OET Reading 11C1

How much fluid should you drink each day for good health? Eight glasses a day has been the widely circulated advice. But recently, two large studies have suggested that’s probably overkill. It turns out that under normal circumstances, you get most of the liquid you need each day from what you routinely eat and drink, including coffee, tea, soft drinks, and even some alcoholic drinks.

So where did this notion of ‘eight glasses a day’ come from? In 1945, the Food and Nutrition Board of the United States National Research Council wrote: ‘A suitable allowance of water for adults is 2.5 liters daily in most instances. Most of this quantity is in prepared foods: But in the 1990’s Dr Heinz Valtin undertook a comprehensive investigation into the myths surrounding water consumption in humans. He found healthy people who drank more water didn’t have a higher ‘output of stool’, and that there was no scientific evidence high fluid intake could relieve constipation. 

And what of the belief that thirst is not a good indicator of a need to drink? Valtin states that while ‘a rise in plasma osmolality’ (which is an internal chemical change) of less than two per cent can elicit thirst, dehydration is defined as a rise of at least five per cent. This is a complicated way of saying you get thirsty before your body starts to dehydrate, so thirst is a good guide.

Australia’s current dietary guidelines don’t recommend a specific amount of water, but simply recommend we ‘drink plenty of water’. “How much water each one of us needs depends on a range of factors,” said CSIRO dietitian Pennie Taylor.” This can include our gender, bodyweight and how much physical activity we do: ‘The guidelines also encourage drinking water over juices, soft drinks, cordials or the like. Also, pregnant or breastfeeding women (who require more fluid), people who live or work in extremely hot climates, and people with high protein diets (the kidneys may need more fluid to help process the increased amount of protein) are encouraged to drink more water. It’s on hot days that most of us notice we’re thirstier than normal. This is because we’re sweating more, and we lose fluid through sweat. “We can lose between 1 to 3 per cent of our fluid quite easily,” Ms Taylor said. 

Associate Professor Ben Desbrow from Griffith University agrees. “Those who work or exercise in hot climates lose the most fluid — up to 2.5 liters of sweat in an hour in extreme circumstances. You need to replace those fluids pretty quickly; otherwise it’s going to fairly rapidly have an effect on your subsequent performance.” Your body will give you some pretty clear signs that you’re not getting dehydrated. So keep an eye out for symptoms such as a dry mouth, headache and feeling dizzy. Also pay attention to your toilet habits, the colour of your urine and how frequently you go to the toilet. It is true that ‘copious and clear’ is a good indicator of healthy wee. But ‘clear’ does not mean colourless. The depth of colour in urine will vary, what you need to look out for is cloudiness — that’s the indicator of a problem. “Your kidneys do a great job in fluid regulation, so frequency of urination and colour of urination are your two best guides,” Associate Professor Desbrow said.

What about the idea that a person may be drinking too much water. There isa thirst control centres in our brain that controls water intake, says Dr Michael McKinley, Senior Fellow at Florey Neuroscience Institute. When we drink water, this part of our brain stops us feeling thirsty long before the water has been fully absorbed into the bloodstream. “Usually if we take in too much water, it’ll suddenly feel like hard work to drink,” he said. However, in some circumstances when people drink a large volume of water, they can over-ride the thirst control centre in the brain. When this happens, their sodium levels can drop too low. This can lead to a condition known as hyponatremia, where the body also starts to retain the excess water. “Normally if we drink too much water, our kidneys would excrete it [as urine],” Dr McKinley said. But sometimes, factors like heat, physical stress or certain drugs can switch off the hormonal signal that causes the kidneys to excrete excess water. Then there is a double whammy effect. Not only have you drunk a lot of water, but you start to hang onto all the water in your body. Drinking more just makes things worse. “This is when things can get dangerous,” Dr McKinley said.