If you engage in regular cardio exercise, whether you’re a group fitness class participant trying to lose weight or a serious competitive triathlete trying to win your age group, you’ve been taught about the importance of workout hydration. And not only have you been taught about it, but you’ve acted on it, taking appropriate steps to prevent dehydration and limit its negative effects on your workouts. The only problem is that what you’ve been taught is wrong, and chances are you’re not hydrating your workouts effectively despite your good intentions.
Believe it or not, before the late 1960s, athletes rarely drank during exercise. Even elite runners would complete entire marathons without swallowing a drop. This changed quickly, however, when the first sports drinks hit the market. Along with these products came a flood of scientific research demonstrating that drinking during exercise not only limited dehydration itself but helped body temperature regulation and even enhanced performance.
A seminal study led by sports drink inventor Robert Cade found that subjects undertaking a seven-mile walk in a hot environment stayed cooler and finished 3 percent faster when they drank a sports drink than when they drank nothing.
Studies like this one led to the notion that people should aim to completely prevent dehydration during exercise, which requires drinking the same amount of fluid that is lost through sweating. But later research called this idea into question. What scientists discovered was that, in many circumstances, it’s really hard — if not impossible — to drink enough to completely prevent dehydration during exercise. Forcing it results in symptoms of gastrointestinal distress such as sloshing, bloating, and nausea.
When individuals are allowed to choose their own drinking rate during exercise, they typically replace only 30 to 70 percent of the fluid they lose through sweating. As a result, they become dehydrated — not as dehydrated as they would if they didn’t drink at all, but more than they would if they tried to drink enough to match their sweat losses. However, recent studies have shown that people perform better when they drink according to their thirst than they do when they are required to drink more, despite being more dehydrated when they finish.
In a 2012 study conducted by English researchers, experienced runners completed a 10-mile time trial 58 seconds faster, on average, when they drank by thirst than when they drank more. And when they drank more, their ratings of gastrointestinal discomfort were much higher.
These findings jive with data from field studies involving athletes in real races. In 2015, French scientists reported that participants in a half-ironman triathlon lost more than 8 pounds between the start and finish due to failure to fully replace fluid lost through sweating. But there was no relationship between how dehydrated athletes became individually and their performance.
Does this mean that dehydration doesn’t matter? Not exactly. It just means that becoming a little dehydrated during exercise is not as costly as drinking more than you can tolerate in an effort to completely prevent dehydration.
In the future it may be possible to bolster the subjective perception of thirst with objective measures that will enable exercisers to pinpoint their own personal optimums for hydration level and drinking rate in different circumstances. For example, wearables that measure hydration in real time may enable you to gather data that demonstrates the drinking rate at which you perform best in a certain type of workout.
In the meantime, it’s important to recognize what most exercisers overlook: that your hydration status at the start of exercise matters more than your hydration status at the end of exercise.
Research has consistently shown that regardless of how little or how much people drink during exercise, they stay cooler and perform better if they are fully hydrated at the start. In a 2013 study, for example, Greek researchers found that cyclists took 5.8 percent longer to complete a 5 km hill climb when they started it in a state of mild dehydration compared to when they started it fully hydrated.
Here’s the bottom line: When it comes to hydrating your workouts, where you start is more important than how you finish. By all means, drink throughout exercise. This will help you stay cooler and feel and perform better. But give equal attention to your hydration during the rest of the day so that you begin your workouts full hydrated and ready to give your best effort.
Cade R, Spooner G, Schlein E, Pickering M, Dean R. J Sports Med Phys Fitness. Effect of fluid, electrolyte, and glucose replacement during exercise on performance, body temperature, rate of sweat loss, and compositional changes of extracellular fluid. 1972 Sep;12(3):150–6.
Daries HN, Noakes TD, Dennis SC. Effect of fluid intake volume on 2-h running performances in a 25 degrees C environment. Med Sci Sports Exerc. 2000 Oct;32(10):1783–9.
Passe D, Horn M, Stofan J, Horswill C, Murray R. Voluntary dehydration in runners despite favorable conditions for fluid intake. Int J Sport Nutr Exerc Metab. 2007 Jun;17(3):284–95.
Roll I, James L, Croft L, Williams C. The effect of carbohydrate-electrolyte beverage drinking strategy on 10-mile running performance. Int J Sport Nutr Exerc Metab. 2012 Oct;22(5):338–46. Epub 2012 Jul 4.
Baillot M, Hue O. Hydration and thermoregulation during a half-ironman performed in tropical climate. J Sports Sci Med. 2015 May 8;14(2):263–8. eCollection 2015.
Bardis CN, Kavouras SA, Arnaoutis G, Panagiotakos DB, Sidossis LS. Mild dehydration and cycling performance during 5-kilometer hill climbing. J Athl Train. 2013 Nov-Dec;48(6):741–7. doi: 10.4085/1062–6050–48.5.01. Epub 2013 Aug 16.