By Maggie Fox, Health and Science Editor

WASHINGTON (Reuters) - Want to help your kids keep the weight off? Just give them water instead of soda and other sugar-sweetened drinks, researchers advised on Monday.

One analysis of the diets of children and teens in the United States showed they drink, on average, 235 "empty" calories in sugar-sweetened beverages each day.

When these drinks are cut out, the average child does not make up for them by eating or drinking more calories elsewhere, the researchers said. In a second study, Dutch researchers found children would cut out sugary drinks before they would exercise or abandon snacks.

"The evidence is now clear that replacing these 'liquid calories' with calorie-free beverage alternatives both at home and in schools represents a key strategy to eliminate excess calories and prevent childhood obesity," Dr. Claire Wang of Columbia University in New York said in a statement.

Writing in the Archives of Pediatrics & Adolescent Medicine, Wang and colleagues said they looked at data from the 2003-2004 National Health and Nutrition Examination Survey that includes detailed questions about diet.

Every 1 percent drop in soft drink intake correlated to more than six fewer calories, they found.

Two other studies showed it is possible to get children to make the change.

Emily Ventura and colleagues of the University of Southern California got 54 overweight Latino teens to take part in a four-month study to improve their diets.

One-third did nothing, another third took one nutrition class a week and one-third did the class plus strength training twice a week.

More than half -- 55 percent -- cut their sugar intake by 47 grams a day, or the equivalent of one can of soda. And 59 percent ate more fiber, up to five grams a day.

This included even the "control" teens who did not do anything extra, Ventura's team reported in the journal.

The children who ate less sugar had healthier blood sugar levels and those who ate more fiber lost on average 10 percent of visceral body fat -- the dangerous type found around the internal organs.

"Our results suggest that intensive interventions may not be necessary to achieve modifications in sugar and fiber intake," Ventura's team wrote.

A program in the Netherlands also cut intake of sweet drinks. Amika Singh and colleagues at VU University Medical Center in Amsterdam tested 1,108 children aged 12 and 13.

They took part in an eight-month program of 11 lessons including biology and physical education. For up to a year afterward the children cut their soft drink intake by on average 10 ounces (more than 200 ml) a day .

The children did not, however, snack less or walk or bike to school more often.

"Reducing intake of sugar-containing beverages should therefore be considered a good behavioral target for future interventions aimed at the prevention of overweight among adolescents," Singh's team concluded.

(Editing by John O'Callaghan)

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