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Humans can survive for weeks without food, but only a few days without water. Water helps balance the chemicals that keep your body working: It fills our cells, helps regulate our body temperature through sweating, flushes waste, forms saliva, lubricates our joints, cushions our spinal cord and brain, and carbohydrates and proteins are metabolized and transported by water in our blood. Athletes can lose 6-10 percent of body weight simply from loss of water during sports or exercise. It's important to drink when thirsty, but also important that you don't push fluids. A study in the Journal of Applied Physiology found a 15 percent reduced muscle endurance during dehydration but no effect on overall muscle strength. Water does not alleviate muscle cramps or stop heat stroke, and too much can cause hyponatremia, or a dilution of salt in the blood -- which can result in death.

That said, a 1.5 percent drop in water can lead to mild dehydration and mess with focus, alertness and short-term memory. Chronic dehydration puts people at a risk for type two diabetes, cancer, high blood pressure and obesity. A study in the journal Physiology and Behavior found that dehydration can cause increased error rates while driving and altered mood states in men. A similar study found that while women don't experience diminished cognitive function, they do see increases in fatigue, tension and anxiety when dehydrated.

And that's just the mild stuff, excessive dehydration is when things get really dangerous. If full dehydration sets in, the body realizes water is not being replaced, and as a result you stop sweating and urinating. This causes the blood to thicken, which leads to increased blood pressure. The heart has to pump harder to push the sludgy blood around the body and cholesterol increases as the water is lost from cells. Ultimately the chances of cardiac arrest increases. It's pretty unlikely for this to happen while playing sports, because eventually you'll have to stop. But, as we get older, the body's thirst mechanism worsens, increasing the risk of dehydration without even knowing we're dehydrated.

Read More:
How Much Water Do People Drink? (theatlantic.com)
"In 1998 the United States averaged 54 gallons of soda consumed per person, to only 42 gallons of water."

Water, Hydration, and Health (nih.gov)
"This review attempts to provide some sense of our current knowledge of water including overall patterns of intake and some factors linked with intake, the complex mechanisms behind water homeostasis, the effects of variation in water intake on health and energy intake, weight, and human performance and functioning."

Athletes should drink only when thirsty, according to new guidelines (eurekalert.org)
"At least 14 deaths of marathon runners, football players and other athletes have been attributed to a condition called exercise-associated hyponatremia, which results from drinking too much water or sports drinks."

Lack Of Drinking Water Deteriorates Human Body: Adverse Effects Of Dehydration (medicaldaily.com)