We all need water to live, but how much do we really know about it? From the truth about drinking eight glasses of water per day to refilling plastic bottles, here's what you should know about water benefits.

For something so seemingly simple and essential as drinking water, plenty of myths and misconceptions exist about possible water benefits and harms.

Learn how to separate the myths from the facts about drinking water.

1. Everyone needs to drink eight glasses of water a day.

Myth. Though water is the easiest and most economical fluid to keep you hydrated, the latest Institute of Medicine recommendation is that women should strive for about two liters or eight glasses a day and men should aim for three liters or 12 glasses a day of any fluid, not just water. “No one can figure out where this ‘eight glasses of water’ came from, but I believe it came from the old RDA [recommended daily allowance] for water that matched water requirements to calorie requirements,” notes Georgia Chavent, MS, RD, director of the Nutrition and Dietetics Program at the University of New Haven in West Haven, Conn. “The new requirement from the Institute of Medicine is much more generous and includes recommendations for total beverage consumption, not just water.”

2. Drinking water flushes toxins from your body.

Fact. Though water doesn’t necessarily neutralize toxins, the kidneys do use water to get rid of certain waste products. If you don’t drink enough water, your kidneys don’t have the amount of fluid they need to do their job properly. “If the body does not have sufficient water, then metabolic wastes will not be removed as efficiently as they should,” explains Amy Hess-Fischl, RD, CDE, of the University of Chicago Kovler Diabetes Center. “In essence, the body would be holding in toxins instead of expelling them, as is required for proper health.”

3. Bottled water can cause tooth decay.

Myth. Bottled water in and of itself doesn’t cause the teeth to decay, but it usually doesn’t contain any fluoride, which is added to tap water to help prevent tooth decay. “Fluoride is an important element in the mineralization of bone and teeth,” says Constance Brown-Riggs, RD, CDE, author of The African American Guide To Living Well With Diabetes and a nutritionist and certified diabetes educator in New York City. “With the increased consumption of bottled water, which is not fluoridated, there has been an increase in dental caries [cavities].”

4. Drinking water can help keep your skin moist.

Myth. While it used to be believed that staying properly hydrated led to youthful, vibrant skin, the reality is that the amount of water you drink probably has very little to do with what your skin looks like. “Unless the individual is severely dehydrated, drinking large quantities of water will not prevent dry skin,” Hess-Fischl says. “Basically, the moisture level of skin is not determined by internal factors. Instead, external factors such as skin cleansing, the environment, the number of oil glands, and the functioning of these oil-producing glands determine how dry the skin is or will become. The water that is consumed internally will not reach the epidermis [the top layer of the skin].”

5. Drinking water helps you lose weight.

Fact. Drinking water won’t specifically trigger weight loss, but it can aid in the process. Water replaces other calorie-laden beverages in the diet, causing you to reduce your overall number of calories. Plus, it can make you feel fuller, so you may eat less at each meal. Water, particularly cold water, may even play a role in increasing your metabolism. “A new study seems to indicate that drinking water actually speeds up weight loss,” says Tanya Zuckerbrot, MS, RD, owner of Tanya Zuckerbrot Nutrition, LLC, in New York City. “Researchers in Germany found that subjects of the study increased their metabolic rates [or the rate at which calories are burned] by 30 percent after drinking approximately 17 ounces of water.”

6. Yellow urine is a sign of dehydration.

Myth. It can be, but not all yellow urine is cause for alarm. “Dark yellow urine may be a sign of dehydration,” says Zuckerbrot. “The kidneys filter waste products and reabsorb water and other useful substances from the blood, so they control the volume and concentration of urine output. Dehydration leads to increased urine concentration, turning your urine dark yellow. Ideally your urine should be straw yellow in color.” Other factors, though, such as taking a multivitamin, can also lead to yellow urine.

7. If you’re thirsty, you are already dehydrated.

Myth. If you start to feel thirsty, then you are headed in the wrong direction and should grab a drink of water, but thirst doesn’t necessarily mean you’re dehydrated. “Thirst begins when the concentration of [substances in the] blood has risen by less than 2 percent, whereas most experts would define dehydration as beginning when that concentration has risen by at least 5 percent,” notes Hess-Fischl.

8. You need sports drinks, not water, to function at a high level in athletics.

Myth. Sports drinks may have fancier advertising campaigns, but water is really all you need to get the fluid necessary to participate in most athletic endeavors. “Adequate fluid, especially water, is most important for athletes of all ages as it is the single most important way the body has to transport nutrients and energy and remove heat during exercise,” says Chavent. “A sports or vitamin beverage may taste better, but is not necessary for hydration and is expensive.” Keep in mind though that people who run marathons or compete in highly strenuous activites may need to supplement their water intake with sports drinks to offset the salt they lose due to heavy sweating over long periods of time. This doesn’t apply to most people who are simply exercising to get fit at the gym, for instance.

9. It’s possible to drink too much water.

Fact. People with certain health conditions can put themselves at risk of complications if they drink too much water. “People with some heart conditions, high blood pressure, or swelling of the lower legs [edema] need to avoid excess water,” says Hess-Fischl. “If you have a history of kidney problems, especially if you have had a transplant, consult your doctor before increasing your fluid intake.” Hess-Fischl adds that you shouldn't drink too much water while eating, as it dilutes your stomach acid and can cause digestion problems.

10. You should not reuse plastic water bottles.

Fact. Plastic water bottles can present a couple of risks to people who drink their contents and then fill them up time and again. “These bottles leach chemicals into your water after multiple uses,” Hess-Fischl explains. “The bottle, if not properly cleaned, may also harbor bacteria from your mouth.”

Water is essential to survival — use these facts to figure out if you need to increase your intake or feel reassured that you’re drinking enough.