If you're still holding onto the notion that eight cups of water a day is sufficient (I bet you also still believe that you need to wait 30 minutes after eating to go for a swim), you're not working at your body's peak performance level.

"Water makes up about 60-70% of our body and plays a role in virtually every function, from keeping our blood flowing and skin healthy right down to our ability to blink our eyes," says Wendy Bazilian, Dr.P.H., R.D., author of The SuperFoodsRx Diet. "Water is our most important nutrient."

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Sure, hydration is important to survival, but if you are healthy and have access to clean water, death by dehydration probably isn't a major fear. So why should you be concerned with hydration? Even mild dehydration can begin to compromise your health and stifle the effectiveness of your workout. Here are the top four reasons to make sure you stay hydrated at all times.

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1) Weight Loss
A few years ago Michael Boschmann, M.D., and colleagues from Berlin's Franz-Volhard Clinical Research Center found that after drinking about 17 ounces of water, the metabolic rates of the study's participants increased by 30%. And the researchers estimated that if a person increases his water consumption by 1.5 liters (51 ounces) a day, he would burn an extra 17,400 calories per year. That's about five pounds just from drinking more water. Additionally, a study by Dr. Brenda Davy, an associate professor at Virginia Tech, found that people who drank water before a meal consumed an average of 75 fewer calories at that meal.

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2) Better Mental Clarity
"Poor hydration leads to increased tiredness, reduced alertness, and impaired short-term memory. Good luck with using that as an excuse to your boss. "Even mild dehydration—being only 1-2% dehydrated has shown to impair cognitive performance, like the ability to perform mental tests and skills requiring thinking," says Bazilian.

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3) Greater Results in the Gym
Not seeing gains in the gym? Perhaps a lack of water is to blame. "If you don't hydrate yourself correctly you won't perform very well and you won't see results," says American Council on Exercise spokesperson and exercise physiologist Fabio Comana, M.S. Muscles are about 80% water and dehydration diminishes blood flow to the brain, the delivery of nutrients to your muscles, and slows down your recovery process. Water also helps reduce the chance of injury and pain because of the key role it plays in keeping joints lubricated.

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4) General Appearance
While water may not be the fountain of youth, it will help improve dry skin, cracked lips, and wrinkles. Water is required to make saliva which is the body's best natural defense against tooth decay, and good hydration helps preserve skin's elasticity and tone.

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How Much Do You Need?
The Beverage Institute for Health & Wellness has created a hydration calculator to help you determine your daily water needs based upon your age, weight, health, environment, and activity level, and you can use the Institute of Medicine's recommendation of 3.7 liters (125 ounces) daily for men as a guideline. But if you're physically active, living in a hot climate or taking workout supplements such as creatine, take precautions to consume extra water.

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Choose the Right Water Bottle
Get in the habit of drinking fluids throughout the day. Help yourself and the environment by carrying a reusable water bottle with you. Stylish BPA-free options include the stainless steel Klean Kanteen 27-ounce Classic ($20), the sportier looking CamelBak 1-L Chute ($14), or a  Sigg Active Top ($29).

Bonus Tip: Recent studies have shown that all hydration does not need to come from fluids. Up to 20% of a person's daily intake can come from food sources such as soup, watermelon, spinach, and more.