A gutsy route to a healthy brain

Gut bacteria and gut heath are phrases we’re hearing more and more.

That’s because scientific research increasingly suggests that the bacteria in our gut play a key role in many – if not all – aspects of our health.

They are not just key to digestion; studies have shown gut bacteria affect everything from our immune system – and therefore our ability to fight off infections – to our weight, mental health and even the risk of developing certain diseases and conditions – including dementia – later in life.

What actually are gut bacteria?

The term gut bacteria refers to the bacteria that live naturally in our gut throughout our lives. You may also have heard of the gut microbiome: this term describes all the different types of microbes – bacteria, viruses and yeasts – found in the gut.

What the vast majority of us don’t realise is just how many microbes are contained within our bodies. In fact, we are comprised of more than 50% microbes and the gut is considered a ‘virtual’ organ.

If all the bacteria in the gut were compressed into an organ, it would weigh around 1.5kg - close to the average weight of the human brain.

These microbes play a crucial role in the metabolism of food and the nutrients in it.

Yet it’s not just how many bacteria we have in our gut; what's crucial is that our gut microbiome is in a state of balance – between so-called ‘good’ and ‘bad’ bacteria.

‘Good’ bacteria are vital in keeping the number and activity of the ‘bad’ bacteria under control – which keeps us in good health.

Several things such as stress, poor diet, illness, and drugs including antibiotics can knock this delicate balance out of kilter.

Having too many ‘bad’ bacteria – or not enough ‘good’ bacteria – puts us at risk of illness. Short term, for example, this could be food poisoning. Longer term, however, this might be linked to conditions such as dementia.

The link between the gut and the brain

One of the most important scientific discoveries of recent years was that gut bacteria play a crucial role in communication between the gut and the brain.

In fact, some studies have suggested that gut bacteria may even act as a ‘second brain’ and may be responsible for – or at least play a part in – the development of age-related diseases affecting the brain such as Alzheimer’s, the most common form of dementia.

Not only that, but research increasingly shows that a poor balance of gut bacteria can directly lead to issues such as depression, and anxiety.

This relationship – or communication – between the gut and the brain is often referred to as the ‘gut-brain’ axis.

The healthier and more diverse the bacteria in our gut are, the better this gut-brain communication pathway will work.

We'll explore how diet can play a role in getting a good balance of gut bacteria (you can also read about how diet affects the brain in the nutrition pillar), but first we can explore how the gut-brain axis works.

How the brain and the gut "talk" to each other

The gut and brain ‘talk’ to each other via what’s called the enteric nervous system – a sub-section of the nervous system embedded in the gut wall.

Beginning at the oesophagus (food pipe) and extending to the anus, the gut wall contains millions of nerve cells called neurons that 'talk' to each other to control all the functions of the gut, such as how food and the nutrients from it are absorbed, the speed at which it moves through the gut, the secretion of mucus to aid digestion, and the flow of blood.

The main way that the enteric nervous system communicates with the brain is via the vagus nerve, which runs between the brain and the gut. In fact, more information is sent by the gut to the brain than vice versa.

Why gut bacteria are so important

Gut bacteria not only help to break down specific nutrients in the food we eat, they also help create other communication signals (or chemical messengers) called neurotransmitters that may modify the activity of the enteric nervous system and therefore how the gut communicates with the brain.

These, in turn, can affect virtually every part of our heath, not least brain health.

For example, gut bacteria generate compounds called amino acids, as well as the neurotransmitters dopamine, GABA, and serotonin - all of which could influence vagus nerve function. They can also influence our immunity as gut cells are the main producers of the chemicals that make up the immune system.

By digesting fibre, gut bacteria also produce lots of short-chain fatty acids – the main source of energy for the cells lining the gut.

One of these, butyrate, helps to strengthen gut immunity and the strength of the gut wall – helping to keep gut bacteria where it should live.

Short-chain fatty acids also stimulate the release of gut hormones which control our appetite and feelings of fullness after eating.

How gut bacteria may affect how the brain ages

Studies have revealed that gut bacteria may have a profound effect on brain health longer term.

One area scientists are increasingly interested in is the relationship between gut bacteria and its role in the various forms of dementia.

For years, it was thought that dementia was caused by clumps (plaques) of protein, building up in the brain.

Now, however, some research suggests that these plaques may occur ‘naturally’ – and that gut problems may increase their deposition or quantity.

As a result, it is thought that perhaps the presence of good bacteria and healthy communication between gut and brain protects against plaque overload.

Research has shown damage to the brain - through the build-up of plaques - could be the result of one, or a combination of, a poor balance of ‘good’ and ‘bad’ bacteria in the gut or inflammation in the gut and elsewhere in the body.

Why ageing makes a difference

One problem is that our balance of ‘good’ bacteria can fall as we age – something confirmed by many scientific studies.

This can be due to a variety of factors such as taking various medications, constipation, illness and a change of diet.

Again, this poor balance of bacteria can have a negative effect on the brain – not least because it can lead to a leaky gut – and many harmful pathogens being able to enter the bloodstream, where they can cause irreversible damage to the brain, as described in the following section about inflammation.

One study, which collected stool samples from patients with Alzheimer’s disease and compared them with healthy people, found that the gut bacteria composition (microbiota) between the two groups was very different.

Specifically, the Alzheimer’s patients had ‘an extreme reduction of microbiota bio-diversity’ (the balance between ‘good’ and ‘bad’ bacteria).

Inflammation

A key way that inflammation may occur is through what’s known as leaky gut syndrome.

This is a digestive condition where bacteria that should remain in the gut ‘leak’ through the intestinal wall, because it has become weakened. Once in the bloodstream, the bacteria trigger inflammation which the affects the brain as well as other organs.

Because the bacteria have leaked out into an area they shouldn’t be – i.e. outside the gut – the body’s immune system is put on high alert, trying to fight these infectious pathogens.

There are suggestions that over time, this infection-fighting process can further trigger the production of plaques.

Additionally, certain molecules, called pro-inflammatory cytokines (different to anti-inflammatory cytokines), trigger the immune system to fight off infection. However, having too many of these cytokines can cause damage to cells - including those in the brain.

Therefore, having a healthy number of good bacteria in the gut helps keep these cytokines at a level that is not detrimental.

There are various ways that the intestinal wall can weaken and become ‘leaky’, or permeable, including an excess of ‘bad’ bacteria, which can be triggered by factors such as stress, too much stomach acid and taking medication. Ageing is another contributing factor.

Not only that, but bacterial components have actually been found in the brain of Alzheimer’s patients.

As a result, some scientists now firmly believe that Alzheimer’s disease may begin in the gut – and is closely related to the imbalance of ‘good’ and ‘bad’ gut bacteria.

How good bacteria can help age-proof brain

Studies have suggested that changes in the composition of bacteria in the gut – either through diet, probiotics or prebiotics – can lead to positive changes in brain function, including those associated with learning and memory, and could be a new area of prevention and treatment for Alzheimer’s.

The most sustainable way to increase the number of healthy gut bacteria is consumption of probiotic foods (foods containing beneficial microbes).

Scientific evidence shows that two of most beneficial bacteria for the gut are Lactobacilli and Bifidobacteria.

  • Lactobacillus

    One type of ‘friendly’ bacteria, Lactobacillus, can produce histamine; this chemical acts as a ‘signalling’ neurotransmitter and also helps to reduce inflammation in the brain.

  • Bifidobacterium

    Similarly, Lactobacillus and another ‘good’ bacteria called Bifidobacterium have been shown to help strengthen gut immunity and the strength of the gut wall, which studies have shown may, in turn, boost memory and learning.

    Beneficial bacteria can also trigger the release of proteins called anti-inflammatory cytokines, which decrease inflammation in the body.

Probiotics

Meanwhile other research has suggested that in people with Alzheimer’s disease, restoring gut bacteria composition to that found in healthy adults will significantly slow down the progression of neurodegeneration, by lowering the level of inflammation in the brain and body.

What is clear is that good bacteria can repair a leaky gut and decrease the level of damaging inflammation in the body and brain.

The best food sources of these are:

  • fermented vegetables such as pickles, kimchi, and sauerkraut;
  • fermented soy foods, including miso, tamari, and tempeh;
  • fermented dairy products such as kefir and yogurt.

Lactobacilli and Bifidobacteria are also the mainstays of probiotic supplements.

Prebiotics

However, what may have an even more beneficial effect are prebiotics - foods that boost the growth of the ‘good’ bacteria that are naturally in your gut - and keep them at healthy levels.

Foods with particularly high prebiotic fibre content include:

  • bamboo shoots
  • artichokes
  • bananas
  • asparagus
  • barley
  • chicory coffee
  • black pepper
  • dark chocolate
  • beetroot
  • fennel root
  • broccoli
  • endives
  • Jerusalem artichokes
  • mustard greens
  • ginger
  • onions
  • garlic
  • tomatoes
  • jicama (Mexican potato)
  • yacón (a natural sweetener)
  • leeks and legumes (especially lentils and lima beans)
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