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Just curious... Some people say 8 glasses of water but some say that is too much and can result in overhydration. How many liters of water should a 16-years-old teenager (boy) drink a day?

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Hi @ArchJ, general nutrition question unrelated to exercise is considered off-topic according to our faq. If you can edit and update your question to fit within the context of the site's scope, I'd be happy to reopen the question. – Matt Chan♦ Mar 30 '12 at 12:41

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Let me start by saying that no amount of water will keep the doctor away. There're still many things that can go wrong.

The 8 glasses of water is not backed up by any meaningful scientific evidence, it's just a rule of thumb. But it includes water from all sources including food.
Making genral recommendations about things like fluid intake is bad idea as it depends on many factors including but not limited to:

  • Climate
    The hotter it is the more water your body loses with perspiration
  • Body Build
    Someone who weights 45Kg, has less water to maintain than a 150Kg one.
  • Physical activity
    During a marathon you need more water than laying in bed. Same is true after a PE class.
  • Water content of food eaten
    If you eat a lot of soup for example than that counts towards your fluid intake

There're two different things that are referred to as dehydration. Acute and Cronic.

  • Acute
    This is what happens if your body is deprived from water for a few days. May result in death.
  • Cronic
    It means that your body is constantly getting less water than it needs. Symptoms include headaches, dry skin and low energy.

You asked about overhydration. It's not the total volume/day that can cause problems but the volume/hour. Healthy kidneys can deal with about a liter of water in an hour. The bottom line is that your body can easily deal with processing 6 liters of water if spread throughout the day but could struggle with 2 liters of consumed in an hour.

It would be a good idea to monitor salt intake. You see the body requires only 3 to 5 grams of salt a day to stay healthy, but most people gobble up 12 to 15 grams of the stuff daily. To rid itself of the overload, the body requires copious amounts of liquid.

Conclusion
If you drink when you're thirsty, don't consume too much salt, and your skin is not as dry as sandpaper than you'll be just fine.

Sources:
8 Common Myths About Dehydration
How much water is too much?
Water Intoxication

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Nice explanation of why the question is way oversimplifying things! – Ivo Flipse♦ Mar 30 '12 at 11:15