Water is the best liquid you can drink to stay hydrated. But caffeinated drinks can help meet your daily fluid needs.
The amount of water your body needs varies. Your age, body size and activity level affect how much water you need. Also, where you live, if you have air conditioning when it's hot, or if you work inside or outside can affect how much you need to drink. Breathing, getting rid of body waste and sweating are ways the body loses water.
As a chemical, caffeine increases production of urine, which means caffeine is a diuretic. But most research suggests that the fluid in caffeinated drinks balances the diuretic effect of typical caffeine levels. High doses of caffeine taken all at once may increase the amount of urine the body makes. This is more likely if you aren't used to caffeine.
Caffeinated drinks are available for every time of day. So it's important to read the labels on these beverages. That will help you stay at a safe level for your total intake of caffeine. The U.S. Food and Drug Administration (FDA) has set a daily limit for total caffeine. The limit is 400 milligrams a day for adults. There is no safe limit identified for others. For kids age 2 or younger, dietary guidelines for Americans suggest avoiding drinks with caffeine. They also suggest that people who are pregnant or nursing talk to their health care providers if they have questions about caffeine.
Very high doses of caffeine, greater than 10 milligrams per kilogram daily, are linked to serious health conditions. If a person weighs 165 pounds (about 75 kilograms), a very high dose is about 750 milligrams of caffeine in a day. Chest pain, irregular heartbeat and stroke have been reported when people take this much caffeine. Caffeine poisoning can happen with 1,200 milligrams of pure caffeine, according to the FDA. That is less than half a teaspoon of caffeine in powder form.
Other, more-common issues people have reported after drinking caffeinated drinks are trouble sleeping, loose stools and upset stomach.
Overall, water is probably your best bet to stay hydrated. It's calorie-free, caffeine-free, inexpensive and easily available.