Fellow junk food eaters can undoubtedly attest to the feeling of craving more, even when one thinks they have finished eating. Whether sweet, salty, or savory, junk food is a food of habit and repetition, compelling us to just have one more bite.
There are, of course, evolutionary reasons that we crave these kinds of simple flavors, embodied in sweets or fatty, salty foods. But there is also a more sinister element to our nation's junk food addiction: food manufacturers are manipulating our minds and bodies — spending millions of dollars over decades to engineer food that tastes good, but not good enough to make us stop eating. In addition, they play off of our own evolution as a species to help us develop dependencies on foods even when we know eating them is not in our best interest.
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"When we taste something and when those nutrients hit our gut, there are signals in the brain — pleasurable signals — that make us think, 'This is really delicious! I like this a lot!'" explains Dr. Alexandra DiFeliceantonio from Fralin Biomedical Research Institute at Virginia Tech Carillon. "That's probably due to a class of chemicals called opioids."
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In addition to these opioids, the body also releases a neurochemical called dopamine, which DiFeliceantonio compared to a "tag," figuratively poking your body and saying, "'Oh, go do that again! That's something that you should eat again. That's something that you should go do.' It has to do with motivation and with learning, and both these signals are really important for our behavior."
In and of itself, there is nothing inherently sinister about this aspect of human neurochemistry. Indeed, the same signals that make a person crave cotton candy or a Big Mac could in theory also draw them toward a crunch carrot, juicy orange or tender strip of lean turkey. Yet according to Dr. Nicole Avena, an assistant professor of neuroscience at Mount Sinai Medical School and a visiting professor of health psychology at Princeton University, there are important differences in how the brain responds to highly-processed junk foods versus how it processes something like a banana or grilled flounder.
"It seems that foods with added sugars are 'enjoyed' differently, as our brain seems to be more sensitive to higher amounts of sugar than we would typically see in nature (like, for example, in an apple)," Avena wrote to Salon. This is evidenced, among other things, by the contrasting ways in which humans eat the highly-processed, chemical-laden foods and the ways in which they eat those that appear in nature.
"People are not experiencing this with things like beans, baked chicken breasts and fruit, even if they really like them," Dr. Ashley Gearhardt, an associate professor at the Department of Psychology at the University of Michigan, told Salon. To illustrate her point, Gearhardt turned to one of her own favorite foods: Watermelon.
"I love watermelon, it's delicious, but nobody sits down and eats the whole watermelon," Gearhardt observed. The difference between a watermelon and something processed by a large company is that, for the latter, those foods are altered in ways to make them literally irresistible. This happens when companies use a combination of salt, sugar and fat to create foods that overstimulate the taste buds — and yet are designed to never quite leave you feeling satisfied. Even though saturating foods in salt, sugar and fat fuels the obesity epidemic — and does not necessarily provide consumers with the best culinary experience — it guarantees that customers will keep coming back for more by overstimulating one's feeling of taste pleasure in exactly the right ways. Since the companies' view their foremost responsibility as being to their shareholders, that is in itself a good enough reason to continue preparing foods with excessive salt, sugar and fat.