A glass of water after waking helps replace fluids lost overnight. But it does not work the same way for everyone. For people who need to manage their blood pressure, it can be a habit that distorts their readings.
The Korean Society of Hypertension recommends that home measurements of morning blood pressure be taken within an hour of waking, after using the bathroom but before breakfast and before taking blood pressure medication, according to medical experts on the 21st. The U.S. Centers for Disease Control and Prevention also advises against eating or drinking anything in the 30 minutes before a measurement. In other words, taking a reading right after drinking water can produce a figure higher than the actual level.
The rise in blood pressure after drinking water is explained by activation of the sympathetic nervous system, which puts the body on alert and raises blood pressure. In a study published in the journal Circulation, systolic blood pressure rose an average of 11 mmHg in 11 healthy older adults who drank 480 milliliters of water. In patients with impaired autonomic function, it climbed by 33 to 37 mmHg after 30 to 35 minutes.
The increase also matters over the long term. In a prospective study that followed 519 older patients with hypertension, the top 10% group whose systolic blood pressure rose most sharply during the two hours after waking had a stroke rate of 19%, compared with 7.3% for the rest. The average follow-up period was 41 months. Another study reported that morning blood pressure was up to 74% higher in people jolted awake by an alarm than in those who woke naturally.
Water temperature is another variable. Japanese researchers had nine older women with an average age of 71.4 and 13 young adults with an average age of 19.9 drink 200 milliliters of water. Cold water and cold carbonated water raised blood pressure more than room-temperature water. The researchers suggested that cold stimulation of the mouth and throat may have activated the sympathetic nervous system.
Patients on fluid restrictions are an exception. People with heart failure or chronic kidney disease may not produce enough urine, allowing fluid to build up in the body. That can lead to swelling, difficulty breathing or higher blood pressure. The National Kidney Foundation advises that hemodialysis patients count not only bottled water but also coffee, soup, ice and ice cream toward their daily total.
The conclusion is clear. Those who measure their blood pressure every morning should take the reading first and drink water afterward. Room-temperature water puts less strain on blood pressure than cold water. Patients told to limit fluids should not add an extra glass in the morning and should manage their intake within the daily allowance.