Adequate hydration is essential for all bodily functions to work well, including the physiological processes of digestion, absorption and excretion.
Water is the largest single component of the body and is an essential component of all body tissues. It plays a key role in the structure and function of your circulatory system and acts as a transport medium for nutrients in your body.
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General Hydration Requirements
Water is naturally filtered out by the kidneys, gastrointestinal and respiratory tract and lost as urine, feces and vapor through the breath or skin. Since our bodies have no way to store water, we need to keep replacing it to keep ourselves healthy and keep bodily functions working efficiently.
A simple way to estimate how much water you should drink in a day is ~4 cups per 1,000 kcal that you eat (or more to compensate for the water lost exercising / sweating).
Babies generally require more as their kidneys continue to develop, while they have a higher percentage of water in their bodies than adults (~75% at birth, dropping to ~65% by 1 year and 55-60% in adults). Their larger surface area to body weight also means there’s more skin through which they can lose water. Nonetheless, babies will still get all their water needs from breastmilk / formula early in life.
Lactating women also need more water than normal – an extra 2-3 cups per day – since a large amount of water is needed to produce milk.
Diluted fruit juices, herbal or teas (caffeine-free) can also be good sources of hydration, while the food we eat also helpfully provides a significant amount of water, particularly fruit and veg.
Water Content of Common Foods
|Green beans, boiled||89|
|Apple (raw, peeled)||84|
|Fish (e.g. baked haddock)||74|
|Chicken (roasted, white meat)||70|
|Beef (e.g. sirloin)||59|
|Cheese (e.g. swiss)||38|
Effects of Dehydration
The first and obvious sign of dehydration is feeling thirsty. So when you feel thirsty, your body already has a water deficit which you should replace. As your body loses more water, health risks steadily increase.
|% of Body Weight Lost||Effect|
|1-2||Thirst, discomfort, loss of appetite|
|3-4||Impaired physical performance, nausea|
|7-9||Dizziness, weakness, labored breathing with exercise|
|10+||Muscle spasms, delirium, eventually failing renal function|
During the first 6 months of age, no additional fluids other than breastmilk/formula are needed unless your baby is unwell and you’ve been advised accordingly by your doctor. In general, a healthy baby will get everything they need from the breastmilk/formula they are already drinking.
After 6 months of age, once (semi)solid food is introduced in the child’s daily nutrition, they should start to get a little extra water:
- ~700 ml/day for 6-month olds (including breastmilk/formula)
- ~800 ml/day between months 7 and 12 (including breastmilk/formula)
- ~1,300 ml/day between years 1 and 3 (including breastmilk/formula)
- ~1,600 ml/day between years 4 and 8
A more reliable and accurate way to calculate hydration requirements in children is according to their body weight.
Hydration Requirements in Children
|Body weight||Hydration requirements|
|< 22 lbs||1.5 oz/lbs|
|22 – 44 lbs||34 oz + 1.5 oz/lbs for each lb above 22 lbs|
|> 44 lbs||50 oz + 0.6 oz/lbs for each lb above 44 lbs|
During the second half of the first year of life (months 6 – 12) when (semi)solid food is introduced, the main source of hydration for infants should be plain water.
Introduction of water should start with a few sips a day from small cups (you can use a shot glass) and then gradually be raised to 3.5 – 7 lbs from a 360 cup / bottle / straw. Fruit juices, sweetened or fizzy drinks and cow’s milk are not ideal before 1 year.
- Charney P. Water, Electrolytes, and Acid-Base Balance. In: Mahan LK, Escott-Stump, S. Krause’s Food & Nutrition Therapy. International Edition, 12e. 2008. pg. 144-159.
- Sawka MN, Cheuvront SN, Carter R. (2005) Human water needs. Nutr Rev. 63:30-39.
- Armstrong LE. (2005) Hydration assessment techniques. Nutr Rev. 63:40-54.
- Gandy J. (2015) Water intake: validity of population assessment and recommendations. Eur J Nutr. 54(2):11-16.
- Meyers RS. (2009) Pediatric Fluid and Electrolyte Therapy. J Pediatr Pharmacol Ther. 14:204-211.