- Why do we love so many foods that don't exactly love us back?

Instead of providing our bodies with the vitamins and minerals we need to grow, move, and survive, we're often drawn to fatty, sugary, and salty snacks and drinks that aren't so great for our long-term health.

(wrapper crinkles) I'm Sheril Kirshenbaum, and on this episode of "Serving Up Science," we're putting junk food under the microscope to figure out the science behind our affinity for highly processed foods that pack a lot of calories with little nutritional value.

(wrapper crinkles) For decades, scientists have been exploring why we're so attracted to sweet and salty stuff.

Let's start with our brains, which respond to drugs and food in surprisingly similar ways.

Dopamine is a natural chemical associated with the expectation of a reward that brings us feelings of pleasure.

When we eat foods that contain lots of fat and sugar, dopamine gives us that rush of elation and desire for more.

Let's just say it hits the sweet spot, sweet spot, sweet spot.

The sugar rush is real, but more may be needed over time to trigger this dopamine release, and when it comes to junk food, the result might be overindulging as these powerful brain chemicals affect how we feel and behave.

- [Homer] More.

- But beyond the dopamine fix, the texture and crispiness of foods may actually fool us into thinking we're eating fresh, healthy snacks even when we're not.

Consider how it sounds to take a bite of this carrot.

Oop, they're stuck, hold on.

(carrot crunches) Mm, very good carrot.

Evolutionary biologists have proposed that when our ancestors were foraging for food, crispness was a signal something was ready to eat and importantly, not yet spoiled.

When fruits and vegetables get a bit older, they become, well, mushier.

And no one, well, no one I know, really enjoys a mealy apple or overripe banana.

Hmm, eww.

That's why.

Fast forward a couple hundred thousand years and our dietary options have expanded tremendously.

Agriculture gave us more stationary societies, and eventually, supermarkets and ballparks, holidays where every month or two we get marshmallow chicks or chocolate coins or jelly powdered donuts.

Our modern environment has become a cornucopia of always available processed junk food, 24 hours a day, seven days a week, but our bodies and brains haven't quite caught up and we're still in foraging mode.

That means clues like crispness or the feel and sound of what we eat may not be as important as they were in the past, but they continue to influence our preferences and food choices.

Give me a minute.

In one experiment, researchers played crunching sounds at various volumes while their subjects ate potato chips.

(man screams) People who heard muffled crunches described chips as stale and soft, while loud crunching sounds were described as crispy and fresh.

Another study found that when it comes to BLT sandwiches, shouldn't I define that for the people who don't eat BLT sandwiches?

- [Woman] No, no.

- Bacon, lettuce, and tomato sandwiches.

Another study found that when it comes to BLT sandwiches, the crispness of the bacon, not the taste or smell, turned out to be the most important in terms of how much someone enjoyed the sandwich.

- [Dog] Bacon, gotta be bacon, only one thing smells like bacon, that's bacon!

- But it's not just the sound of food that nudges us to make unhealthy choices in the 21st century.

The oral sensory experience of what we eat matters as well.

We like the way foods, certain textured foods like cheese puffs feel as they break down in our mouths.

I don't like this, I'm not gonna let it break down in my mouth.

Food companies commit a lot of time and money into evaluating and marketing the texture of their products.

♪ Scrunchous, when it crunches There are even a variety of scientific machines that food manufacturers use related to measuring the crispness and crunch of what we eat.

All right, fine.

Delicious.

Of course, we crave junk food for more reasons than mouthfeel.

- Tell me about mouthfeel, what do you mean by mouthfeel?

- So mouthfeel is in general, how the beer feels in your mouth.

- Billions of dollars go into creating flavor profiles designed to appeal to our innate love of sugar, salt, fat, and crunch.

My hands are like still orange from touching.

Ultraprocessed foods, the meals and snacks that are often pre-packaged, ready to heat or eat, currently make up almost 60% of the calories in the American diet.

You know, actually it wasn't bad, like halfway through, I was like, I kinda like this.

That's how they get ya.

These foods tend to have a lot of additives, preservatives, sweeteners, coloring, flavoring, trans fats, emulsifiers, and more meant to enhance a product's appeal, flavor, or shelf life.

But while that sounds scary, don't get too focused on avoiding the chemicals in our food.

It's virtually impossible.

Also remember that not all chemicals added to food are bad.

We have enough scientific evidence to know junk food is not great for our bodies in large quantities, for all sorts of reasons and might even trick our brains to make us think we need more, like in milkshakes.

Did you know the combination of sugar and fat fools us into not being able to recognize when we're full?

Try it out yourself and guzzle a cup of cream.

My guess is you're likely to stop drinking after a few sips.

- Milk was a bad choice.

- But the creamy milkshake, because of all that extra sugar, it's easy to keep going and going and going without recognizing when enough's enough.

(slurps straw) It's certainly okay to enjoy your favorite cookies and chips now and then, but as my grandmother used to say, everything in moderation.

- [Crew] One summer, I set a record for how many cheese balls I caught in a row in my mouth.

(calm music)