Here’s a great article from The Detroit Free Press.
Drinking enough water seems intuitive, but when it becomes an afterthought, people exercising or working outdoors can easily become dehydrated — often without knowing it.
With schools out and vacations planned, more Americans are heading outdoors. But experts warn many of them will change exercise habits without giving their bodies the time or resources to catch up.
“If you’re not acclimatized to new environmental conditions and you’re increasing your duration or intensity of exercise, you’re at a greater risk of dehydration, one that can even occur over a three- to five-day period,” says Brendon McDermott, an athletic trainer at the University of Connecticut.
“You could be outside mowing the lawn five days into a heat wave and start feeling, say, lightheaded as a result of that dehydration,” he says.
The warning signs include thirst and fatigue, but McDermott says the process begins as soon as the body’s water level starts to decrease. When temperatures start to climb, a person can become dehydrated after 30 minutes, says Michael Sawka, a doctor at the U.S. Army Research Institute of Environmental Medicine in Natick, Mass.
“This causes a reduced exercise performance, maybe a reduced cognitive performance and, likely, a greater susceptibility to heat illnesses,” Sawka says.
Of these illnesses, heat exhaustion is the least serious, Sawka says, associated with moderate or high temperatures and accompanied by hot skin and dehydration. Heat stroke is more serious; it damages the central nervous system and is marked by severe confusion, impaired judgment and seizures or blackouts.
Sawka says heat injury falls in between. It can impede function of the liver, kidney, gut or muscles, and it is characterized by dizziness, fainting and blood or tissue damage.
A drop in body weight, a darker-colored urine and thirst are all evidence dehydration has begun, Sawka says.
Increasing fluid intake starting about two hours before exercising is considered the best preventive step. Cedric Bryant, an exercise physiologist with the American Council on Exercise, recommends drinking 16 to 20 ounces two hours before exercise, six to 10 ounces every 15 to 20 minutes during exercise, and 16 to 24 ounces of fluid afterward for every pound of body weight lost.
McDermott, also a member of the National Athletic Trainers’ Association, says a more specific strategy is monitoring weight before and after exercise and replacing the weight lost with water.
People on diet programs should be particularly cautious, McDermott says. “They could see lower body weight as related to their program, but the body weight could be decreasing because of water loss,” he says.
The trainers association also offers these recommendations for exercising outdoors in a report that is part of the American College of Sports Medicine and the American Medical Association’s “Exercise is Medicine” initiative:
• Gradually increase the intensity and duration of exercise during hot weather.
• Add periods of rest during exercise or between periods of exercise.
• Begin outdoor activities only after you’re properly hydrated, and drink water or sports drinks while active in the heat.
• Monitor urine color, which should look more like lemonade than apple juice.
• Exercise during cooler periods of the day, such as the early morning or late evening.
If you have symptoms that could indicate the onset of a heat-related illness, such as a fever, diarrhea or extreme fatigue, do not exercise. If these occur during your workout, slow down or stop.