What is the gut-brain axis, and why does it matter for your mood in midlife?

Your gut and your brain are in constant communication. Not metaphorically, but through direct physical pathways that affect how you feel, how clearly you think, and how resilient your mood is. In midlife, when hormones are shifting and stress is often at a peak, this connection becomes even more important. Here's what the gut-brain axis is, and what it means for your mental health.

You feel things in your gut for a reason

You have probably noticed it: the tight feeling in your stomach before something stressful, loss of appetite or feeling nauseous when you are anxious, or ‘butterflies in the stomach’ when you are excited or nervous.

Your gut and your brain are physically connected, through a network of nerves, hormones, and immune signals that run in both directions so what happens in your gut affects your brain, and what happens in your brain affects your gut.

This two-way communication system is called the gut-brain axis, and for anyone experiencing low mood, anxiety, or emotional dysregulation, it is one of the most important body systems to understand.

The vagus nerve: your body's communication highway

The main physical link between your gut and your brain is the vagus nerve (pronounced "vay-gus"), and it runs from your brainstem all the way down through your chest and into your digestive tract, carrying signals in both directions.

Around 80 to 90 percent of those signals travel upwards from the gut to the brain, so your gut is sending more information to your brain than your brain is sending to your gut. This means that the state of your digestive system is continuously influencing your mood, your stress response, and your cognitive function. So, a gut that is inflamed, dysbiotic, or under strain is not going to be sending helpful signals upwards

Where your serotonin actually comes from

You may have heard that 90 percent of serotonin is made in the gut, but it does not simply get produced in the gut and travel up to make you happier.  It’s actually a lot more complicated than that.

Serotonin is most often thought of as a brain chemical, hence why antidepressants (often known as SSRIs: selective serotonin reuptake inhibitors) are prescribed to increase it at the level of the brain. But specialised cells lining the gut wall produce serotonin in response to what is happening in the digestive system, and it this gut-made serotonin that regulates gut movement and signals mood, appetite, and wellbeing upward via the vagus nerve.

When the gut microbiome is healthy and diverse, serotonin production is supported, but when gut bacteria are out of balance (dysbiosis) this production is directly affected and is one of the clearest direct mechanisms linking gut health to mood.

Gut dysbiosis and neuroinflammation

Your gut microbiome plays a central role in regulating inflammation throughout the body, including in the brain. When the balance of gut bacteria is disrupted, the gut lining can become more permeable. Bacterial products that should stay inside the gut can cross into the bloodstream and trigger a low-grade inflammatory response. This inflammation does not stay local, but it crosses the blood-brain barrier and contributes to neuroinflammation.

Genetics also play a role here. Some people are more likely to favour a pathway called the kynurenine pathway, particularly under stress. When this happens, tryptophan, the amino acid needed to make serotonin, gets diverted away from serotonin production and towards this inflammatory pathway instead. The result is lower serotonin availability and higher neuroinflammation at the same time.

Neuroinflammation is increasingly recognised as a driver of depression, anxiety, and poor stress resilience. Its connection to gut health is one of the reasons two people with similar life circumstances can have very different mental health experiences.

Why this matters particularly in midlife

Several things converge in midlife that make the gut-brain connection more relevant.

Hormonal shifts in perimenopause affect the gut microbiome directly. Oestrogen plays a role in maintaining the diversity and balance of gut bacteria. As oestrogen declines, microbiome diversity can decrease and gut permeability can increase.

Chronic stress, which is common at this life stage, not only because of competing demands but because of the hormonal shifts themselves, which can dysregulate the stress system several times a day, also directly affects gut health. It alters gut motility, reduces microbiome diversity, and increases intestinal permeability. The gut and the stress system are tightly linked, and each makes the other worse when either is under strain.

There is also a hormonal layer that goes deeper than this. If you want to understand how your gut directly affects your oestrogen levels in midlife, and why that matters for your mood, see the companion piece on Your gut is running your hormones. Here's how.

What to do next

If you are experiencing low mood, anxiety, or fatigue alongside any digestive symptoms, the gut-brain connection is worth exploring properly. Standard GP blood tests and stool tests do not assess microbiome health or intestinal permeability, but functional stool testing can, and it often reveals things that standard testing misses.

If you would like to explore this further, get in touch.