How much fluid should I drink during running?

Hydration is a critical aspect of athletic performance that is often overlooked by runners. While it can be tempting to focus on expensive sports drinks and gels, the truth is that optimizing your fluid intake may have a much greater impact on your running performance. But how much should you drink, and when? In this blog post, we'll take a closer look at the importance of fluid for runners, the effects of over- and under hydration on performance, and how to develop a customized fluid replacement plan based on your own unique needs.

Why is fluid so important for runners?

Water is essential for maintaining body temperature and blood volume, transporting nutrients, and removing waste products from your body. During exercise, your body loses water through sweat, and it's crucial to replace enough fluid to maintain proper hydration levels. With changes in conditions, such as temperature, humidity, exercise intensity and clothing, our sweat rate also changes. This means that to maintain hydration, our fluid requirement will be different during every run.

What are the risks of under- and overhydration?

Dehydration happens when you fail to replenish fluids lost through sweating. This leads to a drop in blood volume, increased heart rate, fatigue, and increases risk of gastrointestinal symptoms such as nausea and diarrhea. Dehydration also increases muscle glycogen use and decreases sweat rate meaning you burn through fuel faster and have increased body temperature. It is crucial to replace fluids, but replacing all losses may not be necessary. For runs lasting up to 3 hours, a 2% body weight reduction is considered acceptable, while for longer events, there is no consensus on how much weight loss is acceptable to avoid performance decline. Losses in excess of 5% of body weight can decrease the capacity for work by about 30%. However, studies have shown that runners who lost up to 5% body weight during a 100-mile race had normal hydration markers. It is important to note that weight loss during exercise greater than 3 hours is due to fluid losses but also substrates losses (muscle glycogen and fat). Dehydration is more common in intense exercise, hot and humid conditions, and when there is limited fluid availability, for example a marathon in the heat.

Overhydration, on the other hand, is less common but can be a problem for ultra-endurance runners. Drinking more fluid than required can lead to hyponatremia (low blood sodium), which can cause fatigue, dizziness, nausea, vomiting, headache, weakness, and swelling of hands and feet. Severe cases can lead to coma and death, though thankfully such instances are rare. Up to 30% of athletes in ultramarathons and Ironman triathlons who have had a blood test after their event, have been found to have hyponatremia. The risk of hyponatraemia is higher in females and slower athletes.

If you are experiencing any symptoms associated with dehydration or overhydration, it is worth checking your fluid intake to ensure that it is at an optimal level for your body's needs.

How do I get the balance of fluid right?

Developing a personalized fluid replacement plan is essential for runners and should be started at least a few weeks before a key event.

To assess fluid losses during exercise lasting up to 3 hours, it is recommended to weigh yourself before and after you run. For exercises lasting beyond this time frame, fuel use will contribute to weight loss, making it difficult to determine how much fluid has been lost. When conducting a sweat rate test, it is important to simulate conditions expected during the event, including pace, weather, clothing, food, and fluid intake. Before the test, prepare for you run, use the toilet, record your weight then head out for your session. As soon as you complete exercise, use the bathroom, and then check your weight again. The change in weight reflects fluid losses (i.e. 1kg = 1L), accounting for any fluid and food consumed and toilet stops taken. Repeat this several times under different weather conditions to determine the amount of fluid you need to replace.

Understanding your personal cues for drinking fluid is also essential, whether it's to drink to thirst or a planned strategy. Some runners are good at tuning into thirst, while others find it challenging to drink on the run, take insufficient fluid, or forget to drink. Even if drinking to thirst is appropriate, a sweat rate test is useful to determine how much fluid to carry during the race or pick up from support crew/aid stations.

What are the best fluids to drink?

The most suitable fluid choice for runners varies depending on a range of factors, such as personal preference, exercise duration, gut tolerance, practicality, fluid availability and fuel options. For endurance events lasting over 60-90 minutes, sports drinks are a great option because they have a high carbohydrate content, and the sodium in them enhances the flavour of the drinks, which can encourage fluid intake. In addition to sports drinks, juice, cordial, and soft drinks can also be appropriate choices for longer duration events. For shorter events, water is generally sufficient. Most importantly, practice your fluid choices and fluid replacement plan in training to determine what is the best option for you.

Summary

For runners, fluid intake is crucial to maintain performance levels and avoid dehydration or overhydration. Finding the optimal amount of fluid intake is key to running faster and farther. It's essential to assess your fluid needs well in advance of a race or event to ensure you are adequately hydrated on the big day. Since there is no universal solution for everyone, experimenting with different fluid intake levels during training can improve your chances of achieving a personal best in your next race.

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References

American Council on Exercise 2021, 'Fluid replacement during exercise', https://www.acefitness.org/education-and-resources/lifestyle/blog/7034/fluid-replacement-during-exercise/.

Casa, DJ et al 2000, 'National Athletic Trainers' Association position statement: Fluid replacement for athletes', Journal of Athletic Training, vol. 35, iss. 2, pp.212–224.

Sawka, MN et al 2007. 'AmericanCollege of Sports Medicine position stand. Exercise and fluid replacement', Medicine and Science in Sports and Exercise, vol. 39 iss. 2, pp. 377–390.

Hew-Butler T, Rosner MH, Fowkes-Godek S, et al 2008. 'Statement of the Second International Exercise-Associated Hyponatremia Consensus Development Conference, Clin J Sport Med, vol.18, iss. 2, pp.111-121.

Jeukendrup AE 2011, 'Nutrition for endurance sports: marathon, triathlon, and road cycling' J Sports Sci., vol. 29 Suppl 1:S91-S99.