New research demonstrates a potential protective role of citrus fruit on the incidence of depression and suggests that Faecalibacterium prausnitzii, a type of bacteria found in the human gut, and its metabolic activity may modulate the influence of citrus and its flavonoids on mood.
Samuthpongtorn et al. report that citrus intake and its components are prospectively associated with a lower risk of depression and altered abundance of 15 gut microbial species, including enriched Faecalibacterium prausnitzii. Image credit: Hans.
Depression is a widespread and debilitating condition, affecting more than 280 million individuals worldwide.
The precise causes of depression are unknown, and treatment is often ineffective.
Seventy percent of patients with depression fail to respond to initial treatment with antidepressant medications and/or develop intolerable side effects to the drugs.
Diet may be a promising avenue for depression prevention and management.
Mediterranean-style diets have been associated with a nearly 35% reduced risk of depression, and Mediterranean diet has similarly shown reductions in mood symptoms.
Although the specific food groups that underlie these findings remain unclear, citrus, including oranges and grapefruits, have recently been linked with lower depression risk.
However, the mechanisms explaining diet-depression relationships remain unknown.
In the new study, Dr. Raaj Mehta, an instructor in medicine at Harvard Medical School and a physician at Massachusetts General Hospital, and colleagues analyzed the interplay between citrus consumption, the gut microbiome, and risk of depression, in over 32,427 participants.
They prospectively examined the long-term intake of citrus in relation to depression and abundance of gut microbial species, and in turn, the associations of these species and their metabolic potential with depression.
“I was working with a fantastic postdoc named Chatpol Samuthpongtorn, who was reading through the literature on depression, looking for an interesting project to take on,” Dr. Mehta explained.
“And he came across this one paper from 2016 that pointed to the possibility that citrus lowers the risk of depression.”
“That piqued our interest because we had access to a rich data set that we could use to follow up on this finding.”
“It’s called the Nurses’ Health Study II (NHS2), and it began in 1989 with the goal of finding risk factors for major chronic diseases in women.”
“It involves over 100,000 women, and roughly every two years they provide researchers with detailed information about their lifestyle, diet, medication use, and health.”
“So we decided to leverage these data to look for evidence that nurses who ate a lot of citrus had lower rates of future depression than those who did not. And that’s what we found!”
The authors found that eating one medium orange a day may lower the risk of developing depression by about 20%.
“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,” Dr. Mehta said.
One unique part of the study is that a subset of participants in the NHS2 gave the researchers several samples of their stool over the course of a year.
“Using DNA sequencing results from these stool samples, we looked for links between citrus intake and particular species of bacteria in the gut microbiome,” Dr. Mehta
“One species of bacteria, Faecalibacterium prausnitzii, stood out.”
“It was more abundant in people who were not depressed than people who were, and consuming a lot of citrus was also associated with high levels of Faecalibacterium prausnitzii.”
“So we think this bacterium may link citrus consumption with good mental health.”
“Because the NHS2 only includes women, we wanted to confirm the findings in a study involving men.”
“So we turned to a similar study, called the Men’s Lifestyle Validation Study, where we also saw increasing levels of Faecalibacterium prausnitzii were inversely correlated with depression risk scores.”
“So then the question became, how is Faecalibacterium prausnitzii making people feel better?”
“One answer, we think, might be that these bacteria use a metabolic pathway known as the S-adenosyl-L-methionine cycle I pathway to influence levels of two neurotransmitters — serotonin and dopamine — produced by human cells in the gut.”
“These neurotransmitters regulate how food passes through the digestive tract, but they can also travel to the brain, where they elevate mood.”
“I hope our results inspire other researchers to look into the link between diet 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, which make ourselves feel better in the short term. But researchers are just starting to understand the specifics.”
A paper on the findings was published in the journal Microbiome.
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C. Samuthpongtorn et al. 2024. F. prausnitzii potentially modulates the association between citrus intake and depression. Microbiome 12, 237; doi: 10.1186/s40168-024-01961-3
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