How do simple carbohydrates affect my body as a teen?

"Simple" in simple carbohydrates is used to connote the ease with which your body can extract the sugars from the carbohydrates you have just eaten. The reason it is so easy for your body to get these sugars is because machines have already done most of the work for you.

As soon as you swallow that first bite of a simple carbohydrate like a doughnut, you experience a sudden leap in your blood sugar level, giving you an instant boost of energy. But your body quickly compensates with an output of insulin (the hormone that metabolizes carbohydrates and regulates blood sugar), lowering your blood sugar level and storing the excess sugar as fat, which leaves you tired and craving more sugar to give you another energy boost. Sounds an awful lot like the effects of morphine or heroin on the body: a brief high followed by a debilitating slump that leaves the victim desperately foraging for her next hit (or bite, in this case). The vicious cycle continues because simple carbohydrates don't keep you full for long and leave you craving more of the same. So you are never full, always tired and lethargic, and aren't getting proper nutrients because all you can think about eating are processed, sugary, fake foods: simple carbs, in a nutshell.