UGent - Hydration plan for marathon runners: Is it okay to cross the finish line dehydrated?

Fluid intake, in addition to carbohydrates, is one of the most decisive factors for athletic performance in endurance sports such as the marathon. During prolonged exertion, the body loses not only fluid through sweat but also important minerals such as sodium. This loss can lead to dehydration, overheating, or the opposite: overhydration. Both extremes negatively affect your performance.

Furthermore, it is a greater challenge for marathoners to maintain their fluid balance than for other endurance athletes like cyclists. While a cyclist can easily carry water bottles and drink regularly, a runner must rely on limited aid stations. In addition, it is a tricky skill to take in small sips while running. That is precisely why a well-thought-out hydration plan is indispensable. Fortunately, the Etixx range with Sport Hydro Tabs, PRO LINE High Carb Drink, and Isotonic Drink, offers several drinks that can help you with this.

How much should you drink? What role do salts play? And is it actually acceptable to reach the finish in a dehydrated state? In this blog, Raf Van Dyck, practical assistant and dietitian at Ghent University, provides scientifically grounded insights and practical tools to help you build your own optimal hydration plan.

Fluid balance in marathoners: why hydration determines your performance

Tokyo 2025. The men’s marathon world championship became a real thriller between the Tanzanian Simbu and the German Petros, ending in a fierce sprint to the finish. Ultimately, Simbu won the gold with just a 0.03-second margin. Petros claimed silver with the same official time as Simbu, a gap more commonly seen in a 100 m race than in a marathon. This minute difference suggests that victory must be sought in “marginal gains”, the idea that performance can be improved by refining small details. Though this concept is very alive in the sports world, every marathoner knows that finishing a race depends on “big gains”: running many kilometers, avoiding injury through stabilization and core training, refining running technique, avoiding gastrointestinal issues, consuming sufficient carbohydrates, and maintaining the body’s fluid and sodium balance.

That fluid balance is an important parameter for a strong marathon performance has long been known. At the 1984 Olympic Games in Los Angeles, the marathon was held under very hot conditions. The women’s race is not remembered primarily as the first Olympic marathon for women but rather because of the Swiss runner Gabriela Andersen‑Schiess, who, severely dehydrated, staggered through the final 400 meters and collapsed after crossing the finish line.

Giving sufficient attention to a balanced fluid and salt balance during both training and the marathon itself is a critical factor in finishing successfully. In marathons under hot conditions (above about 25 °C), it becomes the most important concern to preserve health and to match the performance level achieved in cooler conditions. Learning to drink sufficiently and choosing the right products is essential both during training and in the marathon itself. The question remains: how much should you drink? Should you drink so much during the race that you maintain stable weight, or is it acceptable to reach the finish “dehydrated”?

Dehydration vs overhydration at the finish of a marathon

Running gives a sensation of warmth because, in addition to mechanical work, substantial internal body heat is produced. As long as core temperature remains below 39.0–39.5 °C, neural function and energy production continue at normal levels. When core temperature rises above 39.5 °C, physiological, metabolic, and neural function comes under stress, the worst consequence being brain failure and potentially death.

Of course, the exercising body does everything it can to eliminate this heat, responding by radiating warmth and sweating. Sweat cools the body only if it evaporates, not when it runs off your skin or is wiped away with a towel. The inefficiency becomes clear when large amounts of non‑evaporated sweat drip from your body. When conditions are very hot and especially humid, sweating becomes even less efficient. The amount of sweat lost and the amount of sodium deposited on the skin vary greatly from person to person, and are not strongly correlated with fitness. However, heat training can lead to a strong reduction in sodium concentration in sweat.

When weight is measured at the finish line, some marathoners have lost up to 5 kg, others have maintained weight, and still others have gained a few kilos. The runners who lose weight are often the faster ones with many training kilometers. Runners with fewer training kilometers, those with higher fat percentages, or those with greater muscle mass. In short, the slower runners are more likely to gain weight by overdrinking. Research has shown that with a weight loss of 2 % to 4 % (relative to training weight), excellent marathon performances are possible. So you may cross the finish line slightly dehydrated. Gaining weight is of course also acceptable. But large swings, such as severe dehydration (> 4 % bodyweight loss) or overhydration (weight gain), must be avoided because they can pose life-threatening risks.

The reason is that the body tries to maintain the osmotic pressure of the blood (280–295 mOsm/kg H₂O), which is heavily determined by the amount of sodium and fluid, in order to preserve equilibrium with the working muscle, heart, lung, and brain cells. In dehydration greater than 4 % of body weight, the amount of fluid in the bloodstream is low and sodium concentration can increase (>160 mmol/l). This draws fluid from working cells, which in extreme cases can lead to encephalopathy (disturbed brain function) and death. In overhydration, when little extra sodium is ingested, the blood volume becomes overfilled, resulting in low sodium concentration (<135 mmol/l). Consequently, cells draw in excess fluid and swell. This condition is called EAH (Exercise-Associated Hyponatremia) and can lead to encephalopathy, pulmonary edema, and death.

Since each marathoner responds differently to varying weather conditions, it is important to measure and develop personalized drinking strategies.

Drinking during a marathon: practical tips and strategies

How can you proceed?

1. Measure to know: Build your own dataset of weight loss during training in all weather conditions. Weigh yourself before and after training (in underwear and dried off) so you learn to estimate realistically.
2. Amount to drink: During training, drink between 400 and 1,200 ml per hour so that you aim for dehydration of 1–3 % of body weight.
3. Sports drinks: Use sports drinks such as thirst quenchers and energy drinks with a high sodium concentration, between 230–690 mg, and in warm conditions even between 700–1,150 mg per liter (or 1,170–2,550 mg of salt per liter).
4. Training: Try to drink during every training session (even easy runs) so your body learns to absorb water, salt, and carbohydrates.
5. Drinking technique: If you drink too hastily while running, you risk choking. Relax your mouth while running and shape it into a small scoop. Take small sips so that you can still breathe and swallow at the right moment.
6. Pre-training and hyperhydration: In warm conditions, you can achieve hyperhydration before training or race start by using ORS (Oral Rehydration Salts). These are drinks with a high salt concentration that promote proper absorption and retention of fluid in the body.
7. Marathon plan: To complete a marathon successfully, plan your approach. If you are a casual runner, you may use thirst as a guide. If you aim for a strong time, plan drinking moments every 3–5 kilometers, estimate amounts depending on weather conditions, and try to carry your own drinks.

Raf Van Dyck – Practical Assistant and Dietitian, Ghent University

• Hew-Butler et al., Updated Fluid Recommendation- Position Statement From the International Marathon Medical Directors Association (IMMDA), Clin J Sport Med 2006;16:283–292
• Sawka MN et al., American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 2007;39:377–90.
• Rehrer et al., Fluid Intake and Gastrointestinal Problems in Runners Competing in a 25-km Race and a Marathon, Int J Sports Med 1989; 10: S22-S25
• Almond et al., Hyponatremia among Runners in the Boston Marathon, N Engl J Med 2005;352:1550-1556