(CNN) -- Sugar can easily sneak into the diet, both for you and for your child, even through 100 percent fruit juices.
Many health experts have even expressed concerns that the content of naturally occurring sugars in such juices can have negative health effects on children, such as increasing the risk for obesity.
The relationship between 100 percent fruit juice consumption and weight gain has been analyzed in a study published in the journal Pediatrics on Thursday.
The study suggests that drinking 100 percent fruit juice is associated with a slight amount of weight gain in children 6 and younger who have one serving a day, but no association was found for children 7 and older who have one serving a day.
Yet the study has some limitations, and it recommends drinking 100 percent fruit juice only in moderation.
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"I think caution is definitely in order and that when possible, parents should give whole fruit to kids, instead of fruit juice," said Dr. Brandon Auerbach, a primary care physician and instructor at the University of Washington's Division of General Internal Medicine in Seattle. "Water or low-fat unsweetened milk are other good alternatives to 100 percent fruit juice."
Auerbach, lead author of the new study, said he is the parent of a 7-month-old boy who soon may be offered fruit juices in day care and at school.
"I share the concern that 100 percent fruit juices have a lot of sugar, even though it's naturally occurring sugar," he said. "There are other health concerns about drinking 100 percent fruit juice, besides weight gain, especially related to risk of cavities and risk of future metabolic syndrome or diabetes."
Younger children may face weight gain risk
The study was a systemic review and meta-analysis of eight previous observational studies on 100 percent fruit juice consumption and weight gain among children, based on their body mass indexes, or BMI.
The analysis showed that consuming 100 percent fruit juice was slightly associated with weight gain in children 1 to 6 years old, but not enough to potentially harm health, Auerbach said.
The researchers found that in children ages 1 to 6, consumption of one daily serving was associated with a weight gain of 0.3 pounds or less over one year. In children 7 and older, 100 percent fruit juice was not independently associated with any weight gain.
"While the amount of annual weight gain may be small ... two points should be made," said Dr. Dean Schillinger, a professor of medicine at the University of California, San Francisco, who was not involved in the study.
"For a 5-year-old, 40-pound girl, consuming 4 to 8 ounces of 100 percent fruit juice daily for a year will be associated with her gaining an extra quarter-pound, compared to if she had not been drinking that juice," he said, adding that "the effect of daily consumption appears to be amplified the longer a young child is exposed to daily fruit juice. Consuming fruit juice daily for two years was associated with a significantly greater amount of weight gain."
In other words, the data did not show that gaining a quarter-pound due to drinking fruit juice over two years resulted in a half-pound weight gain. Rather, there appeared to be a risk of much greater weight gain with prolonged fruit juice drinking, Schillinger said of the study.
"This study determined that the individual effect on obesity was 'clinically small.' However, they acknowledge that, from a population health standpoint, such small changes may, in fact, have important public health implications with respect to the obesity and diabetes epidemics," he said. "The evolving consensus is that children should not be exposed to fruit juices, especially in the first 6 years of life."
The researchers also wrote that although the estimate of weight gain in younger children, 2 to 6, was not clinically significant, individual studies in the analysis showed clinically significant weight gain in children younger than 2.
Supporting national guidelines
Overall, the study results showed that one daily 6- to 8-ounce serving increment of 100 percent fruit juice was associated with a small .003 unit increase in body mass index over one year in children of all ages.
"I was somewhat surprised by the results, given that some types of 100 percent fruit juice have comparable amounts of sugar as regular soda," Auerbach said.
He added, however, that the study certainly had some limitations.
"Although we combined evidence from the best available research, the studies were not randomized controlled trials," Auerbach said. "We did not examine other important health outcomes besides weight gain, such as diabetes risk, because too few studies exist on this topic in children."
Also, two studies included in the meta-analysis found significant amounts of weight gain in children ages 1 to 3 associated with drinking one serving of 100 percent fruit juice a day, Auerbach said.
"It may be possible that this age group is at higher risk for weight gain from drinking 100 percent fruit juice than older children," he said.
The American Academy of Pediatrics recommends that children ages 1 to 6 drink no more than 4 to 6 ounces of 100 percent fruit juice a day. For children 7 to 18, juice intake should be limited to 8 to 12 ounces, or 2 servings, per day.
It's not recommended to give fruit juices to infants, Auerbach said.
"In terms of health benefits, children need at least 1 to 2 servings of fruit each day, and 100 percent fruit juice has lots of vitamins, minerals and nutrients like antioxidants," he said. "Our study findings support the current guidelines of the American Academy of Pediatrics on 100 percent fruit juice consumption."
As for adults, the US Dietary Guidelines for Americans recommend drinking no more than one cup, or 8 ounces, of 100 percent fruit juice a day.
Additionally, the dietary guidelines recommend consuming whole fruit instead of 100 percent fruit juice whenever possible for both children and adults, Auerbach said.
Why whole fruit may be better
The nutritional content of a fruit drink can drastically differ from that of an actual piece of fruit. Even though the drink may be 100 percent juice, it's often stripped of the fiber found in whole fruit, according to the Mayo Clinic.
A greater consumption of specific whole fruits, such as grapes or apples, has been linked to a lower risk of type 2 diabetes, whereas a greater consumption of fruit juice has been associated with a higher risk, according to a study in the British Medical Journal in 2013.
"Whole fruit consists of fiber in addition to the vitamins present in the liquid or juice, and fiber we know is associated with a lower glycemic load than fruit juice," said Janet Wojcicki, an associate professor of pediatrics at the University of California, San Francisco, who was not involved in the latest study.
"More and more health experts are concerned about the amount of sugar present in 100 percent fruit juice without the necessary fiber," she said.
In a 2012 paper in the American Journal of Public Health, Wojcicki and co-author Dr. Melvin Heyman recommended that the US Department of Agriculture's Child and Adult Food Care Program promote the elimination of fruit juice in favor of whole fruit for children.
The program manages the types of meals in child care centers, such as Head Start, across the country.
"It's very easy to drink a lot of calories, and I think that's where the concern has historically been with juice," said Katherine Zeratsky, a clinical dietitian at the Mayo Clinic in Rochester, Minnesota.
"A 4-ounce cup of juice is considered a serving of fruit," she said. "I think most consumers of juice are generally drinking more than 4 ounces. Then, the next question is, within the greater quantity, how does that balance within one's diet? And then probably the next question is, is that fruit juice consumption offsetting the intake of other nutritious food?"
To help parents appropriately limit both their children's -- and their own -- consumption of fruit juice, Zeratsky offered some advice:
If your child enjoys using a sippy cup throughout the day, fill the cup with water instead of juice. Your child should avoid snacking or drinking at least 30 minutes before mealtime so they will still have an appetite to consume a nutritious meal. Determine how much fluid your children's cups hold, and be mindful of the portions they are consuming. Try to give your child whole fruit instead of fruit juice. If they ask for fruit juice, try to dilute it with water. Keep more whole fruit than fruit juice in your home to avoid developing the habit of drinking sweet drinks. Check the nutrition labels of juices that you buy. The label should not have any added sugars outside of naturally occurring sugar in the fruit.