The inundation of junk food advertising is contributing to the rising rate of teen obesity -- a public health crisis among a population that is "especially vulnerable" to the messaging, experts say.

In 2016 alone, the food industry spent almost $14 billion on overall advertising to influence Americans' food choices. The U.S. food system is the second-largest advertiser in the American economy, and views adolescents as a major market force, aggressively targeting them to build brand awareness, preference and loyalty.

"These fast food ads get under teens' skin and activate the brain in this unconscious way that is tough to protect themselves from," said Ashley Gearhardt, an associate professor of psychology at the University of Michigan.

Her research has shown that fast food ads activate highly sensitive and still-developing reward pathways in teens' brains. Additionally, foods high in fat, sugar, refined carbohydrates, salt and other flavor enhancers have been shown to have addictive qualities that can increase cravings.

"These ultra-processed foods are engineered to be intensively rewarding," Gearhardt said. "From a biological perspective, adolescents are very vulnerable to things that have an addictive property because the reward systems develop rapidly and peak in adolescence, but the parts of your brain that are the brakes and impose control and restraint develop more slowly."

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