How many glasses of water are in 1.5 liters?

The health recommendation to drink six to eight glasses of water a day is "thoroughly debunked nonsense," a doctor wrote this week in a commentary in the British Medical Journal.

Many health departments and organizations tout the need to drink that much water every day, but there is no high-quality scientific evidence to support the recommendation, wrote Dr. Margaret McCartney, a general practitioner based in Scotland.

Some organizations backed by bottled-water makers -- such as Hydration for Health, created by the makers of Volvic and Evian -- say that it's important to drink 1.5 to 2 liters (about 6 to 8 cups) of water a day, and that being even mildly dehydrated plays a role in disease development, McCartney wrote.

However, no such claims have ever been confirmed in studies, she said, and drinking too much water can actually be dangerous by causing low blood sodium levels (a condition called hyponatraemia) and exposing people to pollutants in the water.

"People still think that we're all going to die or our kidneys will shrivel up if we don't drink eight cups of water a day," McCartney told Postmedia News. "From what I can see, there's never been any evidence in the medical literature about it."

The first recommendation to drink six to eight glasses of water a day has been traced back to a 1945 U.S. research paper, but even that isn't for certain, Postmedia News reported.

The Guardian reported that humans' thirst mechanisms are so sophisticated that if our bodies are in need of water, they'll let us know by making us thirsty.

Drinking extra water is said to reduce urinary tract infections, improve skin tone, help with weight loss (fill up with water first), reduce headaches and fatigue, eliminate constipation and improve concentration. There's no robust evidence for any of this. The kidneys are wonderful things (that don't need flushing with lots of water) and will make concentrated urine to save water.

Dartmouth Medical School physician Dr. Heinz Valtin also told The Huffington Post last month that there aren't any scientific studies supporting the eight-glasses-a-day rule and that, to date, he hasn't seen any additional evidence that would confirm the recommendation. Valtin published a review of the literature in the American Journal of Physiology.

Not all doctors are on board with McCartney's commentary. The commentary wasn't peer-reviewed, one doctor from King's College London pointed out, and it's especially important for the very young and the very old to be appropriately hydrated, particularly by drinking water, the Los Angeles Times reported.

The Mayo Clinic acknowledges that most doctors recommend drinking eight to nine cups of water a day, but said that people need to drink as much water as is necessary to replace water that is lost throughout the day.

In addition, drinking enough to produce about 6.3 cups of clear or slightly yellow urine a day means your fluid intake is probably sufficient, the Mayo Clinic said.

There are factors that can increase or decrease your body's need for water, though, including an increase in physical activity, being sick (vomiting or diarrhea), being in a dry environment or at a high altitude and being in hot or humid weather, according to the Mayo Clinic.

And it's not just water -- fruit and vegetables can also hydrate you, research shows. A University of Aberdeen Medical School study revealed that fruits and vegetables can hydrate the body twice as much as a glass of water because of the salts, sugars and minerals they contain, the Daily Mail reported.

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