What you should drink: a cup of water each mile or every 30 minutes. If you are walking for more than an hour or losing lots of sweat, you should consider drinking an electrolyte (salt) replacement sports drink. The guidelines current as of 2006 are to "drink when thirsty" rather than forcing fluids.
1. Untreated Water from a Lake, Stream, or Spring
In many places, nasty parasites such as Giardia lamblia
are found in "unspoiled" mountain streams because these parasites infest the local squirrels and other animals, who then contaminate the water. If you are going for a hike, carry a water filter or purification tablets and do not drink untreated water from any natural source. Take it from me: I spent years in the hospital microbiology lab helping diagnose people who made this mistake.
2. Alcoholic Beverages
Alcoholic beverages such as beer and wine will make you more dehydrated, as well as impair your athletic ability and judgment. They will also make you more prone to heat sickness and other problems. Save the celebratory drink for after your walk. Before major walks, abstain from alcohol the evening before as well. Drinking during a walk may be a European tradition, but I recommend against it, having seen walkers get into difficulty after drinking alcohol.
Caffeine was long accused of dehydrating you by making you have to urinate more, and it can also act as a laxative. Newer research as of 2002 put that common wisdom into doubt, at least for those who commonly drink caffeinated drinks. If you find yourself making too many restroom stops, you may want to limit your caffeine intake to see if it is causing the problem. Stick to decaf or drink as little of the caffeinated stuff as possible before your walk. Coffee drinkers can get a nasty headache if they go cold turkey, so experiment with how little you really need.
4. Milk and Cream
Some people tolerate milk very well. But many people are lactose intolerant and can have stomach cramps, gas, nausea and diarrhea from milk and milk products. Some people only experience these symptoms during exercise. If you have any such symptoms, avoid milk products for 12 hours before your walk.
5. Carbonated Beverages
Many walkers report gas, belching, and stomach cramps from drinking carbonated beverages while walking. If you experience any of these symptoms, save the sparkling drinks for after the walk.
6. Too Much Water and No Salt Replacement
American College of Sports Medicine guidelines are to "drink when thirsty" rather than drinking constantly and pushing too much water. Studies of slower marathon runners and run/walkers showed that some of them were verging on hyponatremia -- dangerously low sodium levels -- whether they were drinking plain water or water and sports drink. Don't overdo the water. If walking for more than an hour and sweating, you should replace the salt lost in your sweat with an electrolyte-replacement sports drink such as Gatorade or Powerade, or with a snack that contains salt such as mini pretzels or trail mix that includes salted nuts.