The number of U.S. children who drink sugar-free beverages has doubled in the past decade, a new study finds - though the health implications of the trend, if any, are unclear.

Using data from a federal health survey, researchers found that by 2008, 12.5 percent of children were drinking artificially-sweetened beverages. That was up from six percent a decade earlier.

U.S. adults are downing more diet drinks too. One-quarter of Americans surveyed in 2007-2008 said they'd had a diet drink in the past day, versus 19 percent in 1999-2000, the researchers report in the American Journal of Clinical Nutrition.

It's not surprising to see such trends, but the size of the increase among kids was a bit unexpected, according to senior researcher Dr. Miriam B. Vos, of Emory University in Atlanta.

And whether the pattern is good or bad is unknown.

"We do want children to drink less sugar," Vos said in an interview.

"But the challenge," she added, "is that there are no studies that have looked at the long-term health effects of artificial sweeteners in growing children."

That's not to say they are unsafe, Vos said. But she added that animal research has raised some potential concerns. For instance, animals fed artificial sweeteners have shown weight gain - suggesting there could be some effect on metabolism.

"We don't know if anything like that happens in children," Vos stressed.

But before anyone can make widespread recommendations that kids should have diet drinks instead of sugary versions, she said there should be studies into the possible effects on weight and health.

Sugar substitutes include artificial sweeteners like aspartame, saccharin and sucralose; the herbal sweetener stevia is available as a dietary supplement in the U.S.

With the rapid expansion of Americans' collective waistline in the past couple of decades, diet versions of sweet drinks and foods are increasingly popping up on supermarket shelves.

No one knows if that's helping. Vos noted that large population studies have not shown diet-beverage drinkers to be doing better weight-wise - and they tend to weigh more than people who favor water.

But it's hard to know what to make of that, Vos said. People who drink diet beverages may already be overweight and trying to shed some pounds, for instance.

The current findings are based on more than 42,000 Americans who took part in a periodic federal health survey. It included questions on what respondents had to eat and drink in the past 24 hours.

The fact that the questions covered only one day is a limitation of the study, according to Vos. There's no way of knowing how often kids may be having diet drinks, for instance.

Vos said she thinks studies that follow kids over time, to see whether there's a link between diet drinks and weight gain or health, are needed.

Besides the animal research, some studies have found that diet-beverage lovers have increased risks of diabetes, heart problems and stroke. But those reports point only to correlations, and do not prove that artificially-sweetened drinks are to blame.

Vos said that when she counsels families, she suggests that kids stick with water and milk, which has protein, calcium and other nutrients.