New findings add another dimension to “gut feelings”

Eating an orange a day may lower a person’s risk of depression by 20 percent, according to a study led by Raaj Mehta, an instructor in medicine at Harvard Medical School and a physician at Massachusetts General Hospital.

Researchers believe the effect may stem from citrus fruits stimulating the growth of Faecalibacterium prausnitzii (F. prausnitzii), a beneficial gut bacterium that appears to influence the production of serotonin and dopamine — neurotransmitters known to regulate mood and emotional well-being.

In an interview discussing the findings, Mehta highlighted how diet and the gut microbiome may play a significant role in mental health.

What Inspired the Research?

The project began when postdoctoral researcher Chatpol Samuthpongtorn discovered a 2016 study suggesting that citrus consumption might reduce depression risk.

“That piqued our interest because we had access to a rich data set that we could use to follow up on this finding,” Mehta said.

The team turned to the Nurses’ Health Study II (NHS2), a long-running health study involving more than 100,000 women. Researchers analyzed dietary information and health outcomes to determine whether citrus consumption was associated with lower rates of depression.

The results showed that women who consumed more citrus fruits had a lower risk of developing depression over time.

How Significant Is the Effect?

The researchers found that eating one medium orange per day was associated with approximately a 20 percent reduction in depression risk.

“The effect seems to be specific to citrus,” Mehta explained. “When we look at people’s total fruit or vegetable consumption, or at other individual fruits such as apples or bananas, we don’t see any relationship between intake and risk of depression.”

While the findings are promising, Mehta cautioned that the results cannot yet be directly compared with traditional antidepressant medications because the study focused on preventing depression rather than treating existing cases.

The Gut-Brain Connection

A unique aspect of the study involved analyzing stool samples provided by a subset of participants. DNA sequencing allowed researchers to investigate relationships between gut bacteria and depression risk.

One bacterial species stood out: F. prausnitzii. The bacterium was found in greater abundance among participants who were not depressed, and higher citrus consumption was linked to increased levels of the microbe.

“So we think this bacterium may link citrus consumption with good mental health,” Mehta said.

To validate the findings, researchers examined data from the Men’s Lifestyle Validation Study and observed a similar relationship between higher levels of F. prausnitzii and lower depression risk scores.

How Might the Bacteria Influence Mood?

The researchers believe F. prausnitzii may affect mood through a metabolic process known as the S-adenosyl-L-methionine Cycle I pathway.

This pathway may influence the production of serotonin and dopamine in the gut. Although these neurotransmitters help regulate digestion, they can also communicate with the brain and contribute to mood regulation.

The findings add to growing evidence supporting a strong connection between gut health and mental well-being.

Were the Findings Surprising?

Mehta said the results were both expected and unexpected.

“There’s so much evidence now suggesting a strong link between the gut and the brain that I was not surprised to find more,” he said.

However, he noted that citrus fruits had not previously been widely associated with brain health.

“You often hear that fish is ‘brain food,’ but nobody says that oranges are brain food,” Mehta said.

Similarly, F. prausnitzii has long been linked to digestive and immune health, but its potential connection to mental health has received little attention.

What Comes Next?

The researchers hope future clinical trials will determine whether citrus consumption can directly prevent or even help treat depression.

“There’s such a huge unmet need for depression treatments, and eating citrus doesn’t really have any major side effects,” Mehta said. “It would be great to see how much this simple treatment can help.”

Beyond citrus fruits, the team hopes the findings encourage further research into the relationship between diet, the gut microbiome, and mental health.

“I think people know intuitively that the foods we eat impact our mood. We even have a term for this: comfort foods,” Mehta said. “But researchers are just starting to understand the specifics.”

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