Summary: New research from the University of Colorado Boulder shows that consuming a high-fat diet can alter the gut microbiome, change behavior, and affect brain chemistry in ways that may increase anxiety. In an animal model, rats fed a diet high in saturated fat developed less diverse gut bacterial communities and showed increased expression of genes linked to stress-sensitive serotonin signaling in a brainstem region associated with anxiety. These findings indicate that poor dietary choices may have consequences that extend beyond weight gain and metabolic health to influence mood and mental well-being.
Key Facts:
- High-fat diets can disrupt the balance and diversity of gut bacteria, shifting the microbiome toward patterns associated with industrialized diets and obesity.
- Changes in the gut microbiome can communicate with the brain—via immune signaling and neural pathways such as the vagus nerve—and influence neurotransmitter systems linked to anxiety.
- Not all fats are harmful; unsaturated, anti-inflammatory fats found in fish, olive oil, nuts, and seeds support brain health and may mitigate some negative effects of saturated fats.
Source: University of Colorado
When stressed, many people reach for high-fat comfort foods. New research from the University of Colorado Boulder suggests that this habit could worsen anxiety rather than soothe it.
Researchers examined how a high-fat diet affects the microbiome-gut-brain axis and anxiety-related behavior in male rats. Over nine weeks, the team compared adolescent rats fed a typical laboratory diet (about 11% of calories from fat) with rats fed a high-fat diet (45% of calories from fat), where most of the fat came from saturated animal sources.
As expected, the high-fat group gained more weight. More importantly for mood and brain function, those animals developed a less diverse gut microbiome—a marker often linked to poorer overall health—and a distinct shift in bacterial composition. The high-fat diet increased the relative abundance of Firmicutes and decreased Bacteroidetes. Higher Firmicutes-to-Bacteroidetes ratios have been observed in industrialized diets and in some studies of obesity.
Beyond changes in gut bacteria, the high-fat diet produced notable effects in the brain. Animals on the high-fat diet showed elevated expression of three serotonergic genes—tph2, htr1a, and slc6a4—particularly within a subregion of the dorsal raphe nucleus called the caudal dorsomedial dorsal raphe (cDRD). This brainstem area has been implicated in stress and anxiety-related behaviors. Although serotonin is often characterized as a “feel-good” neurotransmitter, specific serotonin neuron populations can drive anxiety-like responses when activated. Increased expression of tph2 in this region has also been associated with mood disorders in human studies.
The investigators propose a mechanism in which an unhealthy microbiome weakens the intestinal barrier, allowing bacterial components and inflammatory signals to reach the body and communicate with the brain—potentially through the vagus nerve and immune pathways—thereby influencing serotonergic circuits that regulate anxiety-related behavior.
Lead author Christopher Lowry, professor of integrative physiology, notes that these results raise the stakes for dietary choices: “If you understand that these foods can impact the brain in ways that promote anxiety, the consequences are not just about weight gain.”
Lowry emphasizes that healthy fats are beneficial. Diets that include sources of unsaturated fats—such as fatty fish, olive oil, nuts, and seeds—support anti-inflammatory processes and brain health. Practical recommendations from the research team include increasing the variety of fruits and vegetables, incorporating fermented foods to support microbial diversity, and reducing consumption of highly processed, saturated-fat-rich foods like fries and heavily processed pizza. Where possible, pairing an occasional indulgence with healthier fats (for example, adding avocado to a burger) may help offset some negative effects.
About this diet and anxiety research news
Author: Lisa Marshall
Source: University of Colorado
Contact: Lisa Marshall – University of Colorado
Image: The image is credited to Neuroscience News
Original Research: Open access. “High-fat diet, microbiome-gut-brain axis signaling, and anxiety-like behavior in male rats” by Christopher Lowry et al., published in Biological Research. The study assesses how a nine-week high-fat diet alters gut microbiome diversity, brainstem serotonergic gene expression (tph2, htr1a, and slc6a4), and anxiety-related defensive behaviors in male rats.
Abstract
High-fat diet, microbiome-gut-brain axis signaling, and anxiety-like behavior in male rats
Obesity and anxiety both increase in modern societies and may be linked through signaling between the gut microbiome and brain serotonin systems. In this nine-week study, male rats consuming a high-fat diet showed decreased alpha diversity and altered community composition of their gut microbiome alongside obesity, increased expression of serotonergic genes in brainstem subregions (including the cDRD), and enhanced anxiety-related defensive behaviors. The high-fat diet raised the Firmicutes/Bacteroidetes ratio and increased specific taxa such as Blautia while reducing Prevotella. Expression of tph2, htr1a, and slc6a4 was elevated in dorsal raphe subregions, and certain bacterial taxa correlated with increased serotonergic gene expression in the cDRD. These findings support the hypothesis that diet-induced obesity alters microbiome-gut-serotonergic brain signaling, contributing to heightened anxiety-like responses in rats.