How Dehydration Affects Exercise Performance

How Dehydration Affects Exercise PerformanceAll athletes have to be concerned about hydration. Exercise performance declines even with modest levels of fluid loss and rises steadily as dehydration levels climb above 2%. Athletes can’t depend on their sense of thirst to tell them when to drink since the urge to drink happens only after fluid levels have already fallen. If you’re exercising and not replacing fluids, it can reduce performance and make exercise feel harder, and if you continue to lose fluid, it can lead to lightheadedness, dizziness, and loss of consciousness. How does dehydration affect your workout, and why is it so important to stay hydrated?

Hydration and Sweating: Why Some People Sweat More Than Others

Everyone sweats at different rates. In general, males have a higher sweat rate than females when they work out at the same intensity, and more conditioned athletes sweat earlier in a workout than less conditioned ones. Genetics play a role, and older people sweat less than younger people when they exert themselves. The environment is also a factor. You’ll sweat more exercising in a hot environment and feel more uncomfortable working out in a humid environment since sweat won’t evaporate as easily from the surface of the skin when there’s too much moisture in the air.

There is a way to estimate how much you sweat during a workout. Weigh yourself without clothing before starting your workout. After exercising for one hour and before consuming fluids or urinating, measure your weight again without clothing. For each pound of weight, you’re down, you lost about 16-ounces of fluid. Of course, your sweat rate will vary depending upon the activity you’re doing and the environmental temperature. So you’ll only get an estimate of how much sweat you’re losing doing a certain activity at a certain temperature.

What Happens if You Don’t Drink Enough Fluid?

Exercise performance starts to suffer at a dehydration level of about 2%. As you reach this level, your core body temperature starts to increase, your heart rate goes up and exercise starts to feel harder. Dehydration also depletes glycogen stores faster, which further affects exercise performance. As more fluid is lost and not replaced, performance degrades even more and exercise feels even more difficult.

Not replacing fluids also increases the risk of heat-related illnesses such as heat stroke and heat exhaustion and contribute to painful muscle cramps that can send you limping to the sidelines in pain.

How to Avoid Dehydration

Start hydrating three to four hours before a workout by drinking 18 to 20 ounces of fluid and eating a salty snack. The sodium helps to hold onto the fluids you’re taking in and stimulates thirst. Drink another 6- 8 ounces of fluid right before exercising and an additional 10 to 20 ounces every 30 minutes while working out. If you’ll be working out for longer than an hour, hydrate with a sports drink to replace sodium and electrolytes. Sodium levels can drop if you sweat profusely, and don’t replace it with a sodium-containing beverage or salty snack. After exercise, drink 16 to 20 ounces of fluid for every pound of weight you’ve lost.

The Bottom Line?

Dehydration makes exercise feel harder and reduces performance. Plus, it increases the risk of heat-related illness and muscle cramps and is a risk to your health. Make fluid replacement a priority when you work out.



American College of Sports Medicine. “Exercise and Fluid Replacement”
Journal of Applied Physiology January 1996 vol. 80 no. 1 363-366.
ACE Fitness. “Healthy Hydration”


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