Water: The Unexpected Blood Pressure Drug

Water also raises blood pressure in older normal subjects, but not in young normal subjects. The studies suggest that water is an important unrecognized factor in clinical studies of blood pressure medications.

"Water is not just a neutral substance, and it cannot be considered a 'placebo' with regards to blood pressure," said Dr. John R. Shannon, instructor of Medicine and Pharmacology. "As physicians, we may ask our hypertensive patients if they just drank a cup of coffee or smoked a cigarette when we measure their blood pressure, but we would never think to ask if they had anything to drink in the last hour. Perhaps we should. It might make the difference in whether or not we adjust their medications."

On the average, 16 ounces of tap water raised blood pressure about 40 millimeters of mercury in patients with autonomic failure. Blood pressure started increasing within two or three minutes after the water was ingested, increased rapidly over the next 15 minutes, and then began to decrease after about 60 minutes. Drinking more water at 60 minutes caused the blood pressure effect to be sustained for another hour.

In older normal subjects, the average increase in blood pressure was 11 millimeters of mercury.

There is something about the water itself that causes the increase in blood pressure; intravenous infusion of 16 ounces of sugar solution did not elicit the effect. In addition, the effect is "dose-dependent" -- the higher the water intake, the larger the increase in blood pressure.

"We do not know how water raises blood pressure, but it is the solute-free water itself, whether warm, room temperature, or cold that elicits the effect," said Dr. Jens Jordan, a former postdoctoral fellow in Clinical Pharmacology and lead author of the study.

"Water might be increasing blood pressure by interacting with osmoreceptors (which sense salt concentrations) or stretch receptors in the stomach or liver," added Jordan, who is currently director of the Clinical Research Center at the Franz Volhard Klinik of Humboldt University in Berlin, Germany.

The studies suggest a new and more effective treatment for the blood pressure abnormalities suffered by patients with autonomic failure. By raising blood pressure, water ingestion during the day can enhance a patient's functional abilities. At Vanderbilt, the careful administration and withholding of water at various times during the day has proven successful as the only form of blood pressure treatment for some patients with autonomic failure.

Although further work is required to understand why water raises blood pressure in older normal subjects, the studies suggest that physicians need to be aware of water as a variable to consider in assessing blood pressure levels. In addition, water intake is a variable that needs to be controlled in short-term pharmacological studies of drugs that increase and decrease blood pressure.