The effects of a high-fat diet — increased body weight and high blood sugar — caused anxiety and depression along with measurable changes in the brain, a new study in mice revealed. Strangely, these high-fat diet fed mice did not benefit from treatment with an antidepressant, the Université Paris Sud researchers said.

The World Health Organization estimates diabetes and depression each affect 350 million people. Are these conditions in some way intertwined? According to the researchers, past studies suggest depressive disorders that develop relatively early in life lead to an increased risk of diabetes, while 10 to 30 percent of diabetic patients suffer from major depression. Prior research also shows how metabolic damage caused by a long-term high-fat diet or a Western diet elicited depressed behavior in animals modeling type 1 and type 2 diabetes mellitus. And, past animal studies have also suggested a high fat diet — consisting mainly of large portions of meat, dairy, and nuts and little to no starches or sugars — might exacerbate depression.

“Collectively, these findings emphasize the existence of common brain circuits and signaling pathways between both metabolic and psychiatric disorders,” wrote Dr. Bruno Guiard, senior author of the study and his co-authors. 

To explore the influence of type 2 diabetes on emotionality, the French research team designed an experiment that worked like this: First, they fed mice a high fat diet and then assessed the rodents for metabolic or psychiatric disorder symptoms, while also examining their brains. Next, the researchers tested to see whether an antidepressant or withdrawal from the high fat diet could reverse the metabolic and behavioral symptoms in the mice. Finally, they analyzed all the results.

Some of what they discovered was expected. The high fat diet increased the body weight of the mice while also raising their blood sugar levels and decreasing their glucose tolerance. Yet these metabolic symptoms also correlated with anxiety/depression-like symptoms in the mice. Examination of the mice also revealed decreased serotonin levels in the hippocampus. (Generally, higher levels of serotonin produce positive moods.)

Importantly, even prolonged use of an antidepressant produced no beneficial effects on the mice. By comparison, stopping the high fat diet completely reversed metabolic impairments and lessened the anxious symptoms, even if some depression-like symptoms remained.

The researchers say their data provide "clear-cut evidence" that metabolic and psychiatric pathologies are not only related, but also linked to discernible changes in the brain. Based on these findings, they call for more experiments to further explore possible therapies for these conditions.

Source: Zemdegs J, Quesseveur G, Jarriault D. High fat diet-induced metabolic disorders impairs serotonergic function and anxiety-like behaviours in mice. BJP. 2015.