It's hard to turn down junk food.However，eating too much is bad for your body，especially if you're between the ages of 10 and 19."Junk food shapes teenage brains in ways that make their ability to think，learn and remember weaker.It can also make it harder to control bad behaviors.It may even up a teen's risk of mental (精神上的) problems，" said Amy Reichelt，a scientist at Western University，Canada.

Reichelt and two other researchers reviewed more than 100 studies about how poor food choices can make a big difference to teenage brains.They found teenagers are more sensitive (敏感的) than any other age group to foods with a lot of fat and sugar，as their brains are not yet fully formed.

Teenage brains are still developing the ability to notice risks and control actions.The prefrontal cortex (前额皮质) is the part of the brain tells us we shouldn't eat chips all the time and helps us control ourselves.However，this part doesn't fully develop until we are in our early 20s.At the same time，teen brains get more excited from prizes.The parts of the brain that make us feel good when we do something happy—like eating tasty foods—are fully developed by the teen years.

As a result，the teen brain has two hits against it when it comes to refusing junk food.Reichelt and her team did an experiment using mice，whose brains develop much like our own.They discovered that "teenage" mice that had highfat foods performed worse on memory tests than those eating normal foods.

So，what's the best way to say no to junk food? The answer is exercise.When we exercise，the brain becomes less sensitive to food，besides this，it also helps our brains grow and become better able to make wise decisions.