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Gut-brain cross-talk: Who's feeding who?

The gut-brain axis is a bi-directional pathway connecting the gastrointestinal tract and the brain. This axis links the brain and the gut microbiota, which are essential for health and wellbeing.

The gut is home to trillions of bacteria, viruses and fungi which digest food and produce vitamins. It is clear that microbial cells outnumber human cells and the total weight of these gut microbes is 1-2 kg, similar in weight to the human brain.

The gastrointestinal tract contains 100 million neurons that communicate back to the brain through neurotransmitters like serotonin and dopamine. These are known as enteric neurons because they are located within the walls of the intestine but outside the spinal cord or brain itself.

Microbiota and their human hosts have co-evolved and are mutually co-dependent for survival.

Stress, infection, diet, lifestyle and environment all play a role in the delicate balance within this vast ecosystem.

When the balance in the gut is disrupted, the resulting gut dysbiosis has been linked to multiple disorders that range from gastrointestinal diseases to mood disorders, behavioural changes and cognitive function.

Susceptibility to stress-related disorders have both genetic and environmental factors, but its become increasingly clear that bacteria are required for normal brain development and brain function throughout our lifespan.

Key processes associated with neuroplasticity in the adult brain such as neurogenesis and microglia activation have been shown to be regulated by the microbiota. (Ogbonnaya et al. 2015) (Erny et al. 2015)

Eating Disorders (ED)such as Anorexia Nervosa (AN) and Bulimia Nervosa (BN) and to some extent, Binge Eating Disorder, are classified as neuropsychiatric disorders. New evidence suggests a link between ED and the gut microbiome.

Serguei Fetissov et al. (2019) have introduced a model the states that AN and BN may originate from altered signalling between gut microbiota and the host immune and neuroendocrine systems regulating feeding behaviour.

They found that plasma of certain ED patients had immunoglobulins (Ig) that react with a-melanocyte-stimulating hormone (a-MSH), a neuropeptide that is expressed in neurons in the hypothalamus. The hypothalamus is a crucial centre for energy balance and regulation of food intake.

a-MSH is a hormone that helps signal satiety. An anorexigenic bacteria called Escherichia coli caseinolytic protease B was found to be responsible for triggering autoantibodies. These antibodies form immune complexes with a-MSH which chronically activate the melanocortin system that regulates feeding behaviour, thus disrupting the physiological control of appetite.

Our gut ecosystem needs to be rich in a wide variety of species for the good of our immune system, but most importantly it keeps our brains healthy.

Without a variety of microorganisms, we would be unable to properly digest foods and absorb nutrients vital to our health.

This symbiotic relationship, as with all relationships, requires nurturing and understanding.

Perhaps today is a good day to introduce an extra serving of vegetables, and make sure you eat or drink something that has been naturally fermented.

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