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The Science of Hydration -Frequently Asked Questions

Q - I never drink during exercise and I feel fine...why do I always hear about hydrating before and during exercise?

A - If sweat loss is minimal, dehydration may not be an issue, but at higher sweat losses, dehydration has unavoidable and negative physiological consequences. For example, dehydration reduces blood volume, makes the blood more viscous (thicker), increases heart rate, and makes it more difficult for the body to radiate heat. In addition, exercise will feel more difficult, causing you to spontaneously decrease exercise intensity and/or end exercise sessions early. The plain truth is that proper hydration during exercise enables you to work harder and longer, getting the most out of each workout.

Q - Is it true that hyponatremia (from over-hydrating) is more dangerous than dehydration?

A - Both conditions are potentially life threatening; dehydration increases the risk of heat illness, which can eventually lead to heat stroke, and hyponatremia (water intoxication) can cause severe central nervous system problems. Both conditions can result in death in severe cases. For most exercisers, heat illness is much more common than hyponatremia. U.S. Army research indicates heat illness is 20 times more like to occur than hyponatremia during physical activity. Hyponatremia occurs when the sodium concentration in the blood is markedly reduced, usually by overdrinking of plain water. It is best to monitor your body weight during the warmer months when exercising, the goal being to weigh the same or slightly less after exercise as you did before. A body weight reduction of 2% or more can impair performance.

Q - I try to drink as much as I can before working out and then top that off by drinking every 10 minutes while exercising. Am I properly hydrating?

A - This approach might be a good one if you're a very heavy sweater. For most folks, a more modest fluid-replacement scheme will do the trick. We are all different in our fluid replacement needs as we all have different sweat rates. Guidelines published by various professional organizations recommend a range of fluid consumption during exercise. It is generally recommended you consume 14 - 20 oz of water two hours before exercising and then 5 - 15 oz of water every 15 to 20 minutes of exercise. Most people lose between 17 to 50 oz of sweat during an hour of vigorous activity.

Q - I prefer water during my workouts because sports drinks have calories and sodium and I'm trying to lose weight.

A - For many exercisers, especially those who do not lose a lot of sweat, water can be an effective fluid replacement. But for heavier sweaters and for those who workout at a high intensity level, a properly formulated sports drink can provide benefits plain water cannot. The carbohydrates and electrolytes in a sports drink provide functional benefits that help in hydration, maintain exercise capacity, and reduced the perceived effort of the activity.

Q - Is it true that I should only drink when I'm thirsty?

A - For sedentary (non-active) individuals, thirst is what maintains fluid balance. But for people who exercise and sweat on a regular basis, thirst is less precise. Here's the physiology: body fluid needs are sensed by the hypothalamus gland in response to changes in blood chemistry. Whenever there is a sufficient increase in saltiness of the blood and a decrease in the volume of blood, a series of events occur that culminate in the perception of "thirst". When adequate fluid and sodium intake occur, thirst is satisfied. The challenge during exercise is that by the time we are aware of thirst, we are already 1% - 2% dehydrated and our capacity to exercise is already reduced. Pro-active and moderate hydration before and during exercise will reduce the likelihood of significant fluid loss leading to dehydration.

From ACSM's Health & Fitness Journal - March/April 2007