You’re watching your favorite TV show and a commercial break starts. First, you see an ad for a candy bar, then one for fast food. Does your ­stomach start growling and does your mouth begin to water? Do you find yourself craving a snack?

If so, you’re not alone. Researchers from McMaster University in Canada recently found that seeing ads for unhealthy foods can have an almost immediate effect on kids’ and teens’ eating habits.

The researchers examined advertising of junk food to more than 6,000 young people through TV commercials and other types of media. (The term junk food is often used to describe foods and beverages that are high in sugar or salt and low in nutritional value.) The researchers found that kids and teens made unhealthy food and drink choices as quickly as 30 minutes after seeing the ads.

Studies like this one have experts concerned about how exposure to all those ads is affecting young ­people’s health.

“There is too much unhealthy food advertising out there,” says Behnam Sadeghirad, one of the authors of the study. “And this is dangerous [for young people].”

If food and beverage ads seem to be everywhere, from TV and the internet to radio and billboards, it’s because companies want to be sure you see their products. They spend about $1 billion a year on 

food ads aimed at teens, accord­ing to a 2012 report. TV may be the hardest place to escape from ads. In 2015, teens watched, on average, 13 TV food ads each day, according to the Nielsen Company. Most of those ads were for unhealthy foods: fast food, candy, sugary drinks, and sweetened ­cereals. On average, teens saw less than one TV ad per week for fruits and vegetables.

“[Unhealthy foods] are not things we should be encouraging kids to eat more often, but unfortunately those are the products being advertised to them the most,” says Jennifer Harris, a University of Connecticut professor who studies food ads aimed at young people. 

The McMaster University study shows that all those junk food ads are affecting which foods kids and teens choose to eat. In other words, those ads are working.

That may be good news for food and beverage companies, but experts say it could spell trouble for you. “One of the potential impacts that unhealthy ads can have is increasing childhood obesity,” says Sadeghirad.

Obesity is the state of being dangerously overweight. People who are obese have a higher risk of many health problems, such as type 2 diabetes and heart disease. More than one-third of kids and teens in the U.S. are overweight or obese, according to the U.S. Centers for Disease Control and Prevention.

Of course, health experts say, it’s OK to treat yourself to a candy bar or a bag of chips sometimes. But what can you do to keep from grabbing an unhealthy snack every time you see an ad for junk food?

Harris says it’s important to learn to spot the techniques that companies use in their ads. (See “Tricks of the Trade.”) She says that will help you make healthier food choices.

“What [you] can do is notice what’s going on,” says Harris. “Think about, ‘Are these the products that really are good for me?’”