The kidneys help to control blood pressure by regulating the amount of salt in the body. If the kidneys become damaged, or unable to function properly because of dehydration, this can lead to high blood pressure.(1)
High blood pressure (hypertension) increases the risk of cardiovascular disease, including heart attacks and strokes, but other symptoms can occur, such as headaches, shortness of breath, sleepiness, confusion and nosebleeds.(2)
Chronic dehydration increases certain blood/body fluid parameters, such as viscosity of the blood, levels of fibrinogen (clotting agent) and haematocrit (blood composition). This can increase the risk of hypertension, atherosclerosis and coronary heart disease.(3,4)
High salt intake is a risk factor for hypertension and restricting salt intake is often recommended to reduce blood pressure.(5) Salt intake promotes water retention, which, in turn can protect against fluid loss, for example in disease which cause diarrhoea.
Research has shown that previous experience of dehydration seems to lead to a preference for salt.(6) Chronic low levels of dehydration within the population may, therefore, be responsible for enhancing people’s preference for salt in the diet. If this is the case, treatments for hypertension should include recommendations about adequate water intake, as well as salt restriction.(7)
Some people suffer significant drops in blood pressure when they stand up, which can even cause them to pass out (orthostatic hypotension). This condition is called primary autonomic failure. In such individuals, drinking between 240ml to 500ml water causes a rise in the lying and seated blood pressure, which can help to alleviate their symptoms. The beneficial effect does not appear merely to be due to rehydration but results from the actual act of drinking, which stimulates the throat, nerves and ultimately the brain.(8,9,10) Water drinking could, therefore, be useful in the management of patients with orthostatic hypotension.(11)
In healthy people, drinking 500ml of water can also help to prevent feelings of 'greying-out' or dizziness when standing upright.(12) Some people experience these types of symptoms after blood donation.(13) Drinking a glass of water before blood donation can therefore help to reduce any adverse effects.(14)
For us to remain healthy, our blood pressure must respond appropriately to changes in our environment, our posture, and our physical and mental activity level. This is known as blood pressure reactivity. Drinking an additional two litres of water per day, in addition to normal fluid intake, over a period of three days, has been shown to improve blood pressure reactivity and help the body cope with the stresses and strains of everyday life.(15) For individuals who have high blood pressure, water ingestion can improve blood pressure reactivity during exercise.(16)
(see also Heart Disease)
Last updated: December 2006
(1) National Kidney Research Fund. Patient Information: Cystitis. NKRF 2001
(2) The National Kidney Research Fund. Blood pressure and kidney disease. NKRF 2001
(3) Chan J, Knutsen SF, Blix GG, Lee JW, Fraser GE. Water, other fluids, and fatal coronary heart disease. Am J Epidemiol 2002;155:827-33
(4) Hattori M, Azami Y. Searching for preventive measures of cardiovascular events in aged Japanese taxi drivers – the daily rhythm of cardiovascular risk factors during a night duty day. J Hum Ergol (Tokyo) 2001;30:321-326
(5) Hooper L, Bartlett C, Davey SG and Ebrahim S. Advice to reduce dietary salt for prevention of cardiovascular disease. Cochrane Database of Systematic Reviews 2004 (1):CD0036565
(6) Fessler DMT. An evolutionary explanation of the platicity of salt preferences: prophylaxis against sudden dehydration. Med hypotheses 2003;61:412-415
(7) Stookey JD. Another look at fuel + O2 ‡ CO2 + H2O. Developing a water-oriented perspective. Med Hypotheses 1999;52:285-290
(8) Endo Y, Yamauchi K, Tsutsui U, Ishihara Z, Yamazaki F, Sagawa S, Shiraki K. Changes in blood pressure and muscle sympathetic nerve activity during water drinking in humans. Jpn J Physiol 2002;52:421-7
(9) Jordan J. Acute effect of water on blood pressure. What do we know? Clin Auton Res 2002;12:250-5
(10) National Research Register, Issue 1, 2002. Investigation of novel therapies in Autonomic dysfunction – water ingestion study. Publication ID: N0241103182, 2002
(11) Mathias CJ and Young TM. Water drinking in the management of orthostatic intolerance due to orthostatis hypotension, vasovagal syncope and the postural tachycardia syndrome. Eur J Neurol 2004;11:613-9
(12) Schroeder C, Bush VE, Northcliffe LJ, Luft FC, Tank J, Jordan J, Hainsworth R. Water drinking acutely improves orthostatic tolerance in healthy subjects. Circulation 2002;106:2806-11
(13) Lu CC, Diedrich A, Tng CS, Parajape SY, Harris PA, Byrne DW, Jordan J, Robertson D. Water ingestion as a prophylaxis against syncope. Circulation 2003;108:2660-5
(14) Hanson SA and France CR. Predonation water ingestion attenuates negative reactions to blood donation. Transfusion 2004;44:924-8
(15) Rochette LM, Patterson SM. Hydration status and cardiovascular function: effects of hydration enhancement on cardiovascular function at rest and during psychological stress. Int J Psychophysiol 2005;56:81-91
(16) Ribeiro GA, Rodrigues LO, Moreira MC, Silami-Garcia E, Pascoa MR, Camargos FF. Thermoregulation in hypertensive men exercising in the heat with water ingestion. Braz J Med Biol Res 2004;37:409-417
