It seems like phrases like "stay hydrated" and "drink plenty of water" are some of the lowest-hanging fruit when it comes to health and wellness advice. But what does that mean? How much water do you really need during exercise? In short, it ultimately depends on the duration of your workout and how much you sweat.
Adequate hydration is key in regulating body temperature, lubricating joints, and transporting nutrients throughout your body. But when we exercise, we sweat, increasing our hydration demand and thereby making our pre-, mid-, and post-workout water intake essential. We’ve talked about the importance of various nutrients for fueling your training sessions. Here we’ll talk fluids to complete the picture.
The science behind sweat
When we exercise, a large amount of energy is created as we metabolize fuel, which our body takes on as heat. And if you’re exercising in an environment where the ambient temperature is higher than skin temperature (hello, summer!), this additional heat will be transferred to the skin. This causes an increase in the body’s core temperature – which signals to your brain that it's time to start figuring out how to cool down. The eccrine glands then kick in to produce sweat!1 When this sweat evaporates from our skin, it takes some heat energy with it, ultimately preventing complications like heat cramps, heat exhaustion, and heat stroke.
In addition to water losses, we lose electrolytes in our sweat, particularly sodium and, to a lesser extent, potassium. The amount of sodium lost will vary depending on things like intensity and duration of an activity, environmental conditions, body size and composition, sex, age, diet and hydration status.1
Hydration and performance
It probably goes without saying, but in order to stay hydrated and stay safe during your workouts, you need to drink water!2 Significant changes in the fluid and electrolyte balance in your body can have detrimental effects on a wide range of body processes, including your ability to exercise. Dehydration of just 2-4% has been shown to reduce VO2max and impair aerobic performance.3 Even a small amount of dehydration, just 1%, can elevate heart rate disproportionately and limit the body’s ability to regulate your body temperature.2 Failing to meet your hydration requirements reduces aerobic capacity, quickens the onset of fatigue, and produces a greater risk of developing heat-related injury, and the more dehydrated you become, the greater the risk of these effects.
Early signs of dehydration include:
- Increased body temperature
- Early onset of fatigue in exercise
- Faster heart rate and breathing than expected
- Increased rate of perceived exertion (RPE)
- Decreased exercise capacity
Later signs of dehydration include:
- Labored breathing
Guidelines for fluid and water intake
Heading into a workout already at a fluid deficit can negatively impact both the duration and power output that you’re able to put into a workout. Drinking about 500 mL (~17 ounces) of water around 2 hours before a workout promotes adequate hydration and allows enough time to hit the bathroom pre-workout if needed.3
Start drinking early in your session! The aim is to drink enough fluid to account for what’s lost from sweat. Performing a simple sweat test can help you determine what your exercise sweat rate is. If you’re training in hot, humid, or sunny conditions, your sweat rate will increase – along with your fluid needs.3
For exercise over one hour, these fluids should include both carbohydrates and electrolytes, specifically sodium. Aim for 600-1200 mL of sports drink that contains 4-8% carbohydrates to maintain blood glucose levels and delay fatigue, plus 500-700mg of sodium per liter to replace sweat losses and prevent hyponatremia.3
Most athletes tend to drink insufficient amounts of fluid post-session to make up for their sweat losses. But this is an important time to rehydrate! If you’re recovering from (or prepping for) a big session or hot conditions, staying on top of your fluid intake during the day is key. A simple way to assess this is your urine color in the morning-- straw or lemonade color is a sign of hydration while dark urine is indicative of inadequate hydration.
The warm and sunny summer days are upon us and that means more sweating, especially in our training sessions! Staying on top of your fluids will not only prevent potential health complications, they’ll also help you make the most out of your training sessions. Most importantly, be aware of the early signs of dehydration and listen to your body.
Some other blog posts we think you'll love:
 Baker LB. Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability. Sports Med. 2017;47(Suppl 1):111–128. doi:10.1007/s40279-017-0691-5
 Convertino V, Armstrong L, Coyle E et al. ACSM Position Stand: Exercise and Fluid Replacement. Medicine & Science in Sports & Exercise. 1996;28(10):i-ix. doi:10.1097/00005768-199610000-00045
 Cheuvront S, Kenefick R, Montain S, Sawka M. Mechanisms of aerobic performance impairment with heat stress and dehydration. J Appl Physiol. 2010;109(6):1989-1995. doi:10.1152/japplphysiol.00367.2010