Learning Lab

The Human Body and the Human Brain – United Not Divided

By Amy Brann

The human brain is the main organ of the body tasked with controlling and regulating our thoughts, feelings and actions throughout the day.

But it doesn’t operate in isolation.

Its connections throughout the body mean that it makes more sense to think of your body and brain as a single system, rather than two separate ones. And the place where this dynamic interaction between brain and body becomes most apparent is in your gut. 

A second brain.

The gut is most commonly associated with all the biological and chemical processes which are necessary to digest the food and drink that we consume as well as helping to regulate our feelings of hunger and satiation. But more recently, scientists have realised that our gut actually does much more than just that.

When you look at it in more detail, the human gut has over 100 million neurons, an even greater number of glia, and an entire microbiome consisting of billions of bacteria, fungi, viruses, and protozoa which alone weighs 2 to 3kg – the same weight as the human brain itself. This is why humans are sometimes called “superorganisms” to reflect the fact that each one of us is carrying around thousands of different types of microorganisms on our external and internal surfaces.

It therefore quickly becomes apparent that the gut is no ordinary organ and explains why it is sometimes called our “second brain”.

So what do all these microorganisms living in our gut do? Why are they so important to the way our brain functions? And is it possible to change our brain functioning, by changing what we eat or how we live our life?

The gut and stress.

Our understanding of the gut-brain axis – as the interaction between the gut and the brain is called in the scientific literature – is still in its infancy, but researchers are starting to understand how the bidirectional pathway between the two works. For example, as well as the brain sending messages down to the gut, telling it what to do and providing appropriate feedback signals, the gut also sends out a complex array of neural, hormonal and immunological messages which can influence not only other parts of our body but also our brain.

More specifically, neuroscience studies have shown that changing the level or health of microorganisms in the gut can influence your body’s stress response – a process that is regulated by our hypothalamic-pituitary-adrenal (HPA) axis. This has led to the suggestion that altering the level of microorganisms in the gut might be one possible route for influencing mood and therefore treating stress-based disorders such as depression.

Oppositely, stress itself is thought to alter the levels of microorganisms living in your gut, illustrating the bidirectional nature of the interaction between our gut bacteria and our stress response. Therefore, it seems like it isn’t just what we eat, or the medications that we take, which can influence the health and survival of these crucial microorganisms, but also our life experiences and the level of stress that we put ourselves under. 

But how does it work? 

However, one difficulty with assessing the impact of this research is that, to date, the majority of the work in this area has been done on animals, rather than humans, and so it is still unknown whether the changes observed in these experiments translate into human behaviour and emotion. What’s more, scientists still don’t yet understand the underlying biology behind how the gut and brain interact at a cellular level, and therefore they aren’t sure whether changes in the gut, which correspond to changes in people’s mood, are causal, or purely correlational.

So there is still a way to go before we understand how our gut is influencing our brain. But for now, we know that our gut is a critical organ for the successful functioning of the rest of our body and also for our brain, especially in relation to regulating our stress levels.

Taking care of the microorganisms inside your gut, through what you eat, your lifestyle and your life experiences is, therefore, something to consider when you go about your daily life.

And it is also a reminder that brain health is not just about your brain. It is about your body too.

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