A Dieticians Perspective: Everyone should drink 8 glasses of water a day

Myth: Everyone should drink 8 glasses of water a day.

Nicole Senior ( Glycemic Index Association Dietitian)

Fact: Most adults need around 2–2½ litres of fluid daily but not all this fluid needs to be water. Fluid needs vary greatly according to climatic conditions, physical activity, body size, diet and your health status.
Eight glasses of water a day is eight metric cups (or eight 8 ounce glasses), or 2 litres. Some experts say there is absolutely no scientific foundation for this oft given advice. This puzzled me because I thought there were principles and guidelines to calculate a person’s fluid requirements and they roughly equate to this 8-glass rule of thumb. Are these guidelines I’ve been using based on good scientific evidence? Before you throw your water bottle against the nearest brick wall, here’s more information to ‘fill out’ the story of how much water we should drink.

Everyone knows we can’t survive long without water. For the more morbid among you, survival time is around 1 week but can be as little as a few hours for a marathon runner experiencing catastrophic heatstroke. Water is essential for life and needed for temperature regulation, digestion, metabolism, absorption of nutrients and excretion of waste. About half the water needed each day goes to sweat and water vapour in our breath. Water accounts for 50–80% of our lean body mass; men have slightly more than women, and the proportion goes down as we age. Replacement of lost water is vital to maintain normal functioning.

The Nutrient Reference Values (NRVs) for Australia and New Zealand acknowledge it is difficult to experimentally derive Estimated Average Requirements (EAR) for water because of individual variation. Because of this, they established an Adequate Intake (AI) based on the median intake of the population. This is a roundabout way of saying they came up with a best guess: 2.1 lites (8 cups) for women and 2.6 litres (10 cups) for men, with clear caveats that people living in hot climates or very physically active need more. You can see how the 8-glass a day rule is starting to sound plausible.

However, what the 8-glass rule fails to recognise is you don’t have to drink all your fluid requirements. There is a lot of water already in food, especially fruits and vegetables as well as the obvious liquid and semi-solid foods like soups, yoghurt and custard. According to Australia’s last national nutrition survey, the intrinsic water in food contributed 700–800ml per day. Water is also a by-product of metabolism: around 250ml (1 metric cup) per day is produced this way. So more accurately, the 8-glass a day rule should be more like 4–6 glasses.

What about thirst? I’ve heard the human thirst mechanism is a poor indicator of our fluid needs and we should drink even though we aren’t thirsty, but is this true? A perusal of the scientific literature suggests this is only true in elite athletes because their fluid needs are higher, but for the rest of us, our thirst serves us well.

So, drink when you feel thirsty and don’t feel you have to gulp down 8 glasses of water a day.

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