Prevention

To prevent dehydration, drink plenty of fluids and eat foods high in water such as fruits and vegetables. Letting thirst be your guide is an adequate daily guideline for most healthy people.

People may need to take in more fluids if they are experiencing conditions such as:

• Vomiting or diarrhea. If your child is vomiting or has diarrhea, start giving extra water or an oral rehydration solution at the first signs of illness. Don’t wait until dehydration occurs.
• Strenuous exercise. In general, it’s best to start hydrating the day before strenuous exercise. Producing lots of clear, dilute urine is a good indication that you’re well-hydrated. During the activity, replenish fluids at regular intervals and continue drinking water or other fluids after you’re finished.
• Hot or cold weather. You need to drink additional water in hot or humid weather to help lower your body temperature and to replace what you lose through sweating. You may also need extra water in cold weather to combat moisture loss from dry air, particularly at higher altitudes
• Illness. Older adults most commonly become dehydrated during minor illnesses — such as influenza, bronchitis or bladder infections. Make sure to drink extra fluids when you’re not feeling well.

http://www.mayoclinic.org/diseases-conditions/dehydration/manage/ptc-20261161