Researchers from the Girona Biomedical Research Institute (IDIBGI) and Pompeu Fabra University (UPF) have identified the role of proline, an amino acid present in many foods, in the depressive state of humans, mice and flies.
This amino acid can be found in foods of animal origin, such as meat, eggs, fish or dairy, and in foods of plant origin, such as fruits and vegetables rich in vitamin C, legumes and cereals.
The study, published in the scientific journal , looked at the type and amount of amino acids in the participants’ diet. These also answered a questionnaire that measures depressive mood.
“We were surprised that what was most associated with depression, evaluated by means of this questionnaire, was the consumption of proline”, affirms Dr. Fernández-Real, from IDIBGI, and also Head of the Endocrinology Section of the Dr. Josep Trueta Hospital. and director of the Department of Medical Sciences at the University of Girona.
But not everyone who had a high proline intake reported the questionnaire to find themselves in a more depressed mood. When the gut microbiota of these people was studied, a relationship was also observed between depression and bacteria, as well as between depression and bacterial genes associated with proline metabolism.
Thus, circulating proline levels were found to depend on the microbiota. “The microbiota of patients with high proline intake but low plasma proline levels was similar to the microbiota associated with low levels of depression and was enriched in bacterial genes involved in proline transport and metabolism,” says Dr. Mayneris -Perxachs, researcher Miguel Servet of the IDIBGI.
From humans to mice and flies
To find out if the presence of proline was a cause or consequence of depressed mood, the microbiota of the participants was transplanted into mice. The rodents that became most depressed were those that received the microbiota from participants with high proline, or from people with more depressed mood. Different genes associated with proline transport were also found in the brain of these mice.
Another confirmatory experiment was performed using fruit flies (Drosophila melanogaster), in which a more depressive mood can be induced. The researchers isolated two types of bacteria from the microbiota associated with proline consumption and added them to the sterilized food of the flies. The flies that ate a diet containing Lactobacillus, which in mice was associated with less depression, showed a greater desire to overcome difficulties that they faced later. In contrast, those who ingested Enterobacter, which in humans is associated with depression, became much more depressed.
Finally, the same experiment was carried out on flies genetically modified to remove the channels that transport proline to the brain. In that case, proline couldn’t get to the brain, and the flies proved to be very resilient in depression.
“These results demonstrate the importance of proline and its influence on people’s depressed mood, an element that until now had not been taken into account,” emphasizes Dr. Fernández-Real. The work also opens the door to propose new studies to find possible diet-based treatments for depression.
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