Children in the United States are not drinking as much water as they should, and the deficiency can have far-reaching implications, a new study suggests.

"Even mild dehydration can affect physiological function, and cause fatigue, muscle weakness, headaches and dry mouth," said Samantha Heller, clinical nutrition coordinator at the Center for Cancer Care at Griffin Hospital in Derby, Conn., who was not involved in the study.

Impaired cognitive and mental performance are also linked to inadequate hydration, said Heller.

According to the study, published in the October issue of the American Journal of Clinical Nutrition, depending on age-only 15 to 60 percent of boys, and 10 to 54 percent of girls drink the minimum amount of water recommended by the U.S. Institute of Medicine.

Too many children are getting much of their water from sweetened beverages rather than plain old H2O, the researchers found. The study also revealed that those who drink water consume fewer sweetened beverages and eat fewer high-calorie foods.

The research looked at the water intake of 3,978 boys and girls, aged 2 to 19 years, who had been included in a national nutrition study from 2005 to 2006.

The investigators found that water intake from all sources varied by age: 2 to 5 year-olds drank 5.9 cups a day. 6 to 11 year-olds got 6.8 cups, and 12 to 1-year-olds consumed 10.1 cups daily. Girls generally drank less than boys.

The findings also suggest that kids of all ages are more likely to drink beverages, such as sodas, tea or milk, and not water at mealtime.

Water makes up 55 to 75 percent of total body weight, said Heller. "We cannot live without water for more than a few days because our bodies cannot store water. Thus, it is essential we replace the water our bodies lose every day."

Heller, a nutritionist and dietitian, advises starting children on water early.

"Give them water instead of sweetened beverages during the day and between meals," she said. To make it more appealing, put sliced cucumbers, oranges, lemons or strawberries in ice water, she suggested.