A study led by Raaj Mehta, a professor of Medicine at Harvard Medical School and a doctor at Massachusetts General Hospital, shows that eating an orange daily could reduce the risk of depression by 20%.

This can be attributed to the fruit's stimulation of the bacterium Faecalibacterium prausnitzii (F. prausnitzii), which resides in the human intestine and influences the production of neurotransmitters serotonin and dopamine, molecules known for their ability to improve your mood.

In an interview with The Harvard Gazette, the researcher explained that the study began in 2016 while working with a postdoctoral fellow. While working, they came across literature suggesting that citrus fruits might reduce the risk of depression, “that sparked our interest because we had access to a very comprehensive dataset that we could use to follow up on this finding,” he said.

“It’s called the Nurses’ Health Study II (NHS2), which began in 1989 to identify risk factors for major chronic diseases in women. More than 100,000 women participated in the study, and approximately every two years, they provided researchers with detailed information about their lifestyle, diet, medication use, and health."

Mehta said they decided to use this data to look for evidence that nurses who ate a lot of citrus fruits had lower rates of future depression compared to those who didn’t eat them, and that’s exactly what they found. 

Oranges and their relationship with depression

When asked how oranges might reduce the risk of depression, Mehta explained that one unique aspect of the study is that a subset of participants in the NHS2 provided researchers with several stool samples over the course of a year.

"Using the results from the DNA sequencing of these stool samples, we looked for links between citrus intake and specific species of bacteria in the gut microbiome. One bacterial species stood out—F. prausnitzii—which was more abundant in people who were not depressed than in those who were. High citrus consumption was also associated with higher levels of F. prausnitzii. Therefore, we believe this bacterium may link citrus consumption to good mental health," he explained.

"Since the NHS2 only includes women, we wanted to confirm our findings in a study that included men," he added. "That's why we turned to a similar study, called the Male Lifestyle Validation Study, where we also found that increasing levels of F. prausnitzii were inversely correlated with depression risk scores."

"The question then is: How does F. prausnitzii make people feel better? We believe one answer could be that these bacteria use a metabolic pathway known as the S-adenosyl-L-methionine cycle to influence the levels of two neurotransmitters (serotonin and dopamine) produced by human cells in the gut. These neurotransmitters regulate the passage of food through the digestive tract, but they can also reach the brain, where they improve mood," he said.

The researcher hopes that the findings will lead to a clinical trial to demonstrate that eating citrus fruits can reduce the risk of depression or perhaps even alleviate the condition in some cases.

"There is a huge unmet need for treatments for depression, and eating citrus fruits actually has no significant side effects, so it would be fantastic to see how much this simple change in lifestyle can help," he stated.