However, they found that this internal mechanism was shut off after two weeks of eating a human diet: They lost their natural inclination to try new foods.
"The interesting thing about this finding is that if the same thing happens in humans, eating junk food may change our responses to signals associated with food rewards," says Margaret Morris, PhD, from the University of New South Wales, Australia. "It's like you've just had ice cream for lunch, yet you still go and eat more when you hear the ice cream van come by."
The researchers attribute the behavior to changes in the reward circuit pathways of the brain. Importantly, reward mechanisms hold important sway over human eating behaviors as well.
"Everything we do—from the time we wake up until we collapse into bed—is driven by reward," explains Pam Peeke, MD, MPH, FACP, author of The Hunger Fix.
Unfortunately, this system is good and bad: "I've seen firsthand how the dopamine rush cuts both ways," she says. "The tremendous high you get from a run in the park can be powerful enough to change your life. But that healthy high occupies the same pathways, and can easily become confused with, the dopamine hit from a snort of cocaine. Clearly, not all rewards are created equal—and some can kill you."
More: Are You Suffering from "False Hunger" Pains?
And junk food has proven to elicit strong addictive properties, eliciting a strong dopamine rush that can lead to addiction. "During continued dopamine flooding, the brain thinks it has 'too much' dopamine—so the brain attempts to compensate for this overabundance by battening down the hatches, decreasing the total number of dopamine receptors to lessen the amount of dopamine your brain absorbs." With fewer receptors, you need more stimulant (in this case, food) to get the same rush as you did the first time.
Fortunately, unlike mice, we have the ability to break the cycle. Dr. Peeke recommends adding these three nutrients into your diets to indulge in dopamine in a healthy way:
"When people don't have enough phenylalanine in their diet, they can feel confused, lack energy, and suffer from depression, decreased alertness, and memory problems," explains Dr. Peeke. Foods with phenylalanine include turkey legs, stewed chicken, salmon, cottage cheese, spinach, watermelon, and chocolate.
Dr. Peeke says, "During times of stress, tyrosine may be drawn away from dopamine production in favor of stress hormones. That's why, in your daily diet, but especially during times of stress, tyrosine-rich foods may help protect your dopamine supply." Foods high in tyrosine include Cornish game hen, cheese, spirulina, mustard greens, avocados, and almonds.
"A diet stocked with tyrosine- and phenylalanine-building foods can get a little protein heavy," admits Dr. Peeke, who notes that luckily, vitamin B6 is also found in many fruits and vegetables. These foods include russet potatoes, red bell peppers, garlic, cantaloupe, and hazelnuts.