Sometimes when reading tips for reducing blood pressure, drinking water is recommended. However, authority websites including the National Heart, Lung and Blood Institute and the Mayo Clinic do not mention drinking water to lower blood pressure, when discussing treatments and lifestyle changes.
Why should some articles say that drinking water lowers blood pressure?
The idea that drinking water will lower blood pressure seems to come from the idea that when lots of water is consumed, that sodium will be flushed out of the body and consequently pressure will drop.
After all, several classes of diuretics are very effective at reducing pressure. These diuretics function by increasing the loss of sodium from the body and an increased volume of urine. Both the loss of sodium from the blood and decreasing the blood volume result in decreased blood pressure.
So, if drugs, that effectively lower blood pressure in most people, work by increasing the volume of urine and the amount of sodium, then drinking more water should do the same thing?
Unfortunately, there is a problem with this reasoning. To understand why, it is necessary to understand that the body very tightly controls the levels of fluid and ions such as sodium and calcium. For optimal functioning, the body has evolved a wide range of control processes that are involved in keeping many ions and fluid levels within a narrow range.
Diuretics act on parts of the system that control sodium. For instance thiazide diuretics bind to, and inhibit, a protein called the Na/Cl symporter (Na=sodium, Cl=chloride) that controls the amount of sodium that is reabsorbed back into the blood from the urine that is being formed. The result is that the body recovers less sodium from the urine as it is being formed, and so more sodium is lost in the urine and there is a slight increase in volume. So diuretics affect the regulatory system and change a part of this.
Water has no impact on the sodium or fluid control system so it will not change the total amount of sodium in the urine, or change the blood volume. Drinking more water will increase the volume of urine as the body regulates fluid levels, to keep the blood volume stable. Additionally, the same amount of sodium (and other ions etc.) in a bigger volume will lead to the urine being more dilute.
Think about the color of urine. If not much fluid has been consumed, or there has been a lot of sweating, there will a small quantity of urine with a strong yellow color (from urobilin). If a person is well hydrated, there is more urine with a pale color. Same thing with sodium. In a healthy individual, the greater the volume of urine due to increased fluid consumption, the lower the concentration of sodium.
So the bottom line is that, generally, increasing the amount of water that is consumed will not increase the amount of sodium lost by the blood, so blood pressure will not be lowered.
In fact, drinking water can actually cause a very short term increase in blood pressure in some people, particularly those with some types of very low blood pressure. This is only temporary and has no long term impact on blood pressure.
Keeping hydrated is good for health, but overhydration has no benefit for lowering blood pressure.