Water is essential for life. It makes up around 66 percent of the human body and is found in the blood, cells, and spaces between them. The body loses water through sweat, urination, defecation, and exhaled breath. It is important to replace this lost water, but overhydration can be dangerous. There have been cases where excessive water intake has led to fatal consequences.

Endurance athletes often go overboard with rehydration. A study in 2005 found that around one-sixth of marathon runners develop hyponatremia, which is a condition where the blood becomes diluted due to excessive water intake. Hyponatremia is a term derived from Latin and Greek roots, meaning “insufficient salt in the blood.” It refers to having a blood sodium concentration below 135 millimoles per liter. The normal concentration ranges between 135 and 145 millimoles per liter. Severe cases of hyponatremia can lead to water intoxication, which causes symptoms such as headache, fatigue, nausea, vomiting, frequent urination, and mental disorientation.

The kidneys in humans regulate the amount of water, salts, and other substances that leave the body by filtering blood through their twisted tubules. If a person drinks excessive water within a short time, the kidneys cannot eliminate it quickly enough, causing the blood to become waterlogged. The excess water is drawn to areas with higher concentrations of salt and other dissolved substances, leading it to leave the blood and enter the cells. As a result, the cells expand like balloons to accommodate the excess water. While most cells have the ability to stretch because they are surrounded by flexible tissues like fat and muscle, this is not the case for neurons. Brain cells are tightly packed within the rigid skull and must share this limited space with blood and cerebrospinal fluid. Wolfgang Liedtke, a clinical neuroscientist at Duke University Medical Center, explains that there is almost no room for expansion or swelling inside the skull.

Brain swelling, also known as brain edema, can have devastating consequences. According to M. Amin Arnaout, a nephrology expert, rapid and severe hyponatremia can cause water to enter brain cells, resulting in brain swelling. This can lead to seizures, coma, respiratory arrest, brain stem herniation, and even death.

Where did the belief that drinking large amounts of water is beneficial come from?
Heinz Valtin, a kidney specialist from Dartmouth Medical School, decided to investigate this common advice. After thoroughly examining the scientific literature, Valtin found no evidence supporting the recommendation to drink eight, eight-ounce (237 ml) glasses of water per day. In fact, he discovered that consuming this much water or more could be harmful. It could potentially lead to dangerous hyponatremia, expose individuals to pollutants, and make people feel guilty for not drinking enough. Valtin published his findings in a review for the American Journal of Physiology—Regulatory, Integrative, and Comparative Physiology in 2002. Since then, no scientific report published in a peer-reviewed publication has contradicted his conclusions, according to Valtin.

How much water should you drink?
According to Joseph Verbalis, chairman of medicine at Georgetown University Medical Center, most cases of water poisoning are not caused by simply drinking too much water. It is usually a combination of excessive fluid intake and increased secretion of vasopressin. Vasopressin is produced by the hypothalamus and secreted into the bloodstream by the posterior pituitary gland. It instructs the kidneys to conserve water, and its secretion increases during periods of physical stress. This can cause the body to conserve water, even if a person is drinking excessive quantities. Additionally, Verbalis explains that a healthy kidney at rest can excrete 800 to 1,000 milliliters of water per hour. Therefore, a person can drink water at a rate of 800 to 1,000 milliliters per hour without experiencing a net gain in water. However, if a person is running a marathon, the stress of the situation can increase vasopressin levels, reducing the kidney’s excretion capacity to as low as 100 milliliters per hour. Under these conditions, drinking 800 to 1,000 milliliters of water per hour can potentially lead to a net gain in water, even with considerable sweating.

During exercise, it is important to balance your fluid intake with your sweat output. This includes being cautious with sports drinks, as consuming too much can lead to hyponatremia. If you are sweating, the amount you should aim to drink is 500 milliliters per hour. However, determining the exact amount of water to consume based on sweat output can be challenging. Whether you are a marathon runner or an average person, as long as you are healthy and have a normal sense of thirst, follow Verbalis’s advice and drink according to your thirst. This is the most reliable indicator.

Sources:
https://www.scientificamerican.com/article/strange-but-true-drinking-too-much-water-can-kill/