Dietary fats entering the brain through the bloodstream may explain the link between obesity and depression, new research suggests.
The study, led by the University of Glasgow in collaboration with the Gladstone Institutes, revealed links between the consumption of diets high in saturated fats that lead to obesity and the development of depression phenotypes.
Scientists also found that by decreasing the release of a specific enzyme called phosphodiesterase, symptoms of obesity-linked depression can be reduced.
Using mouse models, researchers were able to see saturated fatty acids were actually entering the brain via the bloodstream and then accumulated and affected crucial brain signals related to depression.
Mice fed a fat-dense diet were shown to have an influx of fatty acids in the hypothalamus region of the brain, an area related to the metabolic system and known to be linked with depression.
These fatty acids were then able to directly affect the key signalling pathways responsible for the development of depression.
Professor George Baillie, lead author of the study at the University of Glasgow, said: "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.
"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.
"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.
"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.
"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.
"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."
The relationship between obesity and depression is known to be complicated, with overweight patients less likely to respond well to common antidepressant medication.
Such patients also show a substantially slower response to antidepressant treatment with less overall improvement, researchers said.
They hope their findings may lead to antidepressant medications that are more suitable for people who are overweight or obese.
The study is published in Translational Psychiatry.
The work was funded by the Medical Research Council and Onassis Foundation Predoctoral Fellowship.