A recent study places oranges on the list of fruits that boost good mood. Specifically, just one orange a day seems to be enough to reduce the risk of depression by 20%, according to the latest data published in Microbiome.

The study was led by Dr. Raaj Mehta, professor of medicine at Harvard Medical School and physician at Massachusetts General Hospital. According to him, the interest was sparked by an earlier study suggesting that citrus fruits reduce the risk of depression. “This caught our attention because we had access to a rich data set that we could use to track this finding,” he adds.

Thus, he and his research team turned to the Nurses’ Health Study II, a long-term study of about 100,000 female nurses that began in 1989 to identify risk factors for major chronic diseases in women. After analyzing lifestyle, diet, medication use, and overall health, they found that consuming a medium orange daily can reduce the risk of developing depression by about 20%.

The effect seems to be specific to citrus fruits: “When we look at overall fruit or vegetable consumption or other individual fruits like apples or bananas, we see no association between consumption and depression risk,” he notes.

Nevertheless, Dr. Mehta explains that antidepressant treatments remain valuable for managing mental illness: “It is difficult to compare the effectiveness of citrus fruits with traditional antidepressants, such as selective serotonin reuptake inhibitors, because we are talking about preventing depression, while these medications are usually used to treat depression once someone has developed it.”

However, oranges could be a satisfactory preventive measure: “In the future, citrus fruit consumption might become part of a strategy to manage depression alongside more traditional pharmaceutical treatments. But more research is needed before we reach that conclusion.”

Answers in the microbiome

The researchers also looked for connections between citrus intake and specific types of bacteria in the gut microbiome that might be beneficial. By analyzing stool samples and using DNA sequencing results, they identified one standout bacterium: Faecalibacterium prausnitzii (F. prausnitzii). The concentration of this bacterium was abundant in people without depression compared to those with depression, and high citrus consumption was also linked with elevated levels of F. prausnitzii. “Therefore, we believe this bacterium could link citrus consumption with good mental health,” adds Dr. Mehta.

To get a more complete picture for both sexes, the researchers also examined a similar study in men, the Men’s Lifestyle Validation Study. They observed that increasing levels of F. prausnitzii correlated inversely with depression risk scores.

How oranges reduce depression risk

The antidepressant effect of oranges lies in the fact that citrus fruits stimulate the growth of F. prausnitzii, which influences the production of the neurotransmitters serotonin and dopamine — two biological molecules known to boost mood. “These bacteria use a metabolic pathway known as the cycle I of S-adenosyl-L-methionine to affect levels of two neurotransmitters — serotonin and dopamine — which are produced by human cells in the gut. These neurotransmitters regulate how food moves through the digestive tract but can also travel to the brain, where they elevate mood,” explains Dr. Mehta.