Dietary fats can enter the brain through the bloodstream, and cause alterations that could lead to depression, according to a study that may pave the way for new therapies to treat the disorder.
The team also found that by decreasing the expression of a specific enzyme called phosphodiesterase, symptoms of obesity-linked depression can be reduced.
The team from University of Glasgow in the UK conducted a study in mice to show how dietary habits are linked to mental health.
The study, published in the journal Translational Psychiatry, shows that saturated fatty acids enter the brain through the bloodstream, and thereafter accumulate and affect crucial brain signals related to depression.
Obesity and depression have long been linked, with previous clinical studies finding an association between these two conditions.
However, until now, the mechanisms of how obesity affects depression and vice versa have not been fully understood.
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"We often use fatty food to comfort ourselves as it tastes really good, however in the long term, this is likely to affect one's mood in a negative way," said George Baillie, from University of Glasgow.
"Of course, if you are feeling low, then to make yourself feel better you might treat yourself to more fatty foods, which then would consolidate negative feelings," Baillie said in a statement.
Through trials conducted in mice, scientists identified a mechanism linking exposure to a high-fat diet to alterations in hypothalamic functioning.
They found that fatty acids move through bloodstream and gradually accumulate in brain.
The research provides an insight into how making significant dietary changes can also prove beneficial for mental health.
"We all know that a reduction in fatty food intake can lead to many health benefits, but our research suggests that it also promotes a happier disposition," said Baillie.
"Further to that, understanding the types of fats, such as palmitic acid, which are likely to enter the brain and affect key regions and signalling will give people more information about how their diet can potentially affect their mental health," he said.
The relationship between obesity and depression is known to be complicated, with patients with obesity less likely to respond well to common antidepressant medication.
"This research may begin to explain how and why obesity is linked with depression and how we can potentially better treat patients with these conditions," said Baillie.
It could also lead to development of medications suitable for overweight patients.
"This is the first time anyone has observed the direct effects a high-fat diet can have on the signalling areas of the brain related to depression," said Baillie.