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When, what and how much to drink?

Water is more vital to life than food and humans cannot survive long without it. The amount of water the body needs varies according to size, level of physical activity and the atmospheric conditions. The bigger your surface area, the more active you are and the hotter the temperature, the greater your fluid needs. If you’re sweating profusely, you can easily lose a litre of sweat in an hour.

Thirst is generally an excellent indicator of fluid needs, although it may also take some time to replenish the kind of rapid fluid losses that can occur if you’re engaged in strenuous physical activity. The thirst response can also fail in frail aged people, and children may sometimes be reluctant to stop what they’re doing to respond to thirst.  

Eight glasses of water a day?

This recommendation was made by the US National Research Council in 1945, based on the rationale that humans needed 1mL water for each calorie of food. At the time, the average person was assumed to need 2,000 Cals (8,400 kilojoules) a day, which equated to 2,000 mL or 8 x 250 mL glasses of water. That figure has stuck in many people’s heads, although the original statement also noted that much of this water would come from the foods people ate. 

It’s obvious that liquids provide water, but some people may not realise that foods are also a source of water. Many vegetables are over 90% water. Fruits range from 75% water in a banana to over 90% in melons and some tropical fruits. Cooked lean meats are 60-65% water. Cooked pasta is 65-70% water. Pizza is 45-50% water. Even breads are around 35% water. It all adds up.

Needs vary

Eight glasses of water a day will be more than a small or sedentary person in a stable environment would need, but not nearly enough for someone doing strenuous work or endurance sports. The easiest way to tell if you are drinking enough water is to look at the colour of your urine. First thing in the morning, it will be concentrated and yellow but by mid-morning, it should look clear. If it’s still yellow by mid to late morning, it means you haven’t been drinking enough.

Those who have had or are at high risk of kidney stones will also need plenty of water.

Other drinks?

Once upon a time, humans were like all other animals and quenched their thirst with water. Until fairly recently, many people consumed much of their water as tea or coffee. More recently, and with advertising playing a major role, the beverages industry convinced people they needed soft drinks, juices, diet drinks, bottled water, mineral waters and sports drinks to ‘stay hydrated’.

Tea and coffee are perfectly suitable ways to consume 2-4 cups of water a day. The idea that these drinks are dehydrating doesn’t apply at that level. One of the reasons why we need fluid is so the kidneys can use some of it to flush out unwanted substances that arise from our diet or normal metabolism.

For many Australians, a cold beer seems like a very attractive way to deal with thirst. One beer might not cause problems, but it’s wise to quench your thirst with water before starting on any kind of alcoholic drink. For anyone who has been working hard or playing strenuous sport, it’s especially important to replenish lost fluid with water, before alcohol. During many types of sport or heavy work, small injuries occur in muscle fibres. These will normally heal spontaneously within 24 hours. However, if you add alcohol to damaged dehydrated body tissue, small injuries can take about five days to repair. It’s therefore recommended to replenish fluid losses with water before starting on that nice cold beer or chilled white wine.

Sports drinks contain some sugars as well as electrolytes that are useful for elite sports people, especially during and immediately after strenuous physical activity. They are not needed by children or for any moderate workout or for any activity that is less than an hour or so.

Soft drinks and juices will also provide fluid, but it comes with a heap of sugar. An average can of soft drink contains 10-12 teaspoons of sugar. Flavoured mineral waters are slightly less sweet but still count as high sugar drinks.

Juices sound healthier and at least they provide some vitamins, but their sugar content rivals that of soft drinks. When we eat fruit, the sugar is accompanied by dietary fibre. The fibre has many virtues, including the fact that it slows down the rate at which the sugar in the fruit is absorbed. Dietary fibre is also filling. Few people would (or could) munch their way through five apples, but if you juice them, it takes only seconds to take in all their sugar (and kilojoules). 

All soft drinks, including artificially sweetened ones, sports drinks and juices, are highly acidic and that’s a dental hazard. It’s even worse if the drinks are sipped frequently. If you do have a juice or soft drink, it’s best to drink it over a short period and then rinse your mouth with water to prevent the acids eroding tooth enamel. If 

you have been watching the Australian Open, you may notice that tennis players, like many sportspeople, almost always follow their sports drink with a mouthful of water.

Summary points on fluids

We need water, but eight glasses a day is not a sacred number.

Before drinking alcoholic beverages, quench your thirst with water, especially after heavy sweating.

Intersperse alcoholic drinks with water, or mineral or soda water.

Children are more likely to drink water if they see adults drinking water.

Fill a jug or flask with water and chill it in the fridge. After water has been standing for a while, the chlorine evaporates and the taste improves.

Add ice blocks to water or drink plain sparkling water.

Treat yourself to a nice glass. Years ago, when we gave a group of people water in different utensils including a polystyrene cup, a plastic cup, an enamel mug, a coffee mug, a thickish glass, or a wine glass and asked them to rate which water they preferred. The wine glass won hands down. It was all just plain tap water. 

Dr Rosemary Stanton OAM

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