Gut-brain connection: how diet impacts brain health

It's becoming increasingly clear that the gut forms a crucial link between what we eat and how we feel.

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Back in 2004, researchers in Japan made an incredible discovery: mice they had raised in germ-free conditions displayed much more dramatic stress responses than their normal counterparts.

They quickly realised that gut bacteria found in healthy mice are exceptionally important for shaping normal hormonal development and behaviour.

To prove this theory, they gave these germ-free mice lactobacillus bacteria. The mice became much more laid back.

The humble mouse has a lot to tell us about the gut-brain connection.

What the gut health of depressed mice can tell us about dementia

This exciting discovery opened the floodgates for a plethora of research, with scientists around the world taking these experiments even further and yielding some jaw-dropping results.

In one study, Chinese researchers found that by transplanting gut bacteria from patients with severe depression into germ-free mice, the mice themselves began to display symptoms of depression

In case you’re wondering how researchers identify depression in mice, it’s not by examining them for frowns. Instead, they typically put them in a water tank and time how long it takes them to give up swimming, which is considered to mimic the lethargy and hopelessness found in depression.

Why are we telling you about depressed mice? Because it’s becoming increasingly clear that the gut forms a crucial link between what we eat and how we feel. And now, scientists are discovering that poor gut health can actually lead to serious conditions like Alzheimer's, Parkinson's, multiple sclerosis, and chronic pain.

Your gut and your brain talk to each other

Did you know that the gut is often referred to as the "second brain"?  This is because the gut contains a high number of neurons and neurotransmitters (like serotonin and dopamine) that you might also find in the brain. These work to regulate both gut function and our mood and emotions.

Thanks to the enteric nervous system, our two "brains" talk to each other, which means that things that affect one also tend to affect the other.

While the gut-brain connection has been recognised for over a century, we’ve only recently started to understand the extent of its importance for brain health.

So why do our two “brains” talk?

It makes sense on a basic survival level: our brain needs to signal to our stomach when food is coming so it can prepare, and our gut needs to provide feedback on whether we’re full or still hungry. 

But that’s not all. Our gut can also signal an emotional response happening in our brains. You might have experienced this yourself if you’ve ever felt queasy during a stressful situation. 

And as we saw with the mice, it’s becoming all-too-apparent that this also works in the other direction:

The state of our gut health can actually influence our brain and mental health. 

Enter stage left: the gut microbiome

There is a vast and intricate landscape of trillions of bacteria and other microbes living within your gut. And it might sound strange, but these microscopic scavengers in your gut have now been linked to many mental health conditions. 

“The gut microbiome is the most important scientific discovery for human healthcare in recent decades.” - James Kinross, Microbiome Scientist at Imperial College London

Not all gut bacteria are created equal. There’s a mix of ‘good’ and ‘bad’ bacteria in our gut, with the majority being incredibly important for your immune system, heart health, and weight regulation.

However, when things go wrong, the “bad” bacteria can wreak havoc. The reasons for this are complex, but have something to do with increased inflammation in the body that ends up weakening the blood-brain barrier (which is responsible for protecting your brain from toxins circulating in the blood).

Fascinatingly, we actually see different species of bacteria in the poo of people with mental health problems compared with those without.

This fun animated video explains this all further, even heading to the wild, wonderful world of poo transplants (!)

The gut microbiome and dementia

Remember those intriguing experiments with mice?

Well, it turns out that when healthy rats are given the gut bacteria from people with Alzheimer’s, these smart little creatures perform worse on memory tests, and their brains don’t produce as many new cells in memory-related areas. These rats also experience more brain inflammation.

Interestingly, the gut microbiome of people with Alzheimer’s is significantly less diverse than, and distinct from, those without the disease.

It’s becoming increasingly clear that targeting our gut microbiome could hold the key to effective prevention of dementia-promoting conditions like depression and Alzheimer’s disease itself. In fact, a huge study in China found that older people in great health had gut microbiomes as diverse as those in much younger people, and consequently had much lower levels of inflammation.

Help! How do I diversify my gut bacteria?

It’s crucial to diversify our gut bacteria as much as possible by consuming a wide range of foods and nutrients to encourage the coexistence of different types of bacteria.

This Five Lives infographic can help you identify any areas that might be impacting your gut health.

Our next articles will help show you how, whether it’s by eating the rainbow or other science-backed ways to hack the gut-brain connection.

And finally, we’d like to end on the note that an unhealthy gut is just one of many possible contributors to dementia and mental illness, the causes of which are not fully understood. 

Are you looking for a step-by-step guide to improving your brain health? 

The Five Lives app can provide a personalised coaching program to keep you on track.

Take our free and medically-validated brain health assessment to estimate your risk for cognitive decline and dementia in the next 10 years, and to identify actions for change to add more life to your years.

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