Nearly 40 percent of the calories that children consume come from junk food, according to the National Cancer Institute in Bethesda, Md.

Cancer Institute researchers Jill Reedy and Susan Krebs-Smith examined the diets of children and teens ages 2-18.

Half of the unhealthiest calories children consume come from just six foods:

• Soda, sugary fruit drinks and pizza
• Grain desserts like cake, cookies and donuts
• Dairy desserts such as ice cream and whole milk because it contains more fat than skim milk.

"This paper identifies the major sources of overall energy and empty calories, providing context for dietary guidance that could specifically focus on limiting calories from these sources and for changes in the food environment," the researchers say in a statement. "Product reformulation alone is not sufficient."

The article is published in the Journal of the American Dietetic Association.

Some nutritionists blame the food industry for making so many foods with empty calories. They also say most parents and their kids just don't know the nutritional value of the foods they are eating.