Teens 'especially vulnerable' to junk food advertising, experts say US can't address obesity crisis "unless the marketing is drastically lowered."

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.

Food advertising to teens almost exclusively promotes highly processed, unhealthy food loaded with fat and sugar.

According to the CDC, 20.6% of those aged 12 to 19 are obese. Obesity in adolescence can lead to serious long-term physical and mental health consequences, and increase the risk of developing chronic diseases, including heart disease and diabetes, the CDC says.

"There is no way we can address the obesity crisis unless the marketing is drastically lowered," Dr. Jennifer Harris, senior research adviser at the University of Connecticut's Rudd Center for Food Policy & Obesity, told ABC News.

Teens are "especially vulnerable to these advertisements," Harris added. "Peer influence is the most important factor in much of their decision making, and the companies have tapped into that need, and social media, for peer affirmation."

"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."

Harris admitted it's difficult to quantify exactly how much imposing stricter marketing regulations would decrease the rising rates in teenage obesity, but Gearhardt said the "biggest factor is the changing food environment, and what has made it most effective is the large presence of marketing around it."

Companies now also use interactive strategies. Teens are bombarded with promotional material on social media, said Gearhardt.

Teens need guidance, but "parents really have no idea that all of the products that are marketed to kids are unhealthy and don't understand all the ways companies reach their kids," Harris said.

Although parents can't control every food choice made by their children, they "can work to make your home's food environment more of a solace and less of a trigger," Gearhardt added. Parents should stock up on nutritious, minimally processed foods and healthy snacks such as hummus.

Parents also should talk to their children about accounts they follow on social media, including fast food companies or influencers sponsored by those companies. Cutting back on TV watching, so as to avoid ads, or streaming commercial-free offerings also could help.

During these years, teens also are developing their passions and values.

"Parents can capitalize on that to show that the food industry's strategies don't fit with the goals of autonomy and justice of adolescence," Gearhardt said. Reminding teens that products they choose to buy reflect their value systems can lead to better decisions.

"In other countries there has been progress," Harris said. "Chile has comprehensive legislation that protects adolescents from targeted fast food ads, but in America the First Amendment gives companies the right to free speech."

Currently in the U.S., there are no policies or regulations on food advertisements for children 12 or older.

"The only regulation is that companies can't market products in schools that don't meet the nutrition standards of food allowed to be sold at school," Harris added. "We need to challenge the companies. I don't know how they can say they are corporate citizens or socially responsible. The more the message is out there, the more they will have to take responsibility and action."

Eden David, who's studying neuroscience at Columbia University and matriculating to medical school later this year, is a contributor to the ABC News Medical Unit.