The human gastrointestinal tract harbours trillions of microorganisms, consisting of up to 1,000 or so different bacterial species.

These bacteria, known collectively as the gut microbiota, perform a number of vital functions in our body. They defend against pathogens, aid in digestion and nutrient absorption, produce vitamins (K and B), and boost our immune system.

The gut microbiota also has the potential to influence our brain development and behaviour. Our gut and the central nervous system constantly communicate with each other by releasing signalling molecules. The gut microbiota is also involved in this communication process, known as the microbiota-gut–brain axis.

Our microbiota is unique

The composition of the gut microbiota is unique to each individual (even identical twins) and can be affected by many factors including diet, diseases and ageing.

Our gastrointestinal tract is nearly sterile at birth, but quickly transits to one with a diverse microbial community. The composition of this community depends on many factors, including:

• the composition of our mother’s gut microbiota
• the way we are born (vaginal or caesarean delivery)
• our early diet (including being breast of formula fed as an infant)
• early life events such as diseases and stress
• the use of antibiotics and other medications
• hygiene conditions
• the environment.

By the age of three, the gut microbiota stabilises, and its evolution continues at a steadier rate during adulthood.

How gut imbalance affects the mood

An imbalance of beneficial versus harmful gut bacteria, known as “dysbiosis”, has been linked to a number of nervous system, gastrointestinal and psychological disorders.

Exposure to early life stress – including psychological, sexual and physical abuse – can increase the risk of gastrointestinal disorders later in life. The exact reason is unknown but it may be because the establishment of stable gut microbiota is disrupted.

Stress and psychological factors can make these functional gastrointestinal disorders worse. A recent animal study showed that as little as two hours of stress was enough to change the composition of gut microbiota.

Another study showed two weeks of stress could influence the changes in the gut microbiota composition as well as induce some anxiety-related behaviour in mice. Researchers found a correlation between specific elements of anxiety-related behaviour and elements of the gut microbiota.

Research also shows that people suffering from gastrointestinal disorders are more susceptible to anxiety-related disorders such as post-traumatic stress disorder.

Although the underlying mechanism is not fully understood as yet, there is clear evidence of a connection between the microbiota, gut and brain.

Role of probiotics

Probiotics are live microorganisms that have been delivering health benefits for thousands of years by helping to establish healthy gut microbiota. Lactobacillus and Bifidobacterium are the most commonly used probiotics.

The mechanisms responsible for the beneficial impact of probiotics on health are not well known, however accumulating evidence suggests they help colonization of the gut microbiota with beneficial microorganisms.

Treatment of functional gastrointestinal disorders with probiotics has yielded mixed results to date. This may be because not all probiotics are the same. And the same probiotic strain can exert different effects in different hosts.

However, probiotic therapy holds promise for future applications both in functional gastrointestinal disorders and psychiatric illness.

Of particular interest is a recently identified class of probiotics that have been categorised as “psychobiotics” due to their possible antidepressant or qualities. These have been shown to relieve anxiety in patients suffering from functional gastrointestinal disorders.

What else we can do?

While there is an obvious connection between gut microbiota and brain, scientists have more questions than answers about this complex relationship.

What we do know is that promoting the establishment of a healthy gut microbial community in early life is the best way to harness the power of the microbiota throughout life.

Breastfeeding is one of the most important practices to establish friendly gut microbiota in early life. Breastfed infants usually have healthier and more diverse gut microbiota than formula-fed infants. Breast milk also contains healthy microbial communities and materials that stimulate the growth of beneficial gut microbes and probiotics.

We can improve the gut microbiota composition as well as prevent many diseases by simply changing our diet. Modern Western meals are high in sugar and fat, and low in fibre, which may affect the establishment of beneficial gut microbiota.

Fibre-rich foods such as vegetables, fruits and whole-grain cereals are essential to promote the growth and activity of beneficial gut microbiota.