Water constitutes 60% of body weight and it is widely known that humans cannot survive without water for more than few days. Reduced water intake may lead to dehydration while excessive consumption may result in water intoxication. Salt (sodium chloride) is a very essential constituent of the body and it is found in most of the food items. Salt flavour is one of the basic tastes, making salt one of the oldest, most ubiquitous food seasonings. Salting is an important method of food preservation. Sodium has an important role in the regulation of blood pressure and blood volume. It is the most abundant element (cation) in the body fluids. Low sodium level in the blood is called hyponatremia and this condition is characterized by abnormal brain functions. Excess salt intake may worsen blood pressure control, precipitate cardiac failure and worsen kidney disease. There are lot of misconceptions regarding the recommended intake of salt and water in healthy individuals and those with disease conditions. The purpose of this article is to give some insight into the physiology water and salt balance and give few recommendations, regarding the intake in healthy people as well as those with disease conditions.
It is important to ensure that body is adequately hydrated. Good urine output, pale brown urine, absence of significant thirst and healthy skin (normal skin turgor) are markers of adequate hydration. People who are exposed to hot dry weather conditions and those who are involved in strenuous physical activities tend loose a lot of water as sweat. Though thirst mechanism is sufficient to take care of the water needs it is essential that drink plenty of water so that a good urine output is maintained. Patients with kidney stones should also drink plenty of water to prevent crystallization of various salts (calcium oxalate etc) in the urine. In them, more than the quantity of water intake it is important to ensure that they pass at least 2500 ml of urine everyday. They should also make sure that water intake is distributed throughout the day.
How much water a healthy person who is not exposed to hot dry weather and not involved in strenuous physical activities, should drink is a matter of debate. The recommendation that healthy people should drink several glasses of water to keep them healthy and to improve the kidney functions is not based on any scientific observations. There is no evidence to suggest that excess water intake will enhance the removal toxins from the body and improve the functions of internal organs. Scientific recommendation is that one should drink according to thirst and at the same time water intake should be sufficient to avoid dehydration. Increased water intake will certainly increase the urine output and increase the frequency of visits to toilets but is not translated to any improvement in kidney functions.
A healthy average sized adult can consume up to 12-14 litres of water in a day without any features of water overload. Kidney will not be able manage additional water intake beyond this limit and it may lead to a condition called water intoxication. This is a condition characterized by low sodium level, convulsions and loss of consciousness and it can be life threatening. There are some psychiatric conditions, associated with a craving for water intake exceeding this upper limit. Although body can handle up to 12-14 liters of water per day, kidneys can produce only up to 800 ml of urine per hour. Hence if large quantity of water is consumed over a short span of time this could also result in water intoxication. This phenomenon of water intoxication can potentially occur in patients awaiting radiology procedures who are instructed to drink plenty of water to ensure that the urinary bladder is full.
With declining kidney and cardiac functions ability to handle water load comes down. Creatinine level in the blood is the parameter often used for measuring kidney functions. In a healthy adult, creatinine level will be less than 1.4 mg/dl. Though severe kidney failure is associated with a high creatinine level, any value level higher than 2 mg/dl also suggests impaired kidney functions. A significant proportion of patients with chronic diabetes and hypertension have mild to moderately deranged kidney functions. Though they are oedema free and are able to tolerate the usual water load, excess water intake is likely to cause hyponatremia, fluid overload and precipitate heart failure in them as kidney will be unable to handle this extra water load. Patients with heart failure should also restrict water intake as excess fluid may worsen this condition.
The belief that increased water intake will improve kidney functions is only a myth. Everyday, nearly 1200 litres of blood flows through the kidneys, from which 180 litres of fluid is filtered from which eventually 1.5 –2 L of urine is generated. While dehydration will reduce the blood flow to the kidney and reduce kidney functions, increasing hydration beyond a point will not improve the blood supply to the kidney or increase filtration rate and will only increase the urine output without any beneficial effect on the toxin removal.
Salt has an important correlation with blood pressure. Though all mammals share a similar physiology, hypertension, which is uncommon in animals and common in humans, is linked to the increased salt intake among humans. In certain tribal communities of Africa, salt intake is low and the prevalence of hypertension is also low. More the salt intake, the higher are the chances of developing hypertension. Several population studies have shown a direct correlation between salt intake and the prevalence of hypertension, cardiovascular and kidney disorders.
Although in a healthy individual extra salt load can be handled by the kidneys, prolonged high consumption of salt may lead to hypertension, cardiovascular diseases and renal diseases. Hence it is advised that healthy individuals should also restrict salt intake to less than 8g per day. Increased salt consumption can worsen fluid overload and heart failure. Those with hypertension should reduce the intake to 4 g per day and those who require several drugs to control blood pressure and have swelling of the body or kidney or heart failure should restrict the intake even further. Pickles, pappat and other preserved foods are rich in salt and intake of these substances should be restricted. Hypertension is becoming increasingly common in our population and one of the reasons cited is the increased consumption of rich foods with high salt content.
While it is important for patients with hypertension, kidney disease and heart disease to restrict the salt intake it should not be completely avoided from the diet. A healthy individual will be able to conserve sodium even on a severely salt restricted diet while those with disease conditions will continue to loose salt in the urine. The ongoing salt loss in the urine coupled with insufficient intake will eventually lead to the dangerous medical complication of hyponatremia. In fact, hyponatremia is a frequent cause of hospitalization among hypertensive and kidney failure patients and it is often precipitated by insufficient consumption of salt. This condition is more common among women who are also taking a particular type of antihypertensive medication called thiazide diuretics.
Summing up, while adequate water intake to prevent dehydration is important, drinking water in excess quantities has no known scientific benefits. People with compromised kidney and heart functions should restrict fluid intake to prevent fluid overload and hyponatremia. Patients with hypertension and disorders of the kidney and heart should also restrict salt intake, but should not entirely avoid salt.