When we’re stressed, many of us turn to junk food for comfort. But new research from the University of Colorado Boulder suggests this strategy could backfire.
The study found that in animals, a high-fat diet disrupts resident gut bacteria, alters behavior and, through a complex pathway connecting the gut to the brain, affects brain chemicals in ways that fuel anxiety.
“Everyone knows these are not healthy foods, but we tend to think of them strictly in terms of a little bit of weight gain,” said lead author Christopher Lowry, a professor of integrative physiology at CU Boulder. “Understanding that they also affect your brain in ways that can promote anxiety raises the stakes even more.”
Lowry’s team divided adolescent rats into two groups: half were fed a standard diet of about 11% fat for nine weeks; the others were fed a high-fat diet of 45% fat, consisting mainly of saturated fat from animal products.
According to the Centers for Disease Control and Prevention, the typical American diet consists of about 36% fat.
Throughout the study, the researchers collected fecal samples and assessed the animals’ microbiome, or gut bacteria. After nine weeks, the animals underwent behavioral tests.
Compared to the control group, the group eating a high-fat diet unsurprisingly gained weight. But the animals also showed significantly less diversity of intestinal bacteria. In general, greater bacterial diversity is associated with better health, Lowry explains. They also harbored much more of a category of bacteria called Firmicutes and less of a category called Bacteroidetes. A higher ratio of Firmicutes to Bacteroidetes has been associated with the typical industrialized diet and with obesity.
The high-fat diet group also showed higher expression of three genes (tph2, htr1a and slc6a4) involved in the production and signaling of the neurotransmitter serotonin – especially in an area of the brainstem known as the dorsal raphe nucleus cDRD, which is associated is with stress and anxiety.
While serotonin is often labeled as a “feel-good brain chemical,” Lowry notes that certain subsets of serotonin neurons, when activated, can cause anxiety-like responses in animals. Notably, increased expression of tph2, or tryptophan hydroxylase, in the cDRD has been associated with mood disorders and suicide risk in humans.
“It’s extraordinary to think that a high-fat diet alone could change the expression of these genes in the brain,” says Lowry. “The high-fat group essentially had the molecular signature of a high anxiety state in their brains.”
Lowry suspects that an unhealthy microbiome attacks the gut lining, allowing bacteria to enter the body’s bloodstream and communicate with the brain via the vagus nerve, a route from the gastrointestinal tract to the brain.
“If you think about human evolution, it makes sense,” Lowry said. “We’re committed to really noticing things that make us sick so we can avoid those things in the future.”
Lowry emphasizes that not all fats are bad, and that healthy fats such as those found in fish, olive oil, nuts and seeds can be anti-inflammatory and good for the brain.
His advice: Eat as many different fruits and vegetables as possible, add fermented foods to your diet to support a healthy microbiome, and steer clear of pizza and fries. And if you have a hamburger, add a slice of avocado. Some studies show that good fat can counteract some of the bad fats.
