THURSDAY, Sept. 26, 2002 (HealthDayNews) -- Worried about feeling woozy the next time you give blood? Drink some water, and make it a double -- two big glasses may be just the thing to keep you from fainting, say two new studies.
Researchers in Germany and Tennessee put healthy subjects in unusual positions that would lead them to eventually faint. Those who drank 16 ounces of water before the test held up better under the physical stress than those who didn't, according to reports released today at the American Heart Association's 56th Annual High Blood Pressure Research Conference in Orlando, Fla.
The findings make sense because water helps blood flow more smoothly to the brain, says Dr. William Norcross, a professor of clinical family medicine at the University of California at San Diego School of Medicine. "Water is effective because it is so easy and quick to absorb," he explains.
Dr. David Robertson, a professor of medicine at Vanderbilt University and co-author of its study, estimated that 150,000 people faint or nearly faint each year after giving blood. He suspects many of them never return to the donor's table.
The problem is simple -- giving blood means you have less in your body, Norcross says: "Your blood volume is diminished, and there's just not enough to get to the brain. The blood pressure drops, and the lights go out. In about one in 20 people, just one unit of blood -- a pint -- is enough to bring this on."
Blood donors are especially likely to faint after standing up, he says. Blood banks try to counteract the fainting risks by screening donors for diseases such as anemia and making sure they relax for a few moments after giving blood, eat some food and juice, and don't get up too quickly.
The German and American researchers decided to investigate the value of water consumption after an earlier Vanderbilt University study showed that people suffering from chronic fainting problems improved after consuming large amounts of water.
In both studies, researchers created tables, similar to hospital beds, that tilt up and down. The subjects were placed on the tables and then tilted until they were at a 60-degree angle to the floor. They were nearly, but not quite, straight up.
Most people cannot remain at the unusual angle more than 45 minutes without passing out, according to Robertson. In the German study, the subjects had an additional stressor. Their legs were placed in "negative pressure" -- a vacuum -- that made fainting more likely by draining blood from their upper bodies.
Some of the subjects drank 16 ounces of water five or 15 minutes before the test, and some did not. In later tests, the two groups were switched.
Researchers tested the blood pressure and heart rate of the subjects, who went to the brink of fainting but were not allowed to actually pass out.
The Vanderbilt research found that subjects who drank water five minutes before could tolerate the test for about seven more minutes -- a total of 40.9 minutes on average -- than those who didn't. The Germans who drank water 15 minutes before lasted five minutes longer than the other subjects before nearly fainting.
The water probably adds a "jolt" of fluid to the bloodstream, adding to its volume and keeping it flowing, Norcross explains. Water is much more rapidly absorbed than some kinds of food and drink.
Drinking less than 16 ounces may not be effective because water doesn't just go into the bloodstream, he says. "It also gets parsed out to cells and the space between cells."
What To Do
For more about fainting, consult the American Heart Association or the National Institute of Neurological Disorders and Stroke.
