Water. It makes up 60% of the human body and is arguably one of the most important nutrients that we need.  Without it, we cease to exist in approximately three days. Every cell in the body relies on water to function, it acts as a lubricant for joints, regulates body temperature, and helps rid our bodies of waste.

Every individual is constantly losing water through simple acts such as breathing and going to the bathroom.  If you’re an active individual, like a runner or cyclist, you are losing even more water through sweat. This is why it is important for all athletes to stay hydrated. Failure to do so can be fatal as extreme dehydration can create drastic decreases in blood pressure and send your body temperature through the roof.

The Most Common Way Endurance Athletes Hydrate

In the past endurance athletes have been told to drink on a “schedule” during exercise. This method involves taking sips of fluid every 10-15 minutes that usually equates to consuming 16-32 ounces of water, sports drink, or other fluid per hour.  According to the experts, doing so will prevent a 2% reduction in body weight through water lost via sweating which reduces the athletes chance of overheating and having a negative impact on their performance.

Without a doubt, this method of hydration is effective for endurance athletes. However, there is an emerging body of research that demonstrates that “drinking to thirst” may be just as effective and not have a negative impact on endurance performance.

What Does Drinking To Thirst Mean?

As the name implies, drinking to thirst simply means drinking fluid whenever you feel thirsty and stopping when you’re not. Critics of this method have claimed that our thirst mechanism is a poor indicator of when we need to hydrate and will lead to severe hydration in endurance athletes.  But the research does not support that conclusion (we’ll cover that in a minute).

Drinking to thirst does offer some advantages over hydrating on a set schedule. First, it takes the guesswork out of how much and when you should be drinking fluids. Second, you minimize your chance of hyponatremia, the condition where consuming too much fluid (specifically water) can throw your electrolyte balance out of whack and kill you.  

Is It Better To Hydrate On A Set Schedule Or Drink To Thirst?

By now you are probably wondering what hydration method is best. This is what scientists from the University of Sherbrooke in Canada and the University of California Davis Medical Center set out to discover. They wanted to know if endurance athletes perform better when they drink as much as they want, or when they follow a pre-planned hydration strategy aimed at minimizing fluid losses.

The Research On Drinking To Thirst Or Following A Pre-Planned Hydration Strategy

The research, done by Eric Goulet and Martin Hoffman published in 2019 and titled “Impact of Ad Libitum Versus Programmed Drinking on Endurance Performance”, was a systematic and meta-analysis of seven publications that examined ad libitum (ALD = drinking to thirst) versus programmed drinking (PD) on endurance performance.

The eligibility criteria for papers included in the meta-analysis were:

1. Experiments were conducted in a controlled setting (i.e. a lab).
2. The exercise had to be equal to or greater than 60 minutes.
3. Subjects in the studies were in a hydrated state before the start of exercise.
4. Programmed fluid intake during exercise was greater than ad libitum.
5. The electrolytes found in fluids consumed were the same.
6. Carbohydrate intake varied by > 25% between conditions when the exercise was 1 h and matched for exercise > 1 h.

What Were The Results Of The Study Comparing Drinking To Thirst Versus Hydrating On A Set Schedule?

After conducting an in-depth analysis of the seven publications used in the meta-analysis the scientists produced 8 effects estimate involving a total of 82 subjects that included runners and cyclists. These are:

• The mean (± standard deviation) temperature, exercise intensity and length of the experiments were 28 ± 6 °C, 81 ± 12% of maximal heart rate and 96 ± 25 minutes.
• Mean rate of fluid intake for the PD and ALD conditions was 1073 ± 247 milliliter an hour and 505 ± 156 milliliter an hour.
• Mean change in body mass for the PD and ALD conditions was - 1.0 ± 0.5% and - 2.1 ± 0.7%.
• Compared with PD, ALD improved endurance performance by 0.98 ± 0.44% (95% confidence interval 0.11-1.84%).

What does this all mean? In the words of the researchers, “Despite ALD being associated with an hourly rate of fluid consumption half as much as PD, and resulting in a dehydration level considered sufficient to impair EP, both strategies were found to similarly impact 1-2 h cycling or running performances conducted at moderate to high intensity and under temperate to warm ambient conditions.”

So In Non-Geek Speak What Does This All Mean?

Although drinking to thirst improved endurance performance more than programmed drinking, the difference was so trivial it probably doesn't matter too much what method you use to hydrate during endurance exercise. Unless you drink waaaayyyyy too much fluid (again water) that could lead to hyponatremia.  This is why it is recommended to drink an electrolyte based sports drink for any endurance exercise lasting over 1 hour.

How Much Total Fluid Endurance Athletes Should Drink On A Daily Basis

Ryan-Smith and Antonio (2013) suggest total daily fluid amounts will be different for individuals based on the type of athlete, body weight, gender, exercise length and intensity, sweat rate, and type of climate the individual lives/exercises in.

While this makes it difficult to recommend an EXACT amount of fluid daily to promote adequate hydration, a general guideline of 13 -20 ounces of fluid every 2-3 hours will ensure the athlete is adequately hydrated prior to exercise.  

The Best Way To Hydrate Before Exercise

16 to 20 ounces of water or sports drink 2 to 3 hours before exercise and 7 to 10 ounces of water or sports drink 10 to 20 minutes before exercise. This will ensure optimal pre-exercise hydration.

The Best Way To Hydrate After Exercise

Roberts recommends 150% of weight loss replacement with fluid after exercise.  This equals 50 ounces of fluid replaced for each kg of weight loss. It is also important to note, the fluid used to rehydrate after exercise should include carbohydrates and protein to replenish muscle glycogen stores and promote muscle repair.

The Bottom Line On Hydration And Endurance Exercise

Whether you choose to drink to thirst or hydrate on a set schedule, the fact remains that endurance athletes must consume fluids during exercise to perform their best. When exercise is an hour or less water will suffice. As exercise goes beyond 1 hour, an electrolyte/carbohydrate sports drink is recommended.

References:

• Sawka, M. N., Burke, L. M., Eichner, E. R., Maughan, R. J., Montain, S. J., & Stachenfeld, N. S. (2007). American College of Sports Medicine position stand. Exercise and fluid replacement. Medicine and science in sports and exercise, 39(2), 377-390.
• Goulet, E. D., & Hoffman, M. D. (2019). Impact of Ad Libitum Versus Programmed Drinking on Endurance Performance: A Systematic Review with Meta-Analysis. Sports Medicine, 1-12.
• Hew-Butler, T., Loi, V., Pani, A., & Rosner, M. H. (2017). Exercise-associated hyponatremia: 2017 update. Frontiers in medicine, 4, 21.
• Smith-Ryan, A., & Antonio, J. (Eds.). (2013). Sports Nutrition & Performance Enhancing Supplements. Linus Learning.
• Casa, D. J., Armstrong, L. E., Hillman, S. K., Montain, S. J., Reiff, R. V., Rich, B. S., ... & Stone, J. A. (2000). National Athletic Trainers' Association position statement: fluid replacement for athletes. Journal of athletic training,35(2), 212.
• Roberts W. FIMS Position Statement. 2012 Fluid replacement for sports safety and performance. International Sportmed Journal, June 2012;13(2):39-42.