How to Hydrate Properly for Ultracycling and Bikepacking

What science really says and what it means for your next long-distance ride

Whether you are taking part in a 200 km brevet, embarking on a multi-day bikepacking adventure, or competing in an unsupported ultra race, sooner or later the same question always comes up: How much should I actually drink?

The answers are often contradictory. Some recommend immediately replacing every litre of sweat lost, while others advise drinking exclusively according to thirst. There are also numerous myths surrounding electrolytes and sodium.

The good news is that sports science now provides much clearer answers than it did a few years ago. At the same time, it shows that there is no single hydration strategy that works equally well for everyone. Sweat rate, ambient temperature, exercise intensity, and individual physiology vary significantly from one person to another.

This article summarises the current scientific evidence and translates it into practical recommendations for ultracycling and long bikepacking trips. It is based on current reviews and studies on hydration in endurance sports, as well as insights from sports scientist Max Kinzlbauer.

Key takeaways

If you only have a little time, remember these five points:

• A fluid loss of more than 2 % of body weight can measurably reduce endurance performance.^4–6
• Mild dehydration during prolonged exercise is normal and does not need to be fully corrected immediately.⁶
• Drinking more is not automatically better. In extreme cases, excessive fluid intake can be more dangerous than a moderate fluid deficit.^{1, 2, 4}
• Sodium becomes particularly relevant during very long efforts and when large amounts of fluid are consumed, not during every ride.^{10, 11}
• The best hydration strategy is individual and should be tested during training.^{1, 3}

When does fluid loss impair performance?

One of the most important questions in endurance sports is: At what point does fluid loss actually affect performance?

Research provides a relatively clear answer: A loss of more than 2 % of body weight through sweating leads to a measurable reduction in aerobic endurance performance for most people.⁴ More recent studies have confirmed this relationship even under blinded conditions, meaning that the athletes did not know how much fluid they had actually consumed.⁵ The reduction in performance is therefore not merely subjective, but can also be demonstrated physiologically.

Heat intensifies the effect

The extent to which dehydration affects performance depends largely on environmental conditions. Heat intensifies the negative effect because the body must expend additional energy to dissipate heat. When fluid loss reduces the body's ability to regulate temperature, core body temperature rises more quickly, increasing the strain on the body.⁵

Conversely, this relationship helps explain why top marathon performances are often achieved in cool conditions. The body can dissipate heat more efficiently and tolerate moderate fluid loss considerably better.

Remember: Not every fluid loss is problematic. Exercise duration, intensity, and especially ambient temperature are the decisive factors.

Endurance is affected more than strength

Aerobic endurance performance is particularly sensitive to dehydration. Strength and explosive power are affected to a much lesser extent.⁶ At the same time, increasing fluid loss can also impair concentration, decision-making, and technical skills. This is an important consideration during long bikepacking trips, demanding gravel descents, or technical trails.⁶

Preventing dehydration: How to prepare properly

Many riders only start thinking about their fluid intake once they are already on the road. In reality, effective hydration begins before the start.

Studies show that many athletes begin training sessions or competitions in a mildly dehydrated state.^{1, 3} This reduces their fluid reserves before the effort has even begun and can cause performance to decline earlier.

Preparing properly

The current recommendation is to drink approximately 6–8 ml of fluid per kilogram of body weight around two hours before starting, ideally in the form of a sodium-containing drink.² Sodium improves fluid retention, meaning that a larger proportion of the consumed fluid remains in the body instead of being excreted immediately.

A person weighing 70 kg should therefore consume approximately 420–560 ml of fluid.

Urine colour can provide a simple indication of hydration status. Pale yellow urine generally suggests adequate hydration, while very dark urine may indicate a fluid deficit.^{1, 3}

Practical tip: Before long rides, pay attention not only to your fluid intake but also to your carbohydrate intake. Both factors influence how well your body copes with the first few hours of exercise.

During the ride: A slight deficit is normal

A widespread misconception is that every litre of sweat must be replaced immediately. Current sports science does not recommend this approach.

During prolonged exercise, most endurance athletes drink less than they actually lose through sweating. As long as fluid loss remains below approximately 2 % of body weight, this is generally not a problem.⁶ In practice, replacing around 60–70 % of sweat losses initially has proven effective. This reduces the strain on both the stomach and the body's fluid-regulation system.

During very long efforts, such as ultracycling races or multi-day bikepacking adventures, the fluid deficit should not continue increasing indefinitely. At that point, it becomes useful to gradually adjust fluid intake to actual sweat losses in order to keep body-weight loss below approximately 2 %.

Know your sweat rate

How much fluid you actually need depends primarily on your individual sweat rate. Depending on temperature, intensity, and training status, this can range from 0.5 to more than 2.5 litres per hour.^{1, 3}

The simplest method for determining your sweat rate can be used during training:

1. Weigh yourself before the ride.
2. Weigh yourself again after two to four hours under comparable conditions.
3. Take into account the amount of fluid consumed during the ride.
4. Calculate your sweat rate using your weight loss and fluid intake.
5. Subtract approximately 10 % from the calculated sweat rate.

A simple calculation is:

Sweat rate per hour = (body-weight loss in kg + fluid consumed in litres − fluid excreted in litres) ÷ exercise duration in hours

Approximately 10 % should then be subtracted from this result.

The reason is that not all body-weight loss is caused by sweating. A small proportion results from the depletion of glycogen stores, more precisely from the associated metabolic carbon loss, as well as respiratory water loss through breathing. These losses are physiologically separate from actual sweat loss and would otherwise lead to a slight overestimation of the sweat rate.

The measurement should be repeated several times under comparable conditions. Because sweat rate and sodium loss can change throughout the season and with temperature and training status, it is advisable to reassess your sweat rate regularly.^{1, 3, 9}

Can you drink too much?

Yes. In fact, overhydration is often underestimated in endurance sports.

While many athletes are concerned about drinking too little, excessive fluid intake can also pose health risks, especially when the majority of that fluid is plain water.^{1, 2, 4}

When too much water becomes a problem

Anyone who consumes more fluid over a prolonged period than they lose through sweating can develop a condition known as hyponatraemia. This occurs when the sodium concentration in the blood falls because excessive water intake dilutes the blood.^{1, 4}

Symptoms range from headaches, nausea, and confusion to seizures. In very rare cases, severe hyponatraemia can be life-threatening.

However, it is important to put this into context. Hyponatraemia does not occur simply because someone drinks regularly during a long ride. In practice, it usually develops only when significantly more fluid is consumed than is actually lost over an extended period.

In ultracycling, the relevant question is therefore not only how much you drink, but also whether your fluid intake matches your individual sweat rate.

Body weight is a useful indicator

A simple rule of thumb is:

Body weight should not increase during prolonged exercise.

Stable or slightly decreasing body weight is normal. If body weight increases during a race, however, this often indicates excessive fluid intake and should be treated as a warning sign.²

Why you should not replace 100 % of your sweat losses

The recommendation not to replace every litre of sweat immediately may initially appear contradictory. However, there are both physiological and practical reasons for this approach.

The body can tolerate a moderate deficit

Part of the fluid loss comes from glycogen stores and the intracellular compartment. This proportion does not need to be replaced immediately during exercise.⁹ At the same time, very high fluid intake during heavy sweating is often impractical because the stomach and intestines can only absorb limited amounts of fluid.^{4, 7, 9}

Especially in hot summer conditions, sweat rates of 1.5 to more than 2 litres per hour can occur. Fully replacing these losses is often impossible in practice and is not necessary.

Drinking more provides no additional benefit

Athletes who attempt to replace 100 % or more of their sweat losses during exercise increase their risk of overhydration and therefore also their risk of hyponatraemia.^{4, 7}

The current evidence therefore supports a pragmatic approach:

• A moderate fluid deficit is normal during prolonged exercise.
• Fluid intake should be based on individual sweat rate.
• The key objective is to ensure that fluid loss does not rise significantly above approximately 2 % of body weight over time.

Practical tip: If you know your sweat rate, you do not need to calculate every sip precisely. The aim is to maintain a consistent fluid intake that prevents both excessive deficits and excessive water consumption.

Sodium: When electrolytes are genuinely useful

Few topics in endurance sports are discussed as controversially as electrolytes. Some athletes consider sodium tablets essential for every ride, while others believe they are unnecessary. The scientific evidence presents a more nuanced picture.

Why sodium matters

Sodium is the most important electrolyte for regulating the body's fluid balance. It helps retain water in the body and plays a central role in rehydration after prolonged exercise. Drinks with a higher sodium content are demonstrably retained more effectively after exercise than plain water or conventional sports drinks with a low sodium content.^{7, 8}

However, this does not mean that additional sodium must automatically be consumed during every long ride.

When sodium becomes useful during exercise

As long as less fluid is consumed than is lost through sweating, the sodium concentration in the body initially increases slightly. During this phase, there is generally no immediate need to replace sodium specifically.

Sodium becomes particularly relevant when several factors occur together:

• very long exercise duration,
• high sweat rate,
• large fluid intake,
• almost complete replacement of fluid losses.

This combination occurs primarily during ultracycling races, multi-day bikepacking adventures, and ultramarathons.

Everyone loses a different amount of sodium

Another important point is that not everyone loses the same amount of sodium through sweat.

The sodium concentration in sweat can range from approximately 200 to almost 2,000 mg per litre of sweat. This means that athletes can differ by a factor of ten.

General recommendations are therefore only useful to a limited extent.

White salt marks on a jersey or helmet may indicate higher sodium losses, but they do not provide a reliable measure of the amount actually lost. Athletes who want precise values can have them determined under standardised conditions or use modern sweat sensors. For most athletes, however, this is not strictly necessary.

Practical tip: For most bikepacking trips, a standard sports drink or water combined with a balanced diet is entirely sufficient. Regular eating already provides meaningful amounts of sodium.

How much sodium is useful?

For very long efforts, drinks containing approximately 700–800 mg of sodium per litre are often recommended. In practice, this range provides a good starting point for many athletes, particularly when solid food is also consumed.

However, the optimal amount depends on the individual's sweat composition and fluid intake. There is therefore no universally applicable recommendation.

Sodium and muscle cramps

Muscle cramps are often attributed to sodium deficiency. However, the scientific evidence is less conclusive than is commonly assumed.

According to current knowledge, several factors interact. The most important include:

• unfamiliar or very prolonged exercise,
• muscular fatigue,
• declining glycogen stores,
• dehydration,
• electrolyte losses such as sodium.

Sodium may help in certain situations, but it is not the sole cause of, or solution to, muscle cramps.

Individualisation is better than general recommendations

The most important conclusion from current research is:

There is no hydration strategy that works equally well for everyone.^{2, 3, 5}

How much fluid and sodium you need depends on several individual factors:

• body weight,
• individual sweat rate,
• sodium concentration in sweat,
• ambient temperature and humidity,
• exercise intensity,
• exercise duration,
• acclimatisation and training status,
• individual gastrointestinal tolerance.

Athletes who know their sweat rate and regularly test their hydration strategy during training are better prepared than those who rely exclusively on general recommendations.^{1, 3, 9}

Carbohydrates in your drink: Combining hydration and energy

In addition to fluid quantity, the composition of the drink also plays an important role. Carbohydrates provide energy and can help maintain performance over many hours.

For prolonged endurance exercise, 60–90 g of carbohydrates per hour is currently considered a useful target range. Higher amounts are increasingly being discussed, but evidence supporting them during ultra-distance exercise remains limited. The most important objective is to maintain a consistent intake and adapt the quantity to individual tolerance.

Use a weaker mixture in hot conditions

High temperatures change the priorities.

When fluid intake is the main priority, it is advisable to reduce the carbohydrate concentration of the drink. Approximately 30–40 g of carbohydrates per bottle allows faster gastric emptying and improves fluid absorption. The remaining carbohydrates can be consumed through gels, bars, or easily digestible foods.

Highly concentrated drinks, particularly those containing more than approximately 10 % carbohydrate, can slow gastric emptying. As a result, both water and carbohydrates enter the body more slowly. This may lead to increased thirst, stomach discomfort, or nausea.

Practical tip: On hot days, a slightly weaker drink mixture is often the better option. Consume any remaining carbohydrates through gels or solid food instead of making the drink increasingly concentrated.

In moderate temperatures

In cool or moderate conditions, both fluid and carbohydrates can be consumed through the drink without difficulty. The strategy that works best depends primarily on individual tolerance and personal preference.

Multi-day bikepacking trips

As exercise duration increases, nutrition often changes as well.

During high-intensity races, liquid or easily digestible carbohydrates often dominate. During multi-day bikepacking trips or ultra events, the proportion of solid food usually increases. The lower intensity improves blood flow to the digestive system, meaning solid foods are often tolerated better than they are at the beginning of an effort.

Conclusion

Current research clearly shows that there is no perfect hydration strategy.

The key is to adapt your fluid intake to your sweat rate, temperature, exercise duration, and intensity. Mild fluid loss during prolonged exercise is normal and does not need to be fully corrected immediately. At the same time, drinking more is not automatically better. Excessive fluid intake can be just as problematic as significant dehydration.

A nuanced approach is also required when it comes to sodium. For most riders, a balanced diet and a standard sports drink are sufficient. A targeted sodium strategy only becomes more important during very long efforts involving high sweat rates and large fluid intakes.

The most important recommendation is therefore:

Train your hydration strategy just as you train your legs.

Athletes who know their sweat rate, test different conditions during training, and regularly practise their nutrition strategy will begin long bikepacking trips or ultracycling races with significantly better preparation than those who rely exclusively on general recommendations.

References

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3. Maughan, R. J., & Shirreffs, S. M. (2008). Development of individual hydration strategies for athletes. International Journal of Sport Nutrition and Exercise Metabolism.
4. Montain, S. J. (2008). Hydration recommendations for sport 2008. Current Sports Medicine Reports.
5. Jeukendrup, A. (2024). Does dehydration reduce performance? Review article.
6. Maughan, R. J., & Shirreffs, S. M. (2010). Development of hydration strategies to optimize performance for athletes in high-intensity sports and in sports with repeated intense efforts. Scandinavian Journal of Medicine & Science in Sports.
7. Von Duvillard, S. P., Braun, W. A., Markofski, M., Beneke, R., & Leithauser, R. (2004). Fluids and hydration in prolonged endurance performance. Nutrition.
8. Ly, N. Q., Hamstra-Wright, K. L., & Horswill, C. A. (2023). Post-Exercise Rehydration in Athletes: Effects of Sodium and Carbohydrate in Commercial Hydration Beverages. Nutrients.
9. Armstrong, L. E. (2021). Rehydration during Endurance Exercise: Challenges, Research, Options, Methods. Nutrients.
10. McCubbin, A. J., & Costa, R. J. S. (2024). Effect of Personalized Sodium Replacement on Fluid and Sodium Balance and Thermophysiological Strain During and After Ultraendurance Running in the Heat. International Journal of Sports Physiology and Performance.
11. McCubbin, A. J. (2023). Modelling sodium requirements of athletes across a variety of exercise scenarios. European Journal of Sport Science.