Dehydration means lack of water in your body.
How do you know you are dehydrated?
Main symptoms include thirst (not always), dry mouth, tiredness, nausea, headache and, in severe cases, dry skin, muscle cramps, coma or death.
Main signs include sudden loss of weight (the only objective sign), less frequent urination, small amount of dark urine and prolonged skin turgor (the skin on the back of your hand does not bounce back immediately after you have pinched and released it).
What and how much you need to drink to become well hydrated?
Appropriate drinks:
- Water: tap, bottled, mineral, carbonated; rainwater, clean streams or lakes
- Herbal tea
- Decaffeinated coffee
Less appropriate (but still hydrating) drinks:
- High-calorie (sugary) beverages, like fruit juices, ice tea, soda; they slow gastric emptying and thus water absorption, plus, you might do not want to consume a lot of sugar or calories.
- Sport drinks are needed only in hard exercises lasting more than 3-4 hours (marathon). In other circumstances you can replace the lost sodium by eating salty foods.
Even less appropriate (but still hydrating) drinks:
- Caffeinated beverages (coffee, energy drinks, cola, tea); caffeine can make you anxious. Caffeine slightly increases urine excretion, but caffeinated beverages, even in larger amounts, do not likely cause net dehydration (you get more water from the beverage than you lose it by urinating)
- Beer (~4-5% alcohol). If nothing other is available, beer can hydrate you, slower than water, but it can. Alcohol in beer slightly increases diuresis, but again, causes no net dehydration (source: Journal of applied Physiology). Also, by drinking one can (12 oz) of beer per hour, alcohol does not likely accumulates in your blood.
- Beverages high in "net fructose:" apple, pear and mango juice, cola and other beverages sweetened with HFCS. These beverages contain net fructose, which is the amount of fructose greater than glucose (for example, a beverage with 50 g fructose and 30 g glucose contains 20 g net fructose). Every human can absorb only a limited amount of net fructose (~25-50 grams in healthy people, <25 grams in those with fructose malabsorption) "per sitting," the rest stays in the bowel and can trigger diarrhea and can thus worsen dehydration. One liter of pear juice contains ~57 g net fructose, apple juice ~31 g, HFCS-sweetened cola 17 g.
Drinks, inappropriate to treat dehydration:
- Wine (probably...) or any stronger alcoholic beverage.
- After drinking seawater, in order to get rid of excessive salt, you will likely excrete more water with the urine than you have consumed with seawater.
How much do you need to drink to rehydrate yourself?
Roughly, you usually need to drink 1.5 x amount of water you have lost, because you will excrete some water you drink before it will reach your body cells. For example, when you lose 3 kilograms of body weight due to dehydration, you may need to drink 4.5 liters of water to fully rehydrate yourself.
How fast to drink? Use common sense. If you drink fast, you will excrete a lot of water before you will get hydrated. Even in severe dehydration it is probably not wise to drink more than 1.5-2 liters of water per hour.
Eating salty foods along with drinking helps to retain water in your body.
Source: Diagnosis and treatment of dehydration