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Michelle Obama wants you to drink more water. Technically, she was speaking to Americans enamoured with calorific, sugar-loaded beverages when she made her pitch for H2O in Watertown, Wis., in September. But the message – everyone can benefit from drinking more water – resonates on this side of the border.

There's one key component missing from the U.S. first lady's plea, however. Just how much water should we drink, exactly?

The answer isn't so simple.

The eight-glass-a-day myth

Ask anyone how much water the average person should drink in a day. Nine times out of 10, the answer will be eight glasses a day.

Maybe they heard it from a gym teacher, read it in the latest diet book or found it online. It's accepted as a given – but that doesn't make it true.

Health Canada doesn't have a recommendation for water consumption. Instead, the department simply advises people to "drink water regularly."

But the idea that we all should be chugging back eight glasses is so ingrained that few people stop to think about where that pearl of wisdom originated.

Luckily, Dr. Margaret McCartney has. In 2011, McCartney, a general practitioner in Glasgow, Scotland, wrote an editorial published in the British Medical Journal that turned conventional thinking about daily water needs on its head.

McCartney pointed out that there is no clear evidence that simply increasing the amount of water we drink each day is good for our health. And that the idea that everyone needs to aim for eight glasses a day is not rooted in science. She said there's no point worrying about a recommendation that has "mythological origin."

"If we drink according to thirst and comfort, I think we'll be absolutely fine," McCartney said in a recent interview.

So where did the eight-glasses-a-day notion come from?

A 2012 article in the Australian and New Zealand Journal of Public Health tells how the first recommendation to drink eight glasses a day may have been a footnote in the 1945 U.S. Dietary Guidelines. Despite the non-existent scientific evidence backing that particular recommendation, the idea stuck.

Don't drink your calories

However, no one should mistake the questioning of water-intake guidelines for an argument against water consumption.

"I do think we need to drink more water in an effort to drink less sugary drinks," said Dr. Michael Joyner, a physiologist at the Mayo Clinic in Rochester, Minn.

Fruit juice, sports drinks and pop are often singled out by public-health advocates as a contributor to the rising rates of obesity in Canada. Those beverages are often packed with sugar and are high in calories and provide no real nutritional benefit.

Earlier this year, a study from researchers at the Harvard School of Public Health said sugary sweetened drinks contribute to 180,000 deaths around the world each year. Despite the messages about the potential harm, sugary beverages remain popular. For instance, according to a 2008 Statistics Canada report, more than half of boys and 35 per cent of girls between 14 and 18 drink pop on any given day.

In response to criticism, many beverage-makers are putting clearer labels on their products and are promoting consumer education.

Joyner said Michelle Obama's move to promote the consumption of water over other beverages should be commended. But people should let their thirst dictate how much water they drink each day, not a meaningless guideline or promotional campaign.

"It really varies based on environmental conditions," he said. "If you're out in the middle of a desert or involved in a lot of heavy physical activity … you probably need to be drinking a lot more [than eight glasses]," he said.

Straight from the source

Health professionals aren't the only ones who would like to see us drink more water. Bottled-water companies (many of which are owned by makers of pop and sports drinks) also deliver the message that H2O is good for health. Obama's campaign, called Drink Up, is sponsored by numerous bottled-water makers. McCartney has been a vocal critic of Hydration for Health, an international initiative to promote water consumption. The campaign was created by multinational corporation Danone, which sells several brands of bottled water, including Volvic and Evian.

McCartney, who wrote about Hydration for Health in her BMJ article, said the growth of the bottled-water industry is concerning, because transportation and the leftover plastic bottles create unnecessary waste and environmental concerns.

"I'm not a fan of using bottled water," she said. "I think that there's a lot to be said for drinking tap water. It's a good drink. It's cheap, it does us lots of good."