Important Note: The following calculation applies only to healthy, adolescent to middle aged person with no other illness which may alter water balance in the body and living in a temperate climate. If you have renal disease, heart failure, endocrinal disease or generalised/localised edema of any cause or any disease that may affect the water balance and requirement you need to consult with your physician.
The requirement of water per day is based on the daily energy expenditure of the body. The general guideline being that 1ml -1.5ml of water is needed per kcal of energy expenditure per day. To calculate the daily energy expenditure of a person you would need the following
1) Weight of the individual
2) Sex of the individual
3) Activity level of the individual
Note: Your environment climate (too cold or too hot) can drastically affect your water requirement
The initial step is to calculate the resting energy level of the person. The resting energy expenditure (REE) for adult males is 900+10W and that for the adult females is 700+7W, W is the weight in kilograms.
In the next step, the resting energy expenditure (REE) is adjusted to include the activity level of the person to arrive at the actual daily energy expenditure by multiplying the REE by a value corresponding to the activity level of the person.
Sedentary: REE X 1.2
Moderately Active: REE X 1.4
Very Active: REE X 1.8
For example to arrive at the water requirement of a female weighing 50 kg who is moderately active we first calculate the resting energy expenditure
REE for a 50kg female is 700+ (7x50) = 1050 kcal
Actual daily energy expenditure if she is moderately active would be then 1050 x 1.4 = 1470 kcal
The daily water intake requirement for a 50 kg female with moderate activity would be (1-1.5ml per kcal) = 1470 to 2205 ml
(Note: These are only general guidelines and may not apply to your specific situation as it may vary widely depending on the water loss from body. It’s better to take water based on your thirst levels. )
2) How is water lost from the body?
Water is used during normal metabolism. Losses include 50-100ml in the feces, 500-1000 ml by evaporation and during respiration and around 1000ml in the urine.
3) What are the conditions where there is increased need for water?
Fever – with high body temperature the water loss from the body is increased. It is estimated that during fever water is lost at a rate of approximately 200ml/day per degree Celsius raise in temperature.
Diarrhea - results in increased fluid loss from the body. The losses of fluids vary widely depending upon the severity of the diarrhea. It is important that the fluid and electrolytes are replaced to avoid dehydration.
Lactation – You need to take additional water during lactation depending on the milk production. The general guideline is to take 1 ml of water for 1 ml of milk produced. Suppose the milk production is 500 ml per day then you may need to take an additional fluid of around 500 ml as replacement.
Excess Sweating – Suppose you sweat excessively due to the hot external climate or due to hyper metabolic states like hyperthyroidism, then its better to increase the fluid intake.
Vomiting – Vomiting can result in fluid loss and varies depending on the severity and the frequency of vomiting.
Pregnancy – There is not much need for increasing the fluid intake during the pregnancy. Pregnant women can take an additional 50ml per day to maintain appropriate hydration status.
4) What are the conditions where the water intake is restricted?
Your doctor may sometimes advice you to take restricted amounts of fluids in certain medical conditions like chronic renal failure, heart failure, endocrinal problems, generalised edema etc. It entirely depends on the specific situation of the particular patient.