Easiest Way to Lose Weight: Drink Water

Of all the food and beverage choices you face every day, what's calorie-free, virtually cost-free, and, oh yes, essential to keeping you alive? Plain water. But those aren't the only reasons to drink it.

"Water drives basic body performance," says Beth Reardon, director of nutrition for Duke Integrative Medicine, part of the Duke University Health System. "All of the systems in the body require water for proper functioning, and so do 90 percent of all chemical reactions in the body."

Here are the top five reasons to quench your thirst with water.

It will help you de-stress

Why: Being sure to sip water throughout a stressful day can soothe stress-induced symptoms as diverse as headaches, tense muscles, fuzzy thinking, a pounding heart, and low energy. That's because stress taxes all your basic body systems -- and when you're dehydrated, the effects are magnified.

Given that more than half your body weight is water, Reardon says, "just a 2-percent reduction in hydration has a dramatic impact on energy levels and cognitive function." And dehydration further raises levels of cortisol -- the "stress hormone."

Water won't wash your stressors away. But it can provide you with more energy, ease tension, slow breathing, and reduce the strain on your heart.

Water-drinking tip: "Eight by eight -- eight 8-ounce glasses a day -- is a good general rule of thumb," Reardon says, "but it's a myth that's the magic amount for everyone, because there are so many variables." The "right" amount for you depends on factors including your age, your activity level, your health level, medications you're taking, and the weather. So how do you know if you're drinking enough? Follow your thirst, and know that you're on the right track if you have straw-colored urine, Reardon says.

You'll lose weight

Why: In a 2010 study of adults aged 55 to 75, drinking two 8-ounce glasses of water before meals was associated with almost four pounds more weight loss in 12 weeks than in a control group who ate a similar diet but didn't have the pre-meal H20. Participants drank an average of 1.5 cups of water a day before the study.

In part, the Virginia Tech researchers say, water is filling, so you feel fuller and eat less. An earlier study found those who drink water before meals consume an average of 75 fewer calories per meal. (Make that twice a day over a year, and that could add up to 14 pounds!) The Virginia Tech scientists also believe the water drinkers began swapping this zero-calorie beverage for sodas and other caloric beverages.

What's more, when you're well hydrated, your body is working closer to maximum efficiency -- enhancing aspects of weight loss, like digestion and muscle function, when you exercise.

Water-drinking tip: For variety's sake, try flavoring your water. Drop some fruit into a pitcher and let it sit a few minutes -- lemons, oranges, watermelon, and berries all work well. Or let an herbal or flavored green tea bag steep in unheated water to accent the taste.

You'll be less apt to get sick

Why: Hydration keeps your mucus membranes in top working order -- they're gatekeepers to the natural defense system that helps keep out germs such as cold and flu viruses. When these tissues dry out, germs can more easily penetrate to the nasopharynx, where the nasal passages and mouth meet. And if you catch a bug anyway, the severity of your illness is more likely to be lower if you've been drinking a lot of water.

Water is an especially smart health move when you're traveling. Most commercial planes fly at elevations between 30,000 and 35,000 feet, where humidity is 10 percent or lower. That means you're breathing dry air in a tight space filled with germs from dozens of people. Water keeps your mucus membranes moist -- and your defenses high -- even in that challenging situation.

Water-drinking tip: Bring an empty water bottle with you to the airport that you can fill for free once you're past security. Or buy the biggest bottle you can right before you board, and aim to finish it by flight's end. Don't hesitate to ask the flight attendant for refills while you're in the air.

You'll be more comfortable

Why: It doesn't matter if the water you sip is hot or ice-cold. The act of drinking it will keep you warmer on a cold day -- and cool you off on a hot one.

"Your internal thermostat works better when you're well hydrated," Duke nutritionist Beth Reardon says. "Water helps regulate body temperature."

The body's temperature-regulating system, governed by the hypothalamus in the brain, is constantly picking up information that allows it to make adjustments to maintain a fairly steady core temperature. Hot sun? You'll sweat to cool down. Hatless in snow? The hypothalamus will know you're losing heat through your head and work to produce extra energy, such as shivering.

But these mechanisms work less well if you're dehydrated – and dehydration is a common risk for people of all ages, in winter as well as summer. In cold weather, for example, you lose water vapor through your breath. And many people tend to drink less water in cold weather because they don't think they need it as much as on hot, sunny days.

Water-drinking tip: Start by swapping out one soda, cup of coffee, or high-calorie hot chocolate a day with an equal amount of water. Carry a bottle of water with you all day long as a visual reminder to pause and sip. Sip more when you're physically active, whether in water, snow, or any weather condition.

It will help regulate your blood pressure

Why: In 2010, the American Red Cross discovered that when blood donors were given 16 ounces of water to drink before giving blood, there was a 20 percent drop in fainting after the procedure. That was an important finding for them, given that many of those who faint then chalk blood donation up as a bad experience and never return to give again.

It's not entirely clear what mechanism is at work. But the Red Cross was inspired to conduct a study after researchers at Vanderbilt University noticed that drinking water activated the parasympathetic nervous system -- related to the "fight or flight" system that makes you more alert, elevates blood pressure momentarily, and boosts energy. Fainting after donating blood is often connected to a drop in blood pressure, and they theorized that the water would counter that effect.

(Not drinking enough water on a regular basis can also raise blood pressure. That's because dehydration causes blood vessels to constrict as the body strives to conserve water that it loses through perspiration, urination, and breathing. When blood vessels constrict, however, the heart pumps harder, bringing blood pressure up.)

Water-drinking tip: Start your day with a glass of water for a simple energy boost that remedies any dehydration that may have occurred overnight. Keep a filled glass or bottle on your nightstand or an empty one next to the bathroom sink.