This diet increases the risk of depression by changing tryptophan metabolism, which is important for brain function.
Eating a typical Western style diet increases the risk of depression, whereas healthy eating patterns with plenty of fresh fruits and vegetables have been shown to lower depression.
The Western style diet is typically rich in processed foods, sugars, and saturated fats.
A study reveals that higher intake of the Western diet lowers levels of a neuroprotective molecule known as kynurenic acid (KA).
Serotonin and KA are products of tryptophan, an essential amino acid that our body can’t make and so must come from food.
These compounds are important for regions of the brain related to anxiety, cognition, depression, addiction, passivity or violence, and eating behaviours.
The Western diet appears to alter tryptophan metabolism, resulting in lower levels of KA and therefore greater odds of depression.
Dr Edwin Lim, the study’s senior author, said:
“Western-style diets high in fat, sugar and processed foods were already known to increase the risk of depression, but this is the first time a biological link involving the kynurenine pathway has been established.
In this study, we tested participants’ urine for several biological markers, including KA and inflammation, and compared them with how healthy their diet was and the severity of depression symptoms.
People from the group eating an unhealthy diet had lower levels of KA and more severe symptoms of depression.
This indicates that KA may help to protect us against depression.”
The Western diet has already been linked to a wide range of problems including:
- brain damage,
- stress,
- Alzheimer’s disease,
- slower learning and poorer memory,
- immune system problems such as inflammatory skin disease,
- and even more severe COVID infection,
The Western diet and tryptophan
Tryptophan is essential for the human body to function and the typical Western diet is low in nutrients such as tryptophan.
Foods such as milk, fish, cheese, chicken, turkey, eggs, oats, nuts, and seeds are good sources of tryptophan.
Tryptophan breaks down into metabolites delivering various protective functions to the brain.
They are also used by the body for inflammatory responses and cells regulation against disorders such as dementia, cancer, heart disease, and stroke.
In the brain, tryptophan is converted into serotonin and serotonin into melatonin, a hormone that regulates our sleep and mood.
KA is also made by tryptophan via the kynurenine pathway associated with neurological diseases such as Alzheimer’s.
Until now no one knew that the Western style diet can negatively affect tryptophan metabolism even in young and healthy adults.
Dr Lim said:
“Previously, it was believed that changes to tryptophan metabolism were driven by inflammation, despite there not being conclusive clinical evidence for this.
Our study also shows that urine analysis may be a useful alternative to blood tests in collecting valuable biological information on the way our bodies process tryptophan.
This can be a big advantage in that it’s not only simpler—it’s less invasive, which is important for vulnerable people such as children and older adults.”
It is not yet clear if targeting KA would be a treatment option for depression in the future, in a similar way that antidepressants are supposed to boost serotonin levels.
Dr Heather Francis, the study’s first author, said:
“There is, however, a clear relationship between an increased risk of depression and eating an unhealthy diet that is high in fat, sugar and processed foods, giving us all the incentive to eat more fresh vegetables and fruits.”
Like most things, the right amount of kynurenine is the key for the body since elevated levels of KA have been associated with schizophrenia and low serum levels of KA connected to depression.
The study was published in the journal Frontiers in Nutrition (Francis et al., 2022).
