Detailed FAQ on Fluids for Optimal Hydration Status
This answer is brought to you by many of the Australian nutrition professionals who regularly contribute to a nutrition email discussion group.
The following questions and answers address the quantities and types of fluids recommended to maintain appropriate body water levels, and to assist in weight control.
1. How much fluid should I drink each day?
There is a common perception that 'everyone should drink eight glasses of water per day'. Assuming that a typical glass has a volume of ~250 millilitres (mL) this would mean that everyone should be drinking about two litres (2 L) of water each day.
Although it may be true that many people would benefit from drinking more water, it is not possible to specify a quantity that is appropriate for everyone. The requirement to drink water depends on how rapidly it is lost from the body, and also on how much water is obtained from our food. Nearly all 'solid' food actually provides some water--for example, most fruits and vegetables are about 90% water by weight, and we also obtain some water as a result of metabolising protein, fat and carbohydrate.
Water is lost as 'insensible' (i.e. unnoticed) perspiration through the skin, as liquid water in sweat and urine, as water vapour in the breath, and as a component of faeces. Of these, the most variable is the amount of water lost as sweat.
Sweat rates vary enormously. For example, a rate of 3.7 L/hour was measured in Alberto Salazar, the winner of the 1984 Los Angeles Olympic Marathon, which was conducted on a hot summer's day. This means that if Salazar had drunk only 2 L of water during the race, he would have undergone a significant level of dehydration before the race was even half finished. Although this sort of sweat rate is at the high-extreme level, it does illustrate that generalisations about water requirements can be quite misleading.
How much water you need will depend on many factors, including the weather, how much physical work you do, the composition of your diet, and your state of health (e.g. the requirement is much greater during fever, when undergoing treatment for kidney stones, and for people who have a tendency to suffer constipation).
It is also advisable to drink more water than usual in dehydrating environments. These include offices where air-conditioning produces dry air; hot/dry climates (such as occur in much of central and northern Australia); and long-distance flying. In the case of flying, increased water consumption is recommended not only for good hydration status, but also as a preventive measure, along with plenty of physical activity, against deep vein thrombosis (DVT).
2. How can I tell when I need to drink?
For most people, thirst is a pretty good indicator of the need to drink (although it isn't perfect in all situations, as described in detail below). For people who do not readily experience thirst, other relevant indicators include infrequent urination, low volume and dark colour of urine. If you do not have to urinate every few hours, and the urine you produce is much darker than usual, you have probably started to dehydrate.
3. Does it matter what sort of fluid I drink?
For most people water is the most appropriate rehydration fluid under all but exceptional circumstances. Water provides no kilojoules, so it is ideal as the thirst quencher if weight control is an issue (as it is for most Australian adults and an increasing number of children.
Water also has no detrimental effect on the teeth, because it contains no sugar and is not acidic (and if it is fluoridated, it is actually beneficial to dental health).
By way of contrast, many carbohydrate-containing drinks (such as sports drinks, some fruit juices and carbonated soft drinks) not only contain sugars (which increase the risk of tooth decay) but are also quite acidic. People who continually 'bathe' their teeth in sugary, acidic drinks are at high risk of damage to the tooth enamel because of acid erosion.
There are still valid reasons for drinking some carbohydrate-containing fluids. Fruit juice, for example, is a major source of vitamin C in the Australian diet, and sports drinks are thought to be superior to water for rehydration during hard physical work (or exercise), especially in the heat, as described in greater detail in the answer to question 5, below.
Some fluids are not really suitable as the primary sources of water because they contain 'diuretics' (substances that stimulate urine production). Most alcoholic beverages and drinks with high levels of caffeine come into this category. Although the body can cope with moderate quantities of alcohol and caffeine, excessive intakes can have negative effects on hydration status. This applies especially in people who are not used to drinking large volumes of these beverages.
And of course, excessive alcohol consumption also has short-term adverse effects on judgement and coordination, and long-term adverse effects on health.
However, it is worth keeping in mind that small quantities of caffeine are not associated with health risks, and at least one caffeine-containing beverage, tea, is also a useful source of antioxidants, which are thought to be health-promoting.
And the occasional cup of coffee, although not thought to have any particular beneficial effects on health, provides a 'pick-me-up' boost to mental performance through its caffeine content.
5. Are there circumstances in which I should drink fluids other than water?
Fluid intake should be increased when you are exercising or working hard in the heat, and under these circumstances there is good reason to consider using carbohydrate-electrolyte solutions (e.g. 'sports drinks'). It is also appropriate to start drinking even before you feel thirsty, because the sense of thirst may not be activated until well after dehydration has begun.
When sweat rates are high, sports drinks are valuable because they will reduce the likelihood of 'water intoxication' (also known as 'hyponatraemia'). Water intoxication can occur when profuse sweating continues for many hours, with water as the only fluid consumed. Under these circumstances, the salt lost in sweat may not be replaced unless foods containing sodium are consumed or you rehydrate with sports drinks (such as those commercially available).
But remember that commercially-available sports drinks are not only sugary but also acidic, and so can cause considerable damage to teeth if they are consumed inappropriately. With regard to dental health, it is preferable to drink a fairly large volume of carbohydrate-containing fluid (e.g. 500 mL) every 30-60 minutes, and follow this with a mouthful (or two) of plain water to wash the sugar and acid from the teeth, rather than drink small volumes frequently.
Also with regard to fluid intake and dental health, it is advisable to drink fluoridated water and ensure adequate calcium intake (e.g. by drinking three glasses of milk, preferably low- or reduced-fat, or eating an equivalent quantity of milk products daily) to ensure that there is enough calcium in the diet for 'remineralisation' (i.e. repair) of tooth enamel.
6. Are there circumstances when I should limit the amount of fluid I drink?
Under certain medical conditions, some people may need to limit their fluid intake. This may apply if cardiac, renal (i.e. kidney) or liver failure has previously occurred. Your doctor or dietitian can advise you about appropriate levels of fluid intake if you suspect that any of these apply to you.