In normal circumstances, water is the best thing to drink to keep hydrated, but it may not be enough if you're ill, exercising, or in a hot environment. If you feel thirsty, you're already dehydrated. When you are in a situation that you know you run the risk of dehydration, keep a drink by your side and sip from it constantly. Flavored liquids are easier to drink in large quantities than water, making it easier to stay hydrated.
When you become dehydrated from illness or sweating, you lose electrolytes, particularly sodium and potassium. These chemicals become ions when in solution, letting your body conduct electricity through nerve cells and are also used to regulate water inside cells. It's hard to drink enough water to stay hydrated, and drinking only water when you are severely dehydrated can result in hyponatremia, a lack of sodium in the body. This causes dizziness, vomiting, headaches, and in severe cases, death.
So, you want to drink something that's flavored and contains sodium and potassium. In other words, you should drink sports drinks. You can also mix your own. The World Health Organization's old recipe* for oral rehydration fluid was eight teaspoons sugar and one teaspoon salt per liter of water. Adding a half-teaspoon of "light" salt, potassium chloride, will cover your needs for potassium.
Diuretics like caffeine, lemon juice, and alcohol should be avoided. These increase urination, making it hard for your body to stay hydrated. Sugar can also be a diuretic in large amounts. Sports drinks often use an alternative sweetener like dextrose to give athletes energy without having this effect.
*Today, the WHO dispenses a powder similar to sports drinks for combating diarrhea in third-world countries. While we get to see TV ads selling allergy and erectile dysfunction drugs, people in Cambodia see ads with cheery songs about kids with diarrhea:
Posted 1578 day ago