A new study has identified several corresponding lines of evidence which all point like a divining rod to citrus fruit being associated with lower risks of developing depression.
Clinical depression affects hundreds of millions of people worldwide, 70% of whom fail to respond to medication. The causes are varied and difficult to understand, but being that you can’t medicate your way out of a poor diet, one scientist thought he’d look for a dietary solution.
Dr. Raaj Mehta, an instructor in medicine at Harvard Medical School and a physician at Massachusetts General Hospital, has found evidence that just one medium-sized orange can reduce a person’s risk of developing depression by 20%.
“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 to the Harvard Gazette. “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!”
Dr. Mehta’s findings show that total fruit and vegetable consumption was not correlated with anything—citrus stood alone as a deciding factor, and it worked, he later hypothesized, by increasing the total amounts of a gut bacterial species F. prausnitzii. Citrus increased the population of this species in the guts of the nurses who consumed the most citrus.
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Furthermore, because the NHS2 contained only women, he looked for the same parameters and gut microbes in men, and the findings remained: 20% reduced risk of depression, higher levels of F. prausnitzii.
When asked how this data might work to reduce depression, Dr. Mehta suggested the S-adenosyl-L-methionine cycle I pathway may be utilized by these bacteria as a way to influence human neurotransmitters dopamine and seratonin, produced by human cells in the gut.
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“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 told the Gazette. “At the same time, I had not associated citrus with the brain before we got these results. You often hear that fish is ‘brain food,’ but nobody says that oranges are brain food.”
One future question would be whether the effect was unique to oranges. Could it be replicated with lemons, bergamot, grapefruit, tangerines, or limes?
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