The “adequate intakes” officially recommended for total water from all sources each day (for adults between 19-30 years of age) are:
- 3.7 liters (or about 130 fl oz) for men
- 2.7 liters (about 95 fl oz) for women.
These dietary reference intakes, however, are based only on survey results of the average amounts that are consumed by people, on the assumption that these amounts must be about right for optimal hydration.
But the amounts measured for people in the temperate climate of the US, with plenty of access to water, may be too high, and intakes do vary greatly according to activity, environmental conditions (including clothing) and social activities such as drinking with friends.
The recommended amounts are of limited value for another reason – total intake figures fail to give a breakdown of how much can be divided up between different kinds of food and beverage.
Consequently, the common sense guidance that is now gaining ground goes along the lines of this summary from the US National Library of Medicine:
“If you drink fluids when you feel thirsty and have beverages with meals, you should get enough water to keep you hydrated.”
Dietary sources of water intake
Intake from plain water can, of course, come from the tap or bottles, but water is available from other beverages and solid foods, too.
Again, the estimates for the proportions of fluid we obtain from fluids versus foods are based on surveys of average diets, but the individual variety is huge.
One survey of an adult population sample in the US in the late 1970s found that total water intake was made up of 28% from foods, with the same proportion coming from drinking water and 44% from other drinks. Other surveys give a lower figure of about 20% of water intake coming from foodstuffs, with 80% obtained from fluids.
The water content figures for different foods and fluids are much more consistent than the proportions of consumption. Water content ranges along the spectrum of the following examples:
Coffee is popularly thought of as dehydrating, whereas scientific evidence is to the contrary.
- 90-99% water: fat-free milk, juicy fruits such as strawberries, vegetables such as lettuce, celery and spinach
- 80-89%: fruit juice, yogurt, fruits such as apples, pears and oranges, vegetables such as carrots and cooked broccoli
- 70-79%: bananas, avocados, baked potatoes, cottage and ricotta cheeses
- 60-69%: pasta, beans and peas, fish such as salmon, chicken breasts, ice cream
- 30-39%: bread, bagels, cheddar cheese
- 1-9%: nuts, chocolate cookies, crackers, cereals
- 0%: oils, sugars.
Water content does not directly translate into hydrating “power,” however. Milk, for example, is more effective as a source of hydrating fluid than plain water, even though it obviously has a lower water content.
Caffeinated drinks are widely mistaken as being poor at hydrating us because of a belief that they have a diuretic effect on our water balance.
A number of studies to properly test the effect of caffeine on hydration have shown that tea and even coffee are in fact good sources of water and do not dehydrate us. “Advising people to disregard caffeinated beverages as part of the daily fluid intake is not substantiated,” says one study, while another concludes that there is “no evidence of dehydration with moderate daily coffee intake.”
Maintaining safe hydration during exercise10
The messages of sports drinks makers – and even of respected bodies such as the American College of Sports Medicine in their 2007 guidance – would have people serious about exercise worrying about drinking lots of fluids to beat dehydration during physical activity.
The truth, however, is that there is a relatively simple way to ensure that fluid replacement strategies are sufficient and not over-the-top: just follow thirst.
The study about the runners in the 2002 Boston Marathon, for example, produces the following conclusion in the paper published in the New England Journal of Medicine:
“Because runners vary considerably in size and in rates of perspiration, general recommendations regarding specific volumes of fluids and frequencies of intake are probably unsafe and have been superseded by recommendations favoring thirst or individual perspiration rates as a primary guide.”
Measuring the amount of fluid lost as sweat during exercise – as a guide for how much fluid to take under similar conditions – is simple: take a reading of body weight before and after exercise. The difference in grams is the same as the amount of fluid lost in milliliters.
Likewise, this is also a reliable measure of whether too much fluid is being taken in during exercise, because a gain in weight signals too much consumption versus the amount being lost.
This individualized approach would avoid hyponatremia caused by taking in too much water during endurance running. The development of dangerously low blood plasma osmolality varies between individuals and so it is not safe to recommend a fixed volume of fluid intake for everyone.
As the study authors explain, runners weighing themselves before and after training is best because, for example:
“Smaller runners may drink larger volumes of fluids in proportion to their size than larger runners.
“Conversely, in proportion to their size, larger runners may lose less free water than smaller runners through evaporation (by means of sweat), as a result of a lower ratio of surface area to volume.”