From bad breath to car accidents, dehydration is a real health threat

Even being slightly dehydrated affects your ability to put effort into your workout. "A 2% dehydration level in your body causes a 10% decrease in athletic performance," says Goodson. "And the more dehydrated you become, the worse performance gets." Measured by "perceived exertion," how hard you feel you're exercising, you might be working at a 6 but you feel like you are working at an 8, says Goodson.

Keeping skin healthy and glowing requires drinking enough water, says Anne Marie Tremain, MD, a dermatologist with Laser Skin Care Center Dermatology Associates in Long Beach, Calif. "It's best to hydrate from the inside out," she says. "Depending on your lifestyle you may need to adjust your water intake." If you work out every day or are a caffeine fiend, for instance, then you'll need to drink more., because workouts make you sweat and caffeine is a diuretic, which can dehydrate you. For smooth, moisturized skin, Dr. Tremain also suggests keeping showers short (less than five minutes) and using only lukewarm water as hot water can dry your skin out even more.

Few things are more uncomfortable than being stuck in traffic or on a long drive when you need to use the restroom. Logically, it makes sense to simply not drink water before hitting the road. But new research published in Physiology and Behavior shows that the number of driving errors doubled during a two-hour drive when drivers were dehydrated versus hydrated—an effect similar to driving while drunk (defined by most states as .08% blood alcohol). Since often people purposely avoid drinking prior to a long road trip to prevent bathroom stops, dehydration could increase the risk of traffic accidents.

A mid-afternoon slump may have more to do with hydration than you think. "When you're dehydrated your blood pressure drops, heart rate increases, blood flow to the brain slows -- all of which can make you tired," says Luga Podesta, MD, sports medicine specialist at Kerlan-Jobe Orthopaedic Clinic in Los Angeles, Calif. A lack of water to muscles also makes physical tasks feel more difficult and tiring.

Cranky much? Drink a glass of water and your mood may change. "Neurological effects of dehydration can cause irritability," says Dr. Podesta. A small study published in the Journal of Nutrition tested mood and concentration in 25 young women who were either given enough fluids to remain properly hydrated, or who became mildly dehydrated by taking diuretics and exercising. The dehydrated women—who were at a level that was just 1% lower than optimal—reported headaches, loss of focus, and irritability.

It may seem counterintuitive, but dehydration can bring on chills. "This occurs because your body starts to limit blood flow to the skin," says Dr. Podesta. In addition, water holds heat, so if you become hydrated it can be more difficult to regulate your body temperature, which can make you become chilled faster, even when you're not in a cold environment.

A lack of water causes less blood circulation, which can make muscles cramp up, says Ray Casciari, MD, medical director of the La Amistad Family Health Center in Orange, Calif. "The body will protect its vital organs, so it shifts fluid away from muscles and anything that's not vital," he says. Muscle cramps can be extremely painful, making muscles feel harder than normal to the touch. Changes in sodium and potassium through sweat loss can also contribute to cramping.

Your body needs water to keep things moving through your colon. When you're not getting enough H2O, your body compensates by withdrawing more fluid from stool, making it harder and more difficult to pass. That said, it's worth noting that drinking more water when you're already properly hydrated won't necessarily relieve constipation caused by other factors, like the medications you're taking, medical conditions, or a lack of fiber in your diet.

Along with muscles, your brain also gets less blood circulation when you're low on water, which can make you dizzy, says Dr. Casciari. Additionally, mild dehydration may affect your ability to take on mental tasks and cause you to feel foggy headed, according to a study from the British Journal of Nutrition. Interestingly, a study that appeared in the Journal of Nutrition showed greater mood changes in women than in men, both at rest and during exercise.

Dehydration can cause headaches in a couple of different ways. "Lack of water affects your body's serotonin levels, which can give you headaches," says Dr. Casciari. In addition, small blood vessels in the brain respond quickly to hydration levels (which is also behind hangover headaches), leading to dull aches and even full-blown migraines. Try downing a glass or two of water the next time you have a headache and you may discover it disappears. You could also eat fruit, which contains a high percentage of water, Dr. Casciari suggests.