Pizza may be delicious, but it is a major contributor to poor nutrition for children and teens.

Nobody thinks of pizza as a health food. But not many know just how unhealthy pizza can be.

“Not only is pizza one of the things children and teens eat most often, it is packing on extra calories, extra fat, and extra sodium, or salt,” said Lisa M. Powell, Ph.D., professor of Health Policy and Administration at the School of Public Health at the University of Illinois at Chicago.

“If we could help kids eat less pizza, or help them eat healthier pizza, we could have a huge impact on their diet, their nutrition, and their health,” said Powell, who was the lead author for a new study on pizza and American youth.

It’s no surprise that pizza is one America’s favorite foods. Just ask any parent. Plus, scientists have the numbers to back up what we see in school cafeterias, fast food restaurants, homes, supermarkets, and anywhere young people eat and snack.

Pizza is the number two source of calories for children between 2 and 18 years old, said Powell. The number one source of calories is grain desserts, a combination of cakes, cookies, pastries, donuts, and other sweet treats made with wheat and other grains.

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On any given day, over 20 percent of young Americans eat pizza, said Powell. Among kids between the ages of 6 and 19, 22 percent will eat pizza today. That compares to 14 percent for kids ages 2 to 5, and 13 percent for the U.S. population as a whole.

For the study, researchers used the National Health and Nutrition Examination Surveys (NHANES), from 2003 to 2010. Four separate NHANES studies asked kids, or parents of younger children, what they ate on two different days. The researchers used food reports from more than 7,400 children, ages 2 to 11, and 6,500 adolescents ages 12 to 19.

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On days they ate pizza, children 11 years old, or younger, ate 84 more calories than on other days. For adolescents, a pizza day meant they ate 230 calories more than on other days.

What does that mean for weight? One pound of fat is equal to about 3,500 calories. The average kid eats pizza about every five days, or about 73 times a year. The math is easy.

Children 11 and younger eat an extra 84 calories every pizza day, 73 times a year, or an extra 6,132 calories per year. Unless they cut back on another food or get a lot more exercise, they will gain almost two pounds a year just from eating pizza. From age 2 to 11, the average child will pack on nearly 16 extra pounds just from pizza.

It’s worse for teens. They eat an extra 230 calories every pizza day, 73 times a year. That’s 16,790 extra calories from pizza, almost five pounds a year. From age 12 to 19, that’s close to 30 extra pounds from pizza.

Not convinced pizza is unhealthy? The study found that pizza days were bad nutrition days all around. Children ate three grams more saturated fat and 134 milligrams more salt on pizza days. Adolescents ate five grams more saturated fat and 484 milligrams more salt on pizza days.

Bad eating is bad news for health. Too many calories contribute to obesity, which can lead to cardiovascular disease and type 2 diabetes.

Too much saturated fat can increase cholesterol, which increases the risk of cardiovascular disease and type 2 diabetes.

Too much salt increases the risk of high blood pressure, which also increases the risk of cardiovascular disease and type 2 diabetes.

Obesity, cardiovascular disease, and type 2 diabetes are all on the rise in American children, adolescents, and adults. The causes include too many calories, too much saturated fat, and too much salt in the diet.

The American Heart Association (AHA) recommends that people get no more than five to six percent of their daily calories from saturated fat, or about 13 grams per day for a 2,000 calorie diet. The reality is that the average American gets about 12 percent of daily calories from saturated fat. Pizza days make an already poor diet worse.

It’s the same story for salt. AHA recommends that people eat no more than 1,500 milligrams of salt a day, not quite three-quarters of a teaspoon. However, the average diet includes about 3,400 milligrams per day. Pizza days make an already poor diet worse.

The good news is that pizza is most often a snack, not a meal. Helping your kids snack on something healthier than pizza is a good move.

“You don’t want to encourage pizza as a snack,” Powell said. “Snacks like apples are a lot healthier. If you do have pizza, read the product labels, and get the healthiest pizza you can find. Most supermarkets have a whole wall of pizzas to choose from. Not all pizzas are equally unhealthy.”