8 glasses a day, keep the thirst away?

Updated: Mar 17

Our total body water (TBW) is dependent on our age.

Take TBW as a percentage of total body weight, there is approximately 80% TBW in premature infants (less than 37-week-old), 70-75% TBW in term infants (from birth to 1 year-old), 65-70% in toddlers (1 to 3-year-old) and 60% after puberty.

TBW in our body varies with the amount of fat as well, this is because fat has much lower water content. Thus, individuals with more fat have lower TBW.

Where did all the water go?

At a cellular level, the water is separated into 2 main compartments: Intracellular fluid (ICF), about 2/3 of TBW and Extracellular fluid (ECF), around 1/3 of TBW. The fluid in the body helps transport gases like oxygen, nutrients, and wastes. It also acts like a medium for exchange substances between cells and ECF.

For example, a 55 kg young lady with 60% TBW, has around 33L of water around her body, where 22L is in ICF, and 11L is in ECF.

So, if you were the 55 kg young lady, does that mean that you have to drink 33L water a day?

Not quite.

There are a few factors that we need to take into consideration:

  • The recommendation of drinking 8 glasses a day (or more commonly known as the “8x8” rule) was misinterpreted to drink only 8 pure glasses of water a day, ignoring the water intake from other sources.

  • When we are talking about total water intake, that includes the water in the liquid and food that we consume, such as coffee, tea, raw fruits, juice, etc.

  • A warmer season/environment requires more water intake than in the cooler places.

  • If you sweat more e.g., you are an active athlete, you definitely need more water to cover water loss.

  • If you’re pregnant or a breastfeeding mama, you will need to drink extra water as your body is working for 2 (or more).

  • As water gets into our body, it goes out as well. The amount of total water intake also depends on our water output.

We do not lose all 33L of water in a day. Usually, our water outputs are 2-3L per day. Hence, we compensate for what we have lost.

*It is normally assumed that the contribution of food to total dietary water intake is 20–30%, whereas 70–80% are provided by beverages. This relationship is not fixed and depends on the type of beverages and on the choice of foods

+ Foods with a wide range of water content (<40 to >80%)

a. Average total water intakes in sedentary women

b. Average total water intakes in sedentary men

Adapted from Water as an essential nutrient: the physiological basis of hydration

All in all

Water intake can be varied by age, physical condition, temperature, eating habits, etc. Most importantly, we will need to replace what we have lost. If you feel thirsty, drink. If you realize that your urine is dark-yellow in colour, drink. This is because the regulation of water balance is essential for the maintenance of health and life. Stay hydrated!


  1. Jéquier E, Constant F. Water as an essential nutrient: the physiological basis of hydration. Eur J Clin Nutr. 2010;64(2):115-123. doi:10.1038/ejcn.2009.111

  2. Tobias A, Ballard BD, Mohiuddin SS. Physiology, Water Balance. In: StatPearls. StatPearls Publishing; 2022. Accessed March 6, 2022.

  3. General principles of disorders of water balance (hyponatremia and hypernatremia) and sodium balance (hypovolemia and edema) - UpToDate. Accessed March 6, 2022.

*Stock images from Freepik

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