved performance, and faster finishing times. Carbs are the best fuel source for athletes, so it's important to focus on them rather than other macro-nutrients. However, the amount of carbs you should consume per hour depends on various factors like the intensity of the race. If you're aiming to finish the race, 30-60 grams per hour should suffice (one 02 During gel or drink mix per hour). On the other hand, if you're running at a higher pace/intensity, you should consume 90 grams of carbs per hour, which equates to (just) two units of 02 During. A recommended approach is to consume one 02 During drink mix over the hour and a 02 During gel halfway into the hour. 
It's important to be selective when it comes to choosing gels or drink mixes. Avoid any products that use a single source of carbohydrates such as maltodextrin, glucose, or fructose, or primarily use just one of these carbs. These can cause gut issues or gastrointestinal (GI) problems. Instead, opt for a balanced mix of two or three carb sources and aim for a carb ratio of 2:1 or 2:1:1.5. This will make it easier for your gut to process these carbs, resulting in faster gastric emptying (energy availability) and fewer GI/gut issues.
So, in summary, and depending on your training/race intensity:
Low-intensity or sub-2-hour training sessions or race: 1 x 02 During (drink or gel) per hour
High-intensity or super-2-hour training sessions or race: 2 x 02 During (one drink and one gel) per hour

Training your gut during training

We talked about this in a previous blog, but we can’t stress it enough. Training your body to consume large amounts of carbohydrates per hour is crucial if you plan to use carbs as your primary fuel source during a race, such as a marathon. It is essential to consistently use them during your weekly runs, rides, or workouts. Also, it is not wise to switch brands or suddenly increase your hourly intake on race day. Do. Not. Do. That. Your body is already under significant stress during the race, and overloading or stressing your system can be harmful. This is why the off-season is the perfect time to read up, try new brands and products, and train your system to get used to specific carb ratios, formats (such as gels, drinks, bars), and quantities you intend to use during the race.

One final thing: aid stations

Use them for water or cooling, but best avoid the sports drinks they offer as they’re not the products you trained with. They may also offer (carbonated) products that are high in fat, protein or fibres, and you best avoid those. So, bring what you trained with and plan ahead.

02 During

We developed 02 During in response to an extremely confusing and broad offering of gels and drink mixes already available.  Despite the clear science and available information, many products still contain sub-optimal carb ratios and confusing quantities, making it difficult for athletes to optimize their energy management. How can we be expected to consume 90-135 grams per hour when gels typically contain between 18-25 grams of carbohydrates each, especially when such a low-carb gel costs around €2,00-3,50 per unit? This makes things too expensive and highly impractical for most—not to mention the challenge of carrying all those gels.

Both our 02 During drink mix and gel contain 45 grams of carbs per serving, simplifying intake planning and energy management optimisation while significantly reducing cost per hour/training - even when aiming for 135 g/h. 

Our fuel guide

To help you further, we've put together a comprehensive fuel and hydration guide. If you're interested in getting a personalised intake plan, just follow the link below, answer a few questions, and our performance coaches will tailor a plan for you.


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