Proline is an amino acid that is required for a variety of essential functions. Researchers have now found that it's also linked to depression in fruit fly and mouse models, as well as humans. Proline-rich diets appear to increase the risk of depression, according to work reported in Cell Metabolism. In a human study, participants were asked about their mood and their diet. The researchers "were surprised" to find that proline consumption "was most associated with depression,”said Dr. Fernández-Real of the Girona Biomedical Research Institute (IDIBGI).
An analysis of metabolites seemed to confirm these findings, although not everyone that was depressed was consuming a lot of proline; another link was revealed.
The researchers also found that bacterial genes that are linked to proline metabolism, and which would be carried by microbes in the human gut, were also associated with depression. The level of proline in circulation appears to depend not only on the diet, it also depends on the microbes in the gut that metabolize proline.
“The microbiota of patients with high proline consumption but low plasma levels of proline was similar to the microbiota associated with low levels of depression and was enriched in bacterial genes involved in the transport and metabolism of proline," explained Dr. Jordi Mayneris-Perxachs of the IDIBGI.
Still, the researchers wanted to know whether high proline levels would actually cause depression, or if they were a consequence of a depressed mood. So they transplanted gut microbes obtained from the human study participants into a mouse model.
Animals that displayed more depressed behaviors turned out to be those that got gut microbes from study participants carrying high proline levels or people who were more depressed.
Gut microbes may be a better way to tackle this issue for some people as well, because proline is found in so many foods, from meat to peanut butter to soybeans.
The researchers also identified a specific microbe that was linked to depression, Enterobacter. These microbes were fed to a fruit fly model, which then displayed depressive behaviors. That was in contrast to flies given Lactobacillus, a microbe linked to a less depressive mood. When this experiment was repeated with flies that had been genetically engineered to halt the movement of proline to the brain, the depressive behaviors were stopped.
“These results demonstrate the importance of proline and its influence on people’s depressive mood, which so far had not been taken into account," said Fernández-Real. It also suggests that diet could be a way to treat some cases of depression.