WASHINGTON (Reuters) - United States First Lady Michelle Obama, known for urging US children to be more active and eat their vegetables, began a new health push this week by touting the benefits of drinking water.

But exactly how much water to drink remains murky, and some consumer advocates question drinking bottled water at all.

"Just drink up," Mrs Obama urged on Thursday on a visit to the aptly named Watertown, Wisconsin. "Water is the first and best energy drink."

The visit was the kick-off of the "Drink Up" campaign. The push to drink more water is backed by the American Beverage Association and the International Bottled Water Association, igniting scepticism among those who favour tap water over bottled water in most instances.

"Since we started the 'Let's Move!' initiative, I've been looking for as many ways as possible to help families and kids lead healthier lives," the First Lady said in a White House statement.

"If we were going to take just one step to make ourselves and our families healthier, probably the single best thing we could do is to simply drink more water."

There are health benefits to drinking water, but a lot of uncertainty about how much, according to the Mayo Clinic.

Water flushes toxins out of vital organs, carries nutrients to cells and moistens ear, nose and throat tissues, the clinic said on its website. Even mild dehydration can drain energy.

Popular advice has long been to drink eight 240ml glasses of water a day, and while that is easy to remember, it is not supported by hard evidence, the site said.

"Just keep in mind that the rule should be reframed as: 'Drink at least eight 8-ounce (240ml) glasses of fluid a day,' because all fluids count toward the daily total," the clinic said.

The amount of water needed depends on various factors, including general health, how active a person is and where that person lives, according to the clinic.

Ms Emily Wurth of the consumer advocacy group Food & Water Watch applauded the campaign to favor drinking water over sugary drinks but wished Mrs Obama had suggested drinking tap water instead of bottled water.

"Tap water is more regulated than bottled water in the United States, it costs thousands of times less money for consumers and doesn't cause the environmental damage that single-use bottles of water do," Ms Wurth said by telephone.

She said less than one-quarter of all bottles are recycled, with the rest ending up in US waterways and landfills.