How much water to consume?
It is not necessary to measure and consume water. The water in-take can be highly variable depending on the sensation of thirst and cultural practices. Two thirds of the body weight is composed of water. It is freely permeable between the extra cellular and intra cellular compartments depending on osmotic activity. The water input in the body is the total of the liquids including water that we drink and the water generated by the metabolism of food.
The water loss from the body is through the sweat, urine and feces. Normally this is delicately balanced by the brain and kidneys. There is a osmo centre or osmostat in the brain which senses the tonicity (concentration) of blood and produces the Anti Diuretic Harmone (ADH) which stimulates the thirst centre in the brain as well as reduces the water loss in the urine. This is how in spite of a variable water intake the body composition of water remains constant. The kidneys can concentrate urine so that the urine output for the day is as low as 500 ml or it can dilute and produce a urine volume of 10 litres per day. Normally it is not possible to get dehydrated if a person has free access to water and his brain is normal. That is why animals do not calculate but consume as per thirst and live healthily.
However the ability of the kidneys to get rid of excess water from the body when we consume more than directed by the thirst sensation is called free water clearance. So a normal person may be able to clear up to 10 litres per day .But in presence of kidney disease, liver disease, heart disease and intake of certain drugs called diuretics this free water clearance is affected. The body retains the extra water and this can be diagnosed by estimating the serum sodium which drops. This is called hyponatremia (serum sodium less than 135 mEq/lt). This can be a serious condition.
In conclusion the body has an excellent thirst centre which guides us in consuming the right quantity of water. It is not required for a healthy person to measure and consume water. But in those patients who have impaired water clearance water restriction may be required.
- Madras Institute Of Nephrology, Chennai, India