The school year is underway, and many parents see no better way to prepare their children for a taxing day of learning than with a large glass of sunshine, also known as orange juice.

Not so fast, say nutritionists and obesity experts.

That glass of juice — even if it's 100 percent fruit juice — is loaded with unnecessary calories.

"Most parents give their kids fruit juice because of the perception that it's healthy," says Nazrat Mirza, pediatrician and co-director of the Obesity Institute at Children's National Medical Center. "I don't know where that perception came from," she says.

It's true that many juices — particularly orange juice — are fortified with calcium and have a healthful amount of Vitamin C. "But you are getting those vitamins at a high calorie cost," Mirza says.

In fact, if you ate an orange at 60 calories and drank an eight-ounce glass of water you'd be much better off than if you drank one eight-ounce glass of orange juice at about 120 calories, she says.

In addition to being a lower-calorie alternative, the whole fruit and glass of water also are a treat for your digestive system.

"With the whole fruit you are getting fiber and bulk, which makes you feel full — and it keeps things moving," says Kathy Glazer, a Washington area dietician. "Most people, including kids, don't get enough fiber. Whole fruit is packed with fiber."

So if you drink fruit juice or another high-calorie drink, you'll miss out on that feeling of fullness, or satiation, and start looking for something to eat. In other words, you are drinking juice in addition to your normal food intake — not instead of it.

"You register calories differently when you drink them rather than eat them," says Kristen Ciuba, a Washington nutritionist and health coach. "Many people get a third of their total daily calories from sweetened and caloric drinks."

So when she does nutrition consultations, it's one of the first things she suggests eliminating — all sweetened beverages — including 100 percent fruit juices.

But Sarah Ladden, dietician and nutrition communications manager for the Juice Products Association, says 100 percent fruit juice helps Americans get closer to their recommended amount of fruit and vegetable servings. And she says there is no scientific link between obesity and fruit juice consumption.

But in the patient population Mirza sees, overweight and obese children, it isn't unusual to see 800 to 1,200 excess calories a day coming from juices and other sweetened drinks.

Another issue with fruit juice is its impact on the dental health of children, Glazer says. "Definitely don't give children fruit juice in a bottle that they go to bed with," or you'll soon be dealing with cavities.

Juice, milk and soda

Another downside with fruit juices is they might displace something that children really need. Like milk. "You don't want to deprive them of what they need. Milk has protein and calcium, which are very important for growing children," Mirza says.

So, is fruit juice as bad as, say, soda?

Not quite, Mirza says.

"Fruit juice is better than a sweetened soda because you are getting some vitamins," Mirza says. "But the calorie content is about the same."

Actually, it can be less in a soda. In Coca-Cola, for example, eight ounces translates to 97 calories, compared with the 120 calories for the same amount of orange juice.

But sodas also often contain high-fructose corn syrup, which is more taxing for the body to process than naturally occurring sugars, Mirza says.

The bottom line

So, what is the message? Skip fruit juice completely?

"The best thing to do is to try to get kids used to drinking water," Ciuba says. "You can always slice up fruit or add berries to the water to get some flavor into it."

Mirza agrees, saying that parents have an important role in helping develop their children's taste buds.

"Once you have introduced sweet drinks, that is what children will want," she says.

But if you still feel strongly about giving your children fruit juice, the recommendation is to stick with four daily ounces for children ages 1 to 6 and eight daily ounces for children ages 7 and older.

And if you are going with 100 percent fruit juice, go for a calcium-fortified orange juice instead of apple juice, Glazer says.

Ciuba, who has a 7-year-old and a 4-year-old, says at home she serves water with a splash of cranberry juice. She just doesn't really see a case for 100 percent fruit juice — for anyone, adult or child.

"I would say eat your fruit, don't drink it, whenever possible."

BETTER: Fruit is lower on calories and higher on fiber, which makes you feel fuller.

WORSE: Fruit juice is packed with calories and sugar and won't make you feel full.

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Boston is a fitness trainer and freelance writer.