Dehydration

Your body may become dehydrated for any of a number of reasons; the main ones are severe diarrhea, sweating a lot in hot weather or during strenuous activity such as a marathon race, and water evaporating more quickly at lower air pressure, which is a factor in altitude sickness. Dehydration is always uncomfortable and bad for your health; in extreme cases it can kill.

Feelings of thirst are an important early sign of dehydration; others are headache, difficulty with or inability to urinate, and falling blood pressure and associated signs, particularly dizziness. Check the colour of your urine; the lower your body is on water, the darker it gets. If the urine is brown instead of yellow or you are not urinating at all, those are signs that you are seriously low on water; treat this as an emergency.

To maintain hydration, drink sufficient water or oral rehydration solution. Drink several glasses an hour until all the signs mentioned above go away. Also try to reduce whatever stress is leading to the dehydration; in hot weather, find air conditioning or at least shade; at altitude, descend or at least do not ascend further; in a marathon, quit or at least slow down. Other clear liquids, such as weak tea with some sugar, any clear juice or various "sports drinks", are also good. A glass of beer is fine, but avoid overdoing it since alcohol tends to cause dehydration. Caffeine tends to increase urination, so coffee and caffeine-containing soft drinks should be avoided or taken in very moderate amounts.

In some cases it may be difficult to drink enough, or to keep it down if you do. Try drinking many small sips. In extreme cases, health care personnel may give fluids intravenously.

If you are losing a great deal of water you also need to replace electrolytes.

• Hyponatraemia. This is a condition of low blood sodium, caused by losing salt by sweating and drinking lots of water without replacing the sodium. It has sometimes been fatal for long-distance runners. 

If you are undertaking an activity such as high-altitude climbing or major long-distance running, then you need to have a hydration plan that avoids the risks of both dehydration and hyponatraemia. Creating this may require some discussion with a doctor, preferably a sports medicine specialist.

• Water bottle. You could refill bottles got when buying water. Also more sturdy water bottles are widely available, marketed mainly for hikers, tour cyclists and the like. Some are insulated to keep drinks cold. 
• Hydration pack (drink bag). This is a small backpack or a waist pack that lets you carry a water supply that can be sipped through a tube without using your hands. Well-known brands include Camelbak and Source. 
• Oral re-hydration solution. This is sold in many pharmacies and some expedition shops, either as a liquid or as sachets of powder to be mixed with water. 

You can also make rehydration solution yourself (you might want to carry a measuring cup suitable for these quantities, or simply measure a teaspoon and carry that – their volumes vary widely). The medical rehydration recipe is:

• 1 liter of water (boiled or bottled) to:
• 8 teaspoons (40 mL or 30 g) of sugar;
• 1 teaspoon (5 mL or 8 g) of salt, which should be mostly NaCl; and
• (optionally) half a cup of orange juice or mashed banana to add flavour and help replace potassium

"Saudi champagne", a 50-50 mixture of club soda and apple juice, is quite palatable and reasonably close to the right mix for rehydration. Club soda or sparkling mineral water has dissolved salts and minerals, unlike plain or carbonated water, and the apple juice has some sugars. However, the carbonation may not sit well on an upset stomach.