Are you familiar with the gut-brain axis? If you’ve ever felt ‘butterflies’ at the sight of a loved one, or lost your appetite when you’ve been stressed, you might be aware that your mind and stomach are connected. But the gut-brain axis is a real phenomenon, and this constant two-way communication, when out of sync, can trigger gut and other health-related issues.
By and large, the gut-brain axis is a communication system between the brain and the trillions of bacteria, fungi and viruses living within your intestines. Multiple studies have shown that the composition of gut bacteria can have a profound impact on mental health and the functioning of the nervous system. A healthy diet plays a significant role in shaping this microbiome by promoting the growth of beneficial bacteria and stopping the accumulation of harmful ones. Nutrition can also influence the communication along the gut-brain axis, further affecting the links between the gastrointestinal tract and the nervous system.
Read on to explore how different parts of the gut-brain axis work and the role of nutrition in maintaining good gut health.
What is the gut-brain axis?
The gut-brain axis refers to the constant flow of information between gut microbes and the central nervous system. This two-way communication involves multiple different pathways, as well as microbial metabolites such as short-chain fatty acids, branched chain amino acids, and peptidoglycans. It’s a highly sophisticated network that can easily be disrupted by many different factors, such as environmental irritants, stress, antibiotics and even mode of birth delivery.
The gut microbiome is a crucial part of this gut-brain connection. It develops simultaneously with the central nervous system and has a powerful influence over multiple different mental processes. Evidence (opens in new tab) suggests that dysbiosis – a term used to describe a disrupted gut microbiome – may have a significant role in many mental and neurological diseases.
During dysbiosis, the gut-brain axis pathways are dysregulated, which can make the physical barrier between the central nervous system and cardiovascular system more porous. When this blood-brain barrier is leaking, it may lead to inflammation of the brain matter. Gut-related neuroinflammation has been linked (opens in new tab) to the development of diseases such as multiple sclerosis, stroke, Alzheimer’s disease and Parkinson’s disease.
Emerging evidence (opens in new tab) also suggests that a disturbed gut-brain axis may promote weight gain through inducing changes to our metabolism, satiety control and eating behavior. In addition, a recent 2020 study (opens in new tab) demonstrated how the disrupted signaling in the gut-brain axis can create a strong preference for the taste of sugar, but not artificial sweeteners.
But how does the brain-gut axis work? First, let’s break it down into different components.
The vagus nerve
The human gut contains nearly 500 million neurons, which are connected to the brain through nerves. The vagus nerve is one of the biggest nerves connecting your gastrointestinal tract to your nervous system, and plays many important roles in your body. It has a wide-ranging impact on inflammation and the microbiota composition in the gut, yet many factors can affect how well it functions. Psychological stress, for example, has a particularly harmful effect on the vagus nerve and has been shown (opens in new tab) to be involved in the development of gastrointestinal disorders like irritable bowel syndrome and inflammatory bowel disease.
Your gut and your brain also communicate through chemicals called neurotransmitters. Neurotransmitters synthesized by the brain are involved in regulating emotions and the ‘fight or flight’ response. Recent studies (opens in new tab) have shown that these compounds may also play an important role in the gut. Neurotransmitters norepinephrine, epinephrine, dopamine, and serotonin are able to regulate and control not only blood flow, but also affect bowel movements, nutrient absorption, gastrointestinal immune system, and the microbiome.
Many neurotransmitters responsible for maintaining our mental healt, are actually produced either by the gut cells or by the gut microbes.
“The gut produces 90% of our happy hormone serotonin, 50% of our pleasure-seeking dopamine, melatonin – the sleep hormone – and oxytocin, the cuddle hormone’, says Dr Jess Braid, medical doctor and functional medicine practitioner from Audio (opens in new tab). “When the balance of organisms in our gut is wrong, it can affect our mood and our behavior.”
Gut microbes, neurotransmitters and mental disorders influence each other in a bidirectional way, which form a triangle relationship. Dysregulated neurotransmitters may contribute to the onset and progression of inflammatory bowel diseases (opens in new tab) and neurodegenerative conditions (opens in new tab) such as Alzheimer’s and Parkinson’s disease.
Chemicals produced by gut microbes
Gut microbes make a number of chemicals that affect how our brains function. Bacterial fermentation of dietary fiber is the main source of short-chain fatty acids (SCFA) such as butyrate, propionate and acetate. These compounds have been shown (opens in new tab) to prevent digestive issues and reduce the risk of developing obesity and type 2 diabetes.
Short-chain fatty acids are able to migrate across the blood-brain barrier and consequently have an impact on brain structure and function.
“SCFA butyrate plays a key role in the communication between the gut and the brain, protecting the brain against low-grade inflammation,” explains Marilia Chamon, nutritionist and founder of GutfulnessNutrition (opens in new tab). “Butyrate is also the main source of fuel for the cells lining the gut and has many health-promoting effects on the nervous system of the gastrointestinal tract.”
The gut-brain axis and mental health: what’s the link?
Dysbiosis and inflammation in the gastrointestinal tract have been linked (opens in new tab) to a multitude of mental health conditions. Emerging evidence (opens in new tab) suggests that gut microbes play a crucial role in the brain development and flow of information across the nervous system. Poor gut health may contribute to the onset and progression of depression, anxiety, schizophrenia, autism spectrum disorders, migraine, and epilepsy.
There is a significant link between the gut-brain axis and how susceptible we are to stress too. Chronic stress can trigger episodes of depression and anxiety. It’s been speculated that people who have good gut health may be more resilient to pressure than those who struggle with it. Multiple studies (opens in new tab) have also shown how early-life changes to the gut microbiota by way of antibiotic exposure, lack of breastfeeding, birth by C-section, infection, stress exposure, and other environmental influences can result in long-term alterations of stress-related physiology and behaviour.
What foods help the gut-brain axis?
Improving our dietary habits is one of the most important things we can do to benefit our gut microbes.
“For a simple idea of how to improve your gut health naturally, aim for at least 30 different plant-based foods a week, full of fiber and beneficial plant chemicals’, says Dr Megan Rossi, founder of The Gut Health Doctor (opens in new tab). “The more diversity, the better.”
However, certain nutrients are better for our gut than the others and even with a healthy balanced diet we may experience occasional deficiencies. Therefore, it’s worth knowing which foods can help our gut-brain axis the most.
Probiotics are live bacteria strains that bring about various health benefits when consumed in adequate amounts. There are lots of probiotic foods to support your gutbut if you’re not a fan of fermented foods, you can also get probiotic supplements.
Recent studies (opens in new tab) have shown that probiotics may moderate neurological and psychiatric disorders via the gut-brain axis. Strains of probiotics that affect the functioning of the central nervous system the most are often referred to as ‘psychobiotics’. Emerging evidence (opens in new tab) indicates that these so-called psychobiotics may improve cognitive functions as well as symptoms of stress, anxiety and depression.
Prebiotics are certain fractions of dietary fiber and nondigestible carbohydrates that help our good gut bacteria to grow and thrive. Prebiotic foods can include certain vegetables, fruit, wholegrains and seeds.
Prebiotics have a highly beneficial effect on our gastrointestinal tract, but they’ve also been shown to improve brain health, according to a review in Current Pharmaceutical Biotechnology (opens in new tab). Research (opens in new tab) has even suggested that these compounds may help people to recover from traumatic brain injuries and PTSD.
Omega-3 fatty acids
Omega-3 polyunsaturated fatty acids are a type of essential fat that our bodies cannot produce themselves, so we need to get them through diet. Multiple studies (opens in new tab) have shown that omega-3s have a wide-ranging impact on our cardiovascular and metabolic health due to their ability to change the structure and function of cell membranes. What’s more, they can benefit our gut health.
Polyphenols are a diverse group of phytochemicals – organic compounds that naturally occur in plants. Multiple studies (opens in new tab) have pointed towards their beneficial influence on health due to their antioxidant and anti-inflammatory properties.
Certain dietary polyphenols have the ability to impact gut health (opens in new tab) and consequently, protect against Alzheimer’s disease and Parkinson’s disease (opens in new tab). Polyphenols with neuroprotective properties can be found in many different foods, including cocoa, green tea and olive oil.