The Gut-Brain Connection
Microbiome-Gut-Brain Axis
The gastrointestinal (GI) nervous system is often called the ‘second brain’ as we have between 200-600 million neurons lining our GI tract. The gut and brain are actually formed from the same foetal tissue and remain inextricably linked for life through the gut-brain axis.
Over the past few years there has been a huge amount of research into the links between the gut and the brain, which has uncovered clear associations between gut pathology and neurodegenerative and mental health disorders. GI symptoms are surprisingly common in people with mental health and neurological conditions, though they are usually seen as unimportant compared with the altered behaviour.
The importance of GI health to brain function is highlighted in a recent study of more than 1,500 people with GI disorders which found that 84% had anxiety and 27% had depression. In addition, studies in mice, and preliminary work in humans, suggest imbalances in the microbiome are linked to other brain problems like ADHD, autism spectrum disorders and chronic fatigue syndrome.
What happens in vagus doesn’t stay in vagus
The vagus nerve is a major nerve that originates in the brain and extends all the way to the gut. It carries information to the brain about what the gut is doing – what’s being digested, how active it is, and so on.
Studies have shown that severe depression can be remedied by hijacking this system. This treatment option is called vagal nerve stimulation and it works by fitting an electrical pacemaker to the vagus nerve to boost nerve activity, and therefore mood.
But under normal circumstances, the vagus nerve is stimulated by chemicals called neurotransmitters. These neurotransmitters stimulate the vagus nerve to communicate with the brain.
Our gut microbes are able to produce various neurotransmitters, such as serotonin, melatonin, GABA, histamine and acetylcholine. The microbiota therefore plays an important role in regulating nervous activity, including mood, stress reactivity and anxiety levels, using the vagus nerve as a communication channel.
How can eating live bacteria make you feel happier?
One potential mechanism is to do with the ‘happy neurotransmitter’ serotonin. An amazing 95% of serotonin is produced in the gut, where it plays a role in stimulating normal motility, in addition to regulating mood and memory.
Introducing probiotics (live bacteria) to the gut, increases levels of a compound called tryptophan. Tryptophan is of paramount importance for happiness, as it is converted directly into serotonin. Interestingly, the increase in tryptophan is not because the added bacteria make it, but because they prevent the immune system from destroying the body’s supplies of it.
Probiotics have also been shown to improve anxiety and reduce stress. This may be because certain strains of bacteria can increase GABA production, the neurotransmitter that helps with resilience to stress.
A toxic brain
Another mechanism by which our gut microbiota may affect central nervous function is by increasing the leakiness of the gut lining, leading to bacteria and their toxic waste products reaching the brain.
In particular, the bacterial endotoxin LPS is elevated in individuals with depression, and has been shown to increase amygdala activity (the brain region involved in anxious behaviour). LPS-induced brain inflammation is also associated with the accumulation of beta-amyloid plaques which drive cognitive decline.
How to keep your gut bugs in check
The GI tract is home to more than 500 species of bacteria, comprising about 100 trillion bugs altogether. There are ‘good’ bugs called probiotics, which we can keep replenished. These probiotics need nourishing food to help them grow.
Prebiotics are the fibre-rich foods that probiotics feed on in order to grow. Additionally, a compound called butyrate is produced when probiotics break down prebiotic foods in the gut. Butyrate is the preferred form of fuel for the cells that line the gut, and serves to acidify the environment, making it harder for harmful bacteria to survive.
To keep your gut healthy and happy, try to include some more of the following foods in your diet:
Probiotics
· Kefir
· Yogurt (plain, no added sugar, active cultures)
· Kimchi
· Kombucha
· Miso
· Natto
· Pickled vegetables (raw)
· Sauerkraut (unpasteurised)
· Tempeh
Prebiotics
· Apple
· Asparagus
· Banana
· Chicory
· Cocoa
· Aubergine
· Flaxseed
· Garlic
· Jerusalem artichoke
· Leek
· Legumes
· Onion
References
Addolorato, G., et al, State and trait anxiety and depression in patients affected by gastrointestinal diseases: psychometric evaluation of 1641 patients referred to an internal medicine outpatient setting. Int J Clin Pract. 2008 Jul; 62(7): 1063-9.
Bravo J.A., et al. Ingestion of Lactobacillius strain regulates emotional behaviour and central GABA receptor expression in a mouse via the vagus nerve. PNAS. 2011; 108(38): 16050-16055.
Cryan, J.F. and Dinan, T.G. Mind-altering microorganisms: the impact of the gut microbiota on brain and behaviour. Nature Reviews Neuroscience. 2012; 13: 701-712.
Dinan, T.G. and Cryan, J.F. Melancholic microbes: a link between gut microbiota and depression? Neurogastroenterology & Motility. 2013; 25: 713-719.
Furness, J.B., et al. The enteric nervous system and gastrointestinal innervation: integrated local and central control. Adv Exp Med Bio. 2014; 817: 39-71.
Lee, J., et al. Neuro-inflammation induced by lipopolysaccharide causes cognitive impairment through enhancement of beta-amyloid generation. Journal of Neuroinflammation. 2008; 5:38.
Markowiak, P. and Slizewska, K. Effects of probiotics, prebiotics, and synbiotics on human health. Nutrients. 2017; 9(9): 1021.
Montiel-Castro, A.J., et al. The microbiota-gut-brain axis: neurobehavioral correlates, health and sociality. Frontiers in Integrative Neuroscience. 2013; 7: 1-16.
