Fueling and hydration during endurance exercise

In long-term sports, such as marathon, ultramarathon, triathlon and open water swimming, the body does not break down just because it “lacked mental strength”. It often breaks down because there is a lack of fuel, liquid, sodium or strategy. Muscle and liver glycogen function as an internal carbohydrate tank, but this tank is limited. In prolonged efforts, especially over 60 to 90 minutes, carbohydrate dependence increases and the drop in glycogen stores can reduce pace, coordination, decision making, stride strength, stroke power and perception of control. Therefore, VETOR does not treat intra-workout nutrition as an elite detail; treats as part of the prescription. Anyone who runs or swims for a long time needs to train their body, but they also need to train their fueling.

The math starts with duration and intensity. For exercises longer than 1 hour, classic recommendations point to somewhere around 30 to 60 grams of carbohydrates per hour. For longer events, especially over 2.5 to 3 hours, the recommendation can increase to up to 90 grams per hour, as long as the athlete tolerates it well and has trained their intestines to do so. The consensus of the International Olympic Committee also reinforces that, to achieve high intakes, such as up to 90 g/h in events lasting more than approximately 3 hours, the athlete must practice this intake during training, because the gastrointestinal tract needs to adapt. This means that new gel is not tested on the day of the test. Testing is not a laboratory. The long run, long swimming training and specific blocks are the right place to discover quantity, brand, texture, concentration, interval and tolerance. (Taylor & Francis Online⁠)

VETOR's rule of thumb is simple: training sessions of up to 60 minutes generally do not require intra-workout carbohydrates, except in specific cases such as fasting, heat, double sessions or low energy availability. For 60 to 120 minutes, 30 to 60 g/h is usually sufficient for most. Over 2 hours, especially at a strong pace or race, 60 to 90 g/h starts to make sense. In very long events, some trained athletes can go beyond this, but this should not be the starting point for ordinary runners. More carbohydrates are not automatically better; If absorption does not follow intake, the result may be nausea, bloating, colic, diarrhea or decreased performance. The goal is not to copy the professional who ingests absurd numbers; is to find the greatest useful amount that your body can absorb without discomfort.

Choosing the type of carbohydrate matters because the intestine has different “gates” to absorb sugars. Glucose mainly uses the SGLT1 transporter. Fructose mainly uses GLUT5. When the athlete uses only glucose or maltodextrin, there is a practical limit of intestinal absorption. By combining glucose and fructose, it is possible to use different transporters and increase carbohydrate delivery to the blood and muscle. Therefore, strategies with “multiple transportable” carbohydrates have become popular in endurance. A review by Jeukendrup explains that very high intakes, such as 90 g/h, should come from carbohydrates that use different transporters, and studies show that mixtures of glucose and fructose can improve exogenous carbohydrate oxidation and performance compared to glucose alone. (PMC⁠)

The most classic proportion is 2:1, that is, two parts glucose or maltodextrin to one part fructose. In practice, this could mean 60 g/h of glucose or maltodextrin plus 30 g/h of fructose to reach 90 g/h. Some modern products use ratios closer to 1:0.8 or 1:1, but the principle remains the same: combining different sources to increase absorption and reduce overload on a single intestinal transporter. For VETOR, the decision should not be ideological. If the athlete is going to consume 30 to 60 g/h, many sources can work. If you're going to try 75 to 90 g/h, the glucose/fructose mix becomes more important. And if the athlete has a history of gastrointestinal discomfort, progression should be gradual: first 30 g/h, then 45, then 60, then 75, then 90, always testing during training.

Hydration is also not simply “drink as much as possible.” The goal is to limit excessive fluid loss without making the opposite mistake: drinking too much water. Exercise-associated hyponatremia occurs when the concentration of sodium in the blood drops during or up to 24 hours after physical activity, generally due to excess fluid intake in relation to the body's ability to eliminate water. Consensus on hyponatremia in endurance reinforces that the main preventive strategy is to avoid overhydration, and that sodium supplements do not compensate for the error of drinking too much liquid. This is essential for marathon runners and long-term swimmers: hydrating well does not mean loading your stomach with water; It means drinking in a planned way, respecting thirst, climate, sweat rate, duration and gastrointestinal tolerance. (PMC⁠)

Sodium remains important, but it needs to be understood correctly. It helps maintain fluid balance, nerve conduction, muscle contraction and fluid retention, especially in long workouts, heat, high sweating or athletes who lose a lot of salt in sweat. However, cramps should not be explained simply as “lack of sodium”. Reviews on cramps associated with exercise show that the cause is complex and involves both hypotheses of water/electrolyte imbalance, fatigue and changes in neuromuscular control. In other words, electrolytes can help in certain scenarios, especially when there is a lot of sweat and inadequate replacement, but they do not replace conditioning, load progression, pacing, strength, sleep and race strategy. The runner who breaks down at km 32 may be without sodium, without carbohydrates, dehydrated, poorly conditioned, too fast at the beginning or all of these things together. (PMC⁠)

In the VETOR method, intra-workout nutrition becomes a practical prescription: how long does the session last, what is the intensity, what is the climate, what is the athlete's history, what is the objective of the day and what is the next session? A light 90-minute long run may require little. A 2h30 workout with blocks at race pace needs planned carbohydrates. A marathon requires strategy by the hour, not improvisation by sensation. A long swimming session in a pool may result in a bottle on the edge; Open waters require logistics of support points, gel, squeeze or food on the boat. The goal is not to turn every runner into a nutritionist. It is to prevent months of training from being wasted due to a basic fueling error. The body needs an engine, technique and resistance, but in a long race it also needs a full tank, intelligent replacement and a plan that has already been tested before the important day.

Terms explained in simple terms

Glycogen: is the way the body stores carbohydrates in the muscles and liver. Think of it as the “quick tank” of energy for moderate and intense efforts. When falling too much, the athlete may experience a sudden drop in pace, weakness, confusion, loss of coordination and the famous “break”.

Carbohydrate per hour, or g/h: is the amount of carbohydrate that the athlete consumes each hour of exercise. Example: 60 g/h could be 1 30 g gel every 30 minutes, or a sports drink that delivers 60 g over an hour.

Glucose: is a simple sugar used quickly as energy. Many sports products use glucose or maltodextrin, which turns into glucose in the body.

Fructose: is another simple sugar, found naturally in fruits and also used in sports products. It uses a different intestinal transporter than glucose, so it can help when the athlete needs to consume more carbohydrates per hour.

Maltodextrin: is a common carbohydrate in supplements and sports drinks. It is basically a chain of glucose, with a less sweet flavor than regular sugar, and is usually used to deliver plenty of carbohydrates without making the drink too sweet.

2:1 glucose/fructose ratio: means two parts glucose or maltodextrin to one part fructose. Example: 60 g of glucose/maltodextrin + 30 g of fructose = 90 g of total carbohydrate.

Multiple transportable carbohydrates: these are combinations of carbohydrates that use different absorption “gates” in the intestine, such as glucose and fructose. This may allow for greater absorption and less discomfort when intake is high.

Electrolytes: minerals with an electrical charge, such as sodium, potassium, magnesium and calcium. They participate in muscle contraction, nerve conduction and fluid balance. In endurance sports, sodium is usually the most important electrolyte to replace during long training sessions with a lot of sweat.

Sodium: mineral lost in sweat. Helps maintain fluid volume, nerve function and muscle contraction. Important: sodium helps, but does not “save” the athlete who drinks too much water.

Hyponatremia: dangerous drop in sodium concentration in the blood. In endurance, it often happens due to excess water or hypotonic liquid. It can cause nausea, headache, confusion, swelling, seizures and, in severe cases, risk of death.

Gastrointestinal discomfort: any problem in the stomach or intestine during training/race: nausea, reflux, colic, bloating, urge to have a bowel movement or diarrhea. It generally worsens when the athlete consumes too many carbohydrates without intestinal training, a very concentrated drink, a new product or excess liquid.

Train the intestine: practice carbohydrate and liquid intake during long workouts so the body adapts. Just as you train your legs and lungs, you also need to train your digestive system to tolerate fuel on the move.

Reliable sources and what they support

1. Carbohydrates for training and competition — Burke et al., Journal of Sports Sciences Strong source for carbohydrate math. She maintains that 30–60 g/h is a suitable range for long workouts, and that events over 2.5 hours can benefit from intakes of up to 90 g/h.

2. IOC Consensus Statement on Sports Nutrition — International Olympic Committee The IOC consensus reinforces that events lasting more than approximately 3 hours may require high carbohydrate intakes, up to 90 g/h, and that the athlete must practice this strategy in training to develop gastrointestinal tolerance.

3. Carbohydrate Intake During Exercise — Asker Jeukendrup, 2014 This review explains why the recommendation changes depending on the duration of exercise: small amounts can help with efforts lasting about 1 hour; 30–60 g/h is common in endurance; and about 90 g/h can be used in ultraendurance with transportable multiple carbohydrates.

4. Superior endurance performance with ingestion of multiple transportable carbohydrates — Currell & Jeukendrup, 2008 Important study showing that a mixture of glucose and fructose improved endurance performance compared to glucose alone and water. Supports the logic of combining carbohydrates to improve energy delivery.

5. Multiple transportable carbohydrates enhance gastric emptying and fluid delivery — Jeukendrup, 2010 This study compared water, glucose and glucose + fructose in a 2:1 ratio during moderate exercise. It serves to explain why glucose/fructose mixtures may be useful when intake is high and gastrointestinal tolerance matters.

6. Exercise-Associated Hyponatremia — review/consensus, 2019 Reliable source for hyponatremia risk in long events. She reinforces that there is no single fluid and salt recommendation for everyone, and that a prudent guideline is to drink according to thirst and avoid excess fluid.

7. Exercise-Associated Hyponatremia: 2017 Update — Frontiers in Medicine This update reinforces that the main strategy for preventing exercise-associated hyponatremia is to avoid drinking excessive fluids. It also shows that the role of supplemental sodium in prevention is still controversial.

8. Effects of Sodium Intake on Health and Performance in Endurance and Ultra-Endurance Sports — 2022 This review is useful for putting sodium in the right place: it can help mitigate sodium drops in some contexts, but it does not prevent hyponatremia if the athlete is overhydrated. It also warns that excess sodium can be problematic.

9. An Evidence-Based Review of Exercise-Associated Muscle Cramps — 2021 This review explains that exercise-associated cramps are painful involuntary contractions and that the pathophysiology is not yet completely resolved. It serves to avoid simplification like “a cramp is always a lack of salt”.

10. Muscle Cramping During Exercise: Causes, Solutions, and Questions Remaining — Sports Medicine, 2019 Useful source to show that there are different hypotheses for cramps: water/electrolyte imbalance and altered neuromuscular control. This supports a more honest approach: sodium is important, but cramping also depends on fatigue, conditioning and intensity control.

At VETOR, intra-workout nutrition is not “taking a gel when you get tired”. The fuel prescription is: 30–60 g/h for many long training sessions, up to 90 g/h in prolonged events when the athlete has already trained the intestine, preference for glucose/fructose when intake is high, intelligent sodium replacement in sweaty and hot scenarios, and sufficient hydration without making the dangerous mistake of drinking too much water.