The oft-used recommendation to drink six to eight glasses of water a day "is not only nonsense, but is thoroughly debunked nonsense," argues a Scottish physician in a medical journal commentary. But many are already disagreeing.

Dr. Margaret McCartney, a general practitioner, writes in this week's British Medical Journal that there is no clear evidence that drinking lots of water offers any health benefits. Yet, she says, the "myth" persists and is trumpeted by Britain's National Health Service, the equivalent of Health Canada.

Health officials in Canada are among those who tell citizens to aim for eight glasses of water a day. Eat Right Ontario, a joint service of the Dietitians of Canada and Ontario's Ministry of Health Promotion, says water helps to move nutrients and waste through the body, maintain normal blood pressure, and regulate body temperature.

It advocates nine glass of water a day for women and 13 a day for men.

But McCartney insists that too much water can cause problems. It can lead to hyponatremia, which means low sodium levels in the blood. She also worries whether water exposes people to pollutants.

McCartney speculates that the "8 cups of water a day myth" is being propagated by bottled water companies that are out to make a profit.

McCartney notes that a new international health initiative called "Hydration for Health," which promotes drinking more water for a healthier lifestyle, is sponsored by Danone, which markets the Evian and Volvic bottled water brands.

"There are many organizations with vested interests who would like to tell doctors and patients what to do. We should just say no," she says

McCartney argues that there is no high quality published evidence to support claims that drinking increased amounts of water offers benefits. She says reports that increased water can improve concentration and mental performance in kids, for example, have not been confirmed by research studies.

While there are some health conditions that do benefit from drinking more water, such as in people with recurrent kidney stones, the fluid's ability to prevent disease is conflicting, at best, she says.

A number of experts were quick to lash back.

In comments sent in to BMJ.com, Caroline J. Edmonds, a senior lecturer at the School of Psychology at the University of East London said she knows there is "well established literature" about the negative effects of dehydration on mental skills, both in adults and in children, which McCartney didn't mention.

Thomas Sanders, a professor of nutrition and dietetics from King's College London noted that McCartney's article hadn't been peer-reviewed. He also noted that McCartney failed to mention the 2004 report from the influential Institute of Medicine's Food and Nutrition Board.

That report reviewed all the studies done to date and was sponsored by the U.S. Department of Health and Human Services, the National Institutes of Health, Health Canada and other groups.

The report set general recommendations for women at about 2.7 litres of total water from all beverages and foods, a day, and men at 3.7 litres a day. Those who are physically active or who live in hot climates may need to consume more, it noted.

It said that about 80 per cent of people's total water intake comes from beverages -- including caffeinated beverages -- and the other 20 per cent is derived from food.

And good news for coffee lovers -- it said that when it comes to meeting daily hydration needs, caffeinated beverages can contribute as much as non-caffeinated options.

The panel also noted: "The vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide."

"We don't offer any rule of thumb based on how many glasses of water people should drink each day because our hydration needs can be met through a variety of sources in addition to drinking water," Lawrence Appel, chair of the panel that wrote the report, said at the time.

Strangely, that is likely the advice that McCartney herself would offer.