A human can go for more than three weeks without food — Mahatma Gandhi survived 21 days of complete starvation — but water is a different story.
At least 60% of the adult body is made of it and every living cell in the body needs it to keep functioning. Water acts as a lubricant for our joints, regulates our body temperature through sweating and respiration, and helps to flush waste.
The maximum time an individual can go without water seems to be a week, an estimate that is based on observations of people at the end of their lives, when food and water intake has been stopped, Randall K. Packer, a professor of biology at George Washington University told Maggie Fox of NBC News in 2013.
But one week is a generous estimate. Three to four days would be more typical, especially in difficult conditions like broiling heat.
"You can go 100 hours without drinking at an average temperature outdoors," Claude Piantadosi of Duke University told Fox. "If it’s cooler, you can go a little longer. If you are exposed to direct sunlight, it’s less."
Our bodies are constantly losing water, which is why drinking a glass of H20 once a day is not enough to keep the body replenished.
We lose water when we sweat, go to the bathroom, and even when we exhale.
“Under extreme conditions an adult can lose 1 to 1.5 litres of sweat per hour," Packer wrote in 2002 article for Scientific American. "If that lost water is not replaced, the total volume of body fluid can fall quickly and, most dangerously, blood volume may drop."
When you have too little blood circulating in your body, blood pressure falls to levels that can be fatal. Body temperatures also rise when we stop sweating.
Dehydration that causes "a loss of more than 10% of your body weight is a medical emergency," according to the University of Rochester Medical Center, "and if not reversed can lead to death."
We get some water from food, "but drinking water is your main, and best source, of water," according to a website maintained by the US National Institutes of Health.
Other beverages like juice or milk also help keep the body hydrated. The only fluid you would want to stay away from is alcohol because it actually causes the body to lose more water than normal through excessive urination.