High-fat foods have been criticised by dietitians and doctors for decades. And they’re not alone in their criticism. The World Health Organization cites high-fat foods as the number one cause of obesity.
It’s unsurprising then that scientists have begun to investigate other possible effects that high-fat foods could have on the human body, particularly with the increased understanding of the gut microbiome. In fact, it’s this exact understanding that has led to researchers at the University of Colorado Boulder to look into whether high-fat diets can disrupt gut health and increase anxiety, by affecting brain chemistry.
What is anxiety?
Anxiety is a mental health disorder, which affects 301 million people globally.
According to the World Health Organization, people with an anxiety disorder experience excessive fear or worry about a specific situation. For example, a panic attack or social situation or, in the case of generalised anxiety disorder, about a broad range of everyday situations.
Do high-fat foods cause anxiety?
The new study, published in BMC Biological Research, shows that high-fat diets can disrupt gut health and increase anxiety, by affecting brain chemistry. The researchers found that a high-fat diet disrupts resident gut bacteria, alters behaviour and, through a complex pathway connecting the gut to the brain, influences brain chemicals in ways that fuel anxiety.
“Everyone knows that these are not healthy foods, but we tend to think about them strictly in terms of a little weight gain,” said Professor Christopher Lowry, lead author of the study at the University of Colorado Boulder. “If you understand that they also impact your brain in a way that can promote anxiety, that makes the stakes even higher.”
How was the study conducted?
The study was conducted on a group of rats. The team divided the rats, aged five to six weeks, into two groups. Half were fed a ‘standard’ diet, containing approximately 11% fat, for nine weeks. Meanwhile the other half were fed a high-fat diet, containing 45% fat, most of which was saturated fat from animal products.
Throughout the study, the researchers collected faecal samples and assessed the animals’ gut microbiome. And then, after nine weeks, the animals underwent behavioural tests.
The first observation by the team was, perhaps predictably, that the group on the high-fat diet, gained weight. However, they also showed significantly less diversity of gut bacteria. And of the remaining gut bacteria, a large proportion was a form of ‘bad’ bacteria, called Firmicutes, and fewer ‘good’ bacteria called Bacteroidetes.
The high-fat diet group also showed higher levels of genes tph2, htr1a, and slc6a4, which are involved in the production and signalling of the neurotransmitter serotonin, particularly in the region of the brainstem, known as the dorsal raphe nucleus cDRD, which is associated with stress and anxiety. And while serotonin is typically known for its associations to happiness and positivity, certain subsets of serotonin neurons can, when activated, prompt anxiety-like responses in animals.
“To think that just a high-fat diet could alter the expression of these genes in the brain is extraordinary,” said Professor Lowry. “The high-fat group essentially had the molecular signature of a high anxiety state in their brain.”
As more is learned of the gut-brain axis, or more simply put, the communication between the gut and the brain, it is likely that diet will be linked to other mental health issues.
What is the gut microbiome?
Each of us has trillions of microbes or bacteria living in our gut. These are collectively referred to as the gut microbiome. The two most common species of helpful bacteria found in our gut microbiome are Lactobacillus and Bifidobacteria. Maintaining a healthy balance between the helpful (good) bacteria and the unhelpful (bad) bacteria is fundamental in supporting a healthy digestive system, with the gut now understood to be central to health, containing more than 70% of our immune system.
The gut microbiome has been linked not just to gut health, but with the health of the entire body. The gut-brain axis or the communication between the gut and the brain is one fundamental function currently being researched by scientists and also gaining widespread recognition amongst consumers. The gut-skin axis and the gut-liver axis are two other connections which are beginning to be studied and understood. Furthermore, gut health has also been linked to the prevention of colorectal cancer and other chronic diseases.
Not all high-fat foods are bad
It’s important to highlight that while the high-fat diet was linked with an increased risk of anxiety, the rats were predominantly eating saturated fats, which are high in cholesterol and associated with weight gain and heart disease. Foods high in saturated fats include processed meats, butters and cheeses. However, there are many fats which are considered to be good for the body. These include fish, avocados, nuts and seeds, and are associated with health benefits such as reduced inflammation and a lowered risk of developing heart disease. However, these good fats should also be consumed in moderation, as the British Heart Foundation says, “a small amount of fat is good for you.”
Source: High-fat diet, microbiome-gut-brain axis signaling, and anxiety-like behavior in male rats
Published online: 6 May 2024
DOI: https://doi.org/10.1186/s40659-024-00505-1
Authors: Sylvana I. S. Rendeiro de Noronha, Lauro Angelo Gonçalves de Moraes, James E. Hassell Jr. et al.
