Proper hydration becomes even more crucial as the summer heat kicks in, activities are taken outdoors, and the days get longer. Whether you lift weights, do endurance, or are an outdoors enthusiast, knowing the signs of dehydration and having a hydration plan will ensure you perform your best. Even moderate dehydration can negatively impact your physical and cognitive performance.

Aside from the performance benefits of proper hydration, positive medical outcomes such as biologically younger age, longer lifespan, and absence of chronic diseases have been correlated with lifelong, consistent hydration (2).

Symptoms of Dehydration

Dehydration occurs when your body loses more water than it takes in. It negatively affects many physiological functions, such as heart rate and thermoregulation. Knowing the symptoms is crucial to recognize when it is time to rehydrate.

What Effects Hydration?

There isn’t a one-size-fits-all approach to hydration because many variables affect it. Older adults tend to get dehydrated quickly. Men tend to need more water than women. Larger-bodied individuals tend to get dehydrated easier. Aside from all these biological factors, some people have higher sweat rates than others, leading to quicker dehydration.

Age
Gender
Body size
Sweat rate

In addition to biological factors, environmental factors can negatively impact hydration status. It is vital to consider the following factors and plan for more water if they are present.

High temperature and/or humidity level
Low or no availability of shade
Significant elevation gain
Significant load carried
Time of day when the sun is the strongest

Hydration and Performance

Dehydration, which results in losing 2-4% of body weight, negatively impacts endurance and muscle force generation. Cognitive performance also begins to be negatively affected at a loss of 2% body mass from sweat in tasks involving attention, executive function, and motor coordination (3, 5, 6). Mood is also negatively affected by dehydration at a loss of 3-5% of body mass (3, 6).

Aside from performance decreases, the risk for overheating, heat sickness, and heat stroke increases as dehydration increases, putting one at medical risk (4). However, not all activities are at the same risk for dehydration. Low- to moderate-intensity events, such as running over 1 hour, are usually at a higher risk of dehydration than high-intensity short events, such as lifting weights (4).

Guidelines

Before Exercise

The American Council on Exercise recommends drinking about 0.1 oz/ lb (6ml/kg) before exercise begins, ideally at least 4 hours before, to give the body time to absorb the water (20 oz for a 200-lb individual) (1). Drinks and snacks containing sodium can help the body retain fluid. Ensure you don’t start exercising when you are thirsty, but don’t overcompensate; otherwise, unexpected bathroom breaks may occur.

During Exercise

Most research shows that performance begins to decrease once more than 2% of body weight is lost. Therefore, aim not to lose more than 2% of body weight. To monitor sweat loss, measure your before- and after-exercise weight and add urine output and water input, if applicable. With this knowledge, seek to replace every ounce lost to sweat with one ounce of water intake.

If this measurement is impractical, try to drink between 8 and 16 oz of water per hour of activity, adjusting for more water intake if the variables in the What Effects Hydration section are present, such as hotter temperatures (1). For events lasting more than two hours, consume a drink with electrolytes.

After Exercise

If you have at least 12 hours before your next vigorous workout (1), rehydrate using your regular eating and drinking behaviors. If you have less than 12 hours before another vigorous workout, they recommend drinking 0.70 L of fluid per pound of weight lost from sweat during exercise (1).

General Hydration Tips

Invest in a nice water bottle that makes you happy. You’ll be more likely to use it, similar to how a child will be more likely to wear a helmet if they pick out a cool one they like.
Keep water in sight all day long. This will keep you hydrated throughout the day and reduce your risk of dehydration.
Flavor your water with a tea bag, a supplement, or another source of flavor that you enjoy.
When planning how much water to bring, consider environmental (humidity, shade, elevation gain, etc.) and physiological factors (body size, sweat rates, etc.).

Sources

Bryant, C. X., Jo, S., Dalleck, L., Gagliardi, C. S., & Green, D. J. (2020). The Exercise Professional’s Guide to Personal Training: A Client-centered Approach to Inspire Active Lifestyles.
Dmitrieva NI, Gagarin A, Liu D, Wu CO, Boehm M. Middle-age high normal serum sodium as a risk factor for accelerated biological aging, chronic diseases, and premature mortality. EBioMedicine. 2023 Jan;87:104404.
Dube, A., Gouws, C., & Breukelman, G. (2022). Effects of hypohydration and fluid balance in athletes’ cognitive performance: a systematic review. African health sciences, 22(1), 367–376. https://doi.org/10.4314/ahs.v22i1.45
https://www.nsca.com/education/articles/kinetic-select/hydration-and-performance/
Judge, L. W., Bellar, D. M., Popp, J. K., Craig, B. W., Schoeff, M. A., Hoover, D. L., Fox, B., Kistler, B. M., & Al-Nawaiseh, A. M. (2021). Hydration to Maximize Performance and Recovery: Knowledge, Attitudes, and Behaviors Among Collegiate Track and Field Throwers. Journal of Human Kinetics, 79, 111-122. https://doi.org/10.2478/hukin-2021-006
Wittbrodt, M. T., & Millard-Stafford, M. (2018). Dehydration Impairs Cognitive Performance: A Meta-analysis. Medicine and science in sports and exercise, 50(11), 2360–2368. https://doi.org/10.1249/MSS.0000000000001682