This is the second post in a series exploring the neurobiology of the gut-brain connection published in partnership with neuroscientist Dr Amy Reichelt.

Our intestines are home to an ecosystem of trillions of microbes including bacteria, viruses, phages, fungi, protists, and nematodes. Dominant are bacteria from the families Firmicutes and Bacteriodetes. As I talked about in my last blog, the collective community of these microbes in the intestines is the gut ‘microbiome’.

In this blog, we’ll consider the gut microbiome from the perspective of obesity research. Changes in the gut microbiome are associated with several disease states including obesity. Obesity-associated microbes affect insulin resistance, inflammation, fat deposition, metabolism, and appetite. And the gut microbiome is becoming a target for new anti-obesity therapies.

How does the microbiome influence our health?

The gut microbiome responds to its surrounding environment including to the foods we eat. Healthy versus unhealthy diets have a profound effect on the microbiome including which species dominate.

For example, eating an unhealthy diet full of sugar and saturated fat alters the balance between ‘good’ Bactereoides and ‘bad’ Firmicutes bacterial species. Firmicutes are needed to digest fats, and eating a high-fat diet encourages their population to increase, leading in turn to weight gain. Bactereoides digest soluble fibre, so people who consume fibre-rich diets have more Bactereoides than Firmicutes.

Research in germ-free mice, which lack gut microbes, found altering the balance between Bactereoides and Firmicutes dictated how obese or lean mice became. And a propensity for obesity or leanness could be transmitted from mouse to mouse via gut microbes alone.

The brain regulates appetite, feeding behaviours, and energy balance, and although the gut microbiome contributes to obesity and metabolic syndromes such as diabetes, the exact mechanisms underlying this relationship are uncertain. To date, the link between the microbiome and obesity in humans is “weak”.

How might gut bugs speak to the brain?

The gut provides an environment for bacteria to ferment digested food into all kinds of chemicals and molecules that inform the brain of our current nutritional state. As mentioned in the previous blog, the microbiome signals to the brain via pathways such as vagus nerve, immune and hormone systems, and also via the production of bioactive microbial metabolites released into the bloodstream.

Short-chain fatty acids (SCFAs) are metabolites produced in the gut by microbial fermentation of undigested dietary fibre and complex carbohydrates. SFCAs can cross the blood-brain barrier and modulate brain activity to regulate appetite and food intake. In the brain, SCFAs are neuroprotective.

One such SCFA is butyric acid or butyrate. Butyrate is an anti-inflammatory agent in the brain and the gut and reduces levels of harmful toxins that can damage neurons. Butyrate also fortifies the blood–brain barrier by tightening connections between cells, which prevents harmful toxins entering the brain causing neuron-damaging inflammation. A leaky blood-brain barrier is often seen in obesity and neurodegenerative disorders such as Alzheimer’s disease. Butyrate also boosts the production of molecules that are important for neuroplasticity including brain-derived neurotrophic factor (BDNF). Boosted levels of BDNF leads to enhanced memory performance in lab rats. Scientists have found that eating a junk food diets reduces SCFA levels so it is important to eat a healthy diet to maintain optimal brain function.

How can we change our gut microbiome?

I think of the microbiome like the different soils that plants grow in — certain plants grow well in nutrient-rich compost, but they won’t survive in dry, sandy soil. Western diets, especially those with low levels of fibre, have possibly reduced the diversity of microbiota over generations. By contrast, traditional diets high in fibre and low in sugar and fat increase microbiome diversity.

So what can we do to ensure our microbiome supports our health?

Researchers suggest the diversity and function of microbiome are established by your genetic background and external factors, including way you were delivered at birth and how you exercise. It’s suggested the key determinant affecting the composition and activity of the gut microbiome is your diet — explaining about 57% of variations in total gut microbiota.

We can influence the populations of gut microbes by making our gut environment hospitable to them by eating probiotic and prebiotic foods.

Probiotics are foods or dietary supplements containing specific gut-beneficial microbes (such as Lactobacillus or Bifidobacterium) to stimulate the growth of these microorganisms in the gut. Probiotic-rich foods include live yoghurt, kombucha, kimchi, and fermented vegetables such as sauerkraut.

Prebiotics are foods stimulate the growth of beneficial bacteria. I think of them as the nutrient-rich fertiliser that helps plants grow. Prebiotic-rich foods include bananas, onions, wheat, flaxseeds, legumes and leafy green vegetables, e.g. spinach, kale, and broccoli. Fermented foods contain microbes that aid gut health and are also prebiotic. The live microbes have already worked on digesting these foods, unlocking extra nutrients.

Harnessing the gut microbiome to boost the brain

A healthy gut microbiota is crucial for proper metabolic function and homoeostasis. It’s a win-win for us and our microbes — in exchange for living and proliferating in the gut, they keep us healthy.

Changing our gut microbiome using probiotics and prebiotics has exciting potential to improve brain function. In my next blog, I’ll talk about a whole new field called ‘psychobiotics’ that could provide a novel therapy for people suffering from mood disorders.

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