Now that we've established that food is something we can do without for a reasonable amount of time, we can move on to water. Living without water is very different from living without food. In hot conditions with no water, dehydration can set in within an hour. A baby locked in a hot car or someone who is physically overexerted in the heat without replacing fluids can actually die in a period of several hours.
Humans need water to live, plain and simple. We lose water through sweat, urine, feces and even breathing. This water needs to be replaced in order for our organs to continue to work properly. In severe heat, an adult can lose as much as 1.5 liters of water through sweat alone [source: Scientific American]. The main risk without water in high heat is that your body temperature will continue to rise and you'll suffer from heat stroke. Drinking water will cool you down and lower your core temperature.
With mild dehydration, you'll experience the following:
- Lack of saliva
- Decreased frequency of urine
- Decreased output of urine
- Deep color and strong odor in urine
- Even less urine
- Dry mouth
- Dry and sunken eyes
- Rapid heartbeat
- No urine
- Lethargy and irritability
- Vomiting and diarrhea
The final stage of dehydration is shock. This is characterized by blue-gray skin that's cold to the touch. A severe drop in blood pressure produces this coolness.
Now back to the question at hand. How long can you go without water? Assuming you're in reasonable shape and in ideal conditions -- that is, not in the heat or cold and not exerting, a human can probably live for about 3 to 5 days without any water. Healthier humans can live another day or so longer.
This isn't something you should test. While people may fast or try a body cleanse without food, you should absolutely never go without water for more than a day. The Mayo Clinic recommends drinking about eight cups of water a day, although there's some debate about this number [source: Mayo Clinic]. Some physicians say less is fine, while others say the number should be closer to 10 cups or even more.
For more information on food, water and survival, please dive into the material on the following page.