How gut bacteria affect mood, focus and brain health

The human gut is home to trillions of bacteria, viruses, yeasts and fungi that together make up the gut microbiome, and the balance and diversity of these gut bacteria are key to our health and well-being. Our digestive system is also lined with 100 million nerve cells, and the vagus nerve provides a two-way communication between the gut and the brain. We are only just beginning to understand how certain microbes support and upset the health of the brain and the nervous system.

"Bacteria and other micro-organisms regulate how our guts work," says Dr Joseph Murray, a gastroenterologist and scientist at the Mayo Clinic. "They help with speed of transit and absorption of nutrients as food passes through, but importantly they also take up all the parking spots: these are places where nasty bacteria - like salmonella, for example - can take up residence and then invade us. So one way we're protected is by our 'good' bacteria. They take up all the entry points, so bad bugs have a harder time getting in."

The gut contains almost 70% of the body's immune cells which exist in a fine balance with the resident microbiota, offering us protection against invading micro-organisms that may be detrimental to health.

"Disruptions to the microbiota can trigger inflammation which has the potential to impact brain physiology, mood and cognitive impairment," explains Professor Jonathan Swann, a professor of metabolism at the University of Southampton's Faculty of Medicine. "The microbiota produce a diverse range of chemicals that interact with the immune system and the nerves in the gut: they can also be absorbed through the intestine into the bloodstream and reach the brain.

"Recent studies in mice have demonstrated that chemicals from the mother's gut microbiome can reach the developing fetus to influence neurodevelopment. Preliminary studies are also linking age-related microbial changes with cognitive decline."

Depression and low mood

Gut health plays a key role in how we feel emotionally as well as physically. Gut bacteria produce neurotransmitters (chemical substances which allow messages to pass between nerve cells) such as serotonin, dopamine, GABA, norepinephrine and acetylcholine. All of these can affect mood, motivation, focus and reward. Around 90% of our serotonin is produced in the gut rather than the brain. Chemicals released by gut bacteria have also been implicated in the onset of eating disorders and obesity as well as depression.

"Changes to intestinal function are commonly seen in individuals with anxiety and depression," says Swann. "Conversely, the microbes in the gut can produce chemical signals that impact the brain and modify behaviour and cognitive function. Depression and anxiety are often seen in individuals with gut disorders such as irritable bowel syndrome (IBS)."

Several scientific studies have confirmed the link between gut microbes and state of mind, with one study suggesting that probiotics (so-called 'good' bacteria) could be cautiously recommended to treat depression. Another research paper links gut bacteria with well-being in the workplace and suggests that a better understanding of the brain-gut axis could support the optimal cognitive functioning and mental well-being of staff.

"The last decade has seen the emergence of a new class of probiotics, termed psychobiotics, targeted towards the gut-brain axis," says Swann. "To date, clinical trials using a variety of probiotic strains have had mixed success for depression and anxiety. However, as our understanding of psychobiotics increases, the effectiveness of these interventions is anticipated to improve."

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Neurological diseases

Researchers from the Universities of Edinburgh and Dundee have identified a probiotic which prevents the build-up of a protein linked with Parkinson's disease. The strains L. salivarius LS01 and L. acidophilus were most effective and inhibited the growth of potentially pathogenic bacteria such as Escherichia coli and Klebsiella pneumoniae.

A link between Alzheimer's disease and 'bad' gut bacteria has also been mooted. One study demonstrates that chronic infection by the gut microbe Helicobacter pylori triggers massive inflammatory mediators that adversely affect brain health and cognitive function.

"In terms of Alzheimer's disease, there are increasing data demonstrating a link between disease development, brain inflammation, and the microbiota," confirms Swann. In Parkinson's disease, sleep disorders are common and emerging data show a relationship between sleep patterns and the gut bacteria. This indicates that while microbial manipulation may not necessarily prevent or treat Parkinson’s disease, it may be useful for managing related issues."

Future research

Murray suggests that developing successful probiotic treatments for neurological diseases is still some way off, but we may see positive developments within the next five years.

"With multiple sclerosis, for example, scientists have looked at the microbiome of the intestine in people with and without the disease and there seem to be differences there," he acknowledges. "The issue is whether these differences are a cause or a consequence of the disease changing the microbiome.

"The challenge of diseases such as Parkinson's and Alzeimer's is that they are decades in the making. So if gut bacteria are involved in onset it could take years to map our understanding of that process, and the importance of viruses and fungi should also be considered. Ultimately though, we may develop far more useful drugs from examining the human stool and the microbiome than from exploring the Amazon for new plants and synthesising new compounds."

How to keep your gut healthy

Eat less

A calorie restriction study published in 2018 demonstrated that reducing calorific intake by 15% over two years slowed the ageing process and protected against age-related diseases such as Alzheimer's, Parkinson's, cancer and diabetes. Another study in 2020 proved that calorie reduction alleviates ageing-related accumulation of inflammatory cells throughout the body.

Reduce your intake of preservatives and sugar

A diet that is high in processed food and sugar increases inflammation and reduces the amount of beneficial bacteria in the gut. Preservatives are also found in many processed foods including some breads, margarines, cakes and biscuits.

"Preservatives prevent bacteria from growing in our food, so they are also likely to impact upon the bacteria and microbiome in our gut," observes Murray. "Better to eat fresh foods and those preserved by natural fermentation such as live yoghurt, kefir, kimchi and sauerkraut. Naturally fermented foods are rich in beneficial bacteria and enzymes. Although these microbes pass through us quickly they interact positively with our immune system."

Eat more plants

Eating a diet rich in fruit, vegetables, nuts and pulses has been proven to promote the development of more diverse and stable microbial systems. A recent study shows that polyphenols, which are abundant in plant foods, increase Bifidobacterium and Lactobacillus, which 'provide anti-pathogenic and anti-inflammatory effects and cardiovascular protection'.

"Bacterial breakdown products from dietary fibres (the indigestible compounds of plants) can acidify the intestinal environment to restrict the growth of harmful bacteria," explains Swann, "while also fortifying the barrier function of the gut and the brain."

Consume good-quality prebiotics and probiotics

"While the development of psychobiotics is in its infancy, the beneficial effects of prebiotics and probiotics on gut health are well established," says Swann.

Prebiotics are found in foods such as whole grains, greens, onions, garlic, soya and bananas and probiotics are present in some fermented foods. It is important to note that not all synthesised probiotics will survive the acidity of the stomach to reach the lower intestines. If buying a probiotic supplement, always choose a good-quality product that is proven to remain 'live' as it moves through the digestive tract. Visit charity Guts UK for further information about prebiotics and probiotics.

Reduce stress

While acute stress has been shown to increase immune response in some contexts, chronic stress and anxiety can increase motility of food through the gut; this reduces the absorption of essential nutrients and can exacerbate symptoms of IBS and reduce immune function. So finding ways to relax and reduce stress is crucial in ensuring optimum health for the gut-brain axis.