The healing power of the brain-gut connection

Anyone who's ever tried to cure the blues with Ben and Jerry's knows that there is a link between our stomachs and our moods. Foods high in fat or sugar release pleasure chemicals into the brain in much the same way that drugs do, and chocolate in particular is frequently touted as a mood-elevating treat. Now research from a team of pharmacologists in Ireland provides new support for this brain-gut connection, showing that probiotic bacteria, like those found in many strains of yogurt, can help to elevate mood and reduce anxiety. Formally referred to as the microbiome-gut-brain axis, this system has been implicated in stress responding, with gut microflora affecting the HPA (hypothalamic-pituitary-adrenal) axis and altering stress and anxiety responses. In the current study researchers gave mice Lactobacillus rhamnosus and then subjected them to various stress-inducing tests. Mice who had been fed the probiotic solution demonstrated less freezing or fear-response behaviors compared to those who were given plain broth. They were also more likely to explore exposed novel environments in an elevated maze, an indication of security and lack of anxiety. Finally, on a depression assessment mice were placed in a forced swim test (also called the behavioral despair test), where they were submerged in water and had to struggle to stay afloat. Lack of effort and time not spent attempting to swim are seen as indicators of depression and hopelessness, and probiotic-fed mice had less immobility time than broth-fed mice. Corroborating these behavioral results, test mice also had lower levels of corticosterone after being stressed than control mice.

This interaction between the brain and the gut is facilitated by the vagus nerve, the cranial nerve implicated in transmitting sensory information from internal organs to the brain. When this nerve was cut the effects of the probiotics disappeared and test mice had decreased exploratory behavior and greater periods of immobility, similar to the broth-fed mice.

The anxiolytic effects of L. rhamnosus seem to be tied to GABA, an inhibitory neurotransmitter involved in anxiety. Probiotic administration altered levels of GABA mRNA expression in regions of the brain including the amygdala, hippocampus and prefrontal cortex. In depressed individuals, GABA levels in the frontal cortex are shown to be reduced, however in the probiotic-fed animals cortical GABA levels were elevated. This led researchers to theorize that L. rhamnosus might help to protect against stressful or anxiety-producing events. GABA levels in the amygdala are also commonly elevated in depressed individuals, and GABA antagonists (which reduce the levels of the neurotransmitter in the brain) are sometimes used as antidepressants. In the current study, lower levels of GABA were found in the hippocampus and amygdala after probiotic consumption, suggesting an interaction between L. rhamnosus and the memory and emotional centers of the brain, potentially increasing associative learning and memory consolidation, and decreasing fear responding to stressful events.

The connection between diet and behavior doesn't just apply to stress. Certain highly specified restrictive diets have also been used to help treat and control a variety of neurological disorders, most notably epilepsy. First pioneered in the 1920s at John's Hopkins Children's Hospital, extreme high-fat/low-carbohydrate diets are gaining support as a possible alternative for drug-resistant epilepsy, though some physicians are still skeptical. The diet works by invoking ketosis, the process in which the body burns fat stores rather than carbohydrates for fuel. This typically occurs when the body is in a starvation state, and is also the premise on which low-carb diets are based. However, ketogenic diets also appear to have an antiepileptic effect, particularly effective in cases of severe pediatric epilepsy. Doctors are not sure why the treatment works, but one theory is that the ketone bodies produced by the liver when the body burns fat protect neurons from damage, though how or why this happens is still unknown.

A keystone paper from University College London published in 2008 was the first to empirically report the efficacy of the ketogenic diet, and a provocative profile in the New York Times of a family dealing with epilepsy, "keto" and the trials it brings has brought this treatment to national attention. The diet itself is strictly regimented and incredibly difficult to follow. It requires exact caloric measurements and proportions of protein, carbohydrates and fats, with roughly a 3-to-1 fat to carb/protein ratio. This relates to a diet of roughly 90% fat which can be dangerous, potentially triggering kidney problems and malnutrition. However the effectiveness of the treatment is gaining recognition, and patients who are on the ketogenic diet (mostly children) are carefully monitored for cholesterol levels and cardiovascular health.

Keto is now being looked at to potentially treat other serious medical disorders as well, such as Parkinson's disease and cancerous tumors. It is important to note though, that individuals being treated with a prescribed diet are also frequently on concomitant medication. Diet alone will not be able to cure all ailments, but the connection between diet and mental and physical health can not be denied, and in the very least it is a good place to start keeping yourself well and taking preventative action.

Dana Smith

PhD student in Experimental Psychology at the University of Cambridge