The gut-brain axis – what is it?

We have probably all heard about this, but what exactly is the gut-brain axis, and what does it do? In short, it describes a newly discovered two-way functional link between the function of the gut and cognitive health.

We are well-aware of what happens when children don’t have a happy digestive system – they become irritable, colicky, moody and have trouble sleeping. In more serious situations, problems in the gut like infections and diarrhea can cause pain, discomfort and loss of appetite.

There is emerging evidence that this goes deeper, through relationships between slight changes in the gut microbiome and changes in behaviour and emotional development as kids get older.

How is the gut connected to the brain?

In healthy infants and children, gut function is controlled by the directly connectted nerve signals from the brain and spinal cord. In fact, the gut is often called the “second brain”, because it has such a rich supply of nerves [1]. These nerves control the movements of the gut that are involved in digestion. The same nerves also send feedback signals to the brain

The gut is also controlled indirectly by brain hormones that circulate around the bloodstream. These regulate appetite and the feeling of fullness after feeding/eating, and the cycles of sleeping and waking. Some hormones are also made locally in the liver and pancreas, to control appetite and increase the digestive process when food arrives.

Roles of the gut microbiome

The gut microbiome provides another indirect link between the gut and brain. It is known that gut bacteria can digest long chain fats to produce short chain fatty acids (SCFAs), and amino acids from protein, to produce factors such as gamma-aminobutyric acid (GABA) [2] and glutamate. They also stimulate gut cells to make serotonin (5-HT) and other byproducts, such as tryptophan. These substances act in the brain to cause a more relaxed and less stressed effect.

The gut microbiome also controls the gut immune system, and prevents gut inflammation, which is associated with an increased production of stress hormones. These have an opposite effect on the brain.

Cognitive and behavioural problems

There is mounting evidence that imbalances in the gut microbiome are related to cognitive and behavioural problems, including autism spectrum disorder [3] and attention-deficit hyperactivity disorder (ADHD) [4].

An important study of infant microbiomes in healthy infants found that there were basic profiles of bacteria, as measured in the feces, that were able to predict the cognitive abilities of the same children at 2 years of age (Carlson et al, 2018)[5]. Interestingly, those with profiles that related to cognitive function were likely to be breastfed, had a higher birth weight, and were born by caesarean section.

There is further evidence that prebiotic support can shift the profile of the microbiome to a more favourable profile. In a study that compared the effects of a GOS-based prebiotic on bacterial profiles in autistic children [6], the prebiotic use resulted in a profile that resembled that of non-Autistic children, and increased the levels of Bifidobacteria, a type of bacterium promoted by breastfeeding.

A separate study published by the same researchers <sup”>[7] also found that autistic children who were give a GOS prebiotic showed behavioural improvements, as well as a bacterial profile that was similar to the profiles reported by Carlson et al. (2018).

Taken together, these studies suggest that improving the gut microbiome may lead to improvements in behaviour, in children with behavioural problems via the gut-brain axis.

  1. Miller et al. Microb Ecol Health Dis. 2018 Nov 8;29(1):1542921.
  2. Silva et al. Front Endocrinol (Lausanne). 2020 Jan 31;11:25.
  3. Vuong & Hsiao. Biol Psychiatry. 2017 Mar 1;81(5):411-423.
  4. Checa-Ros Nutrients. 2021 Jan 16;13(1):249.
  5. Carlson et al. Biol Psychiatry. 2018 Jan 15;83(2):148-159.
  6. Grimaldi et al. FEMS Microbiol Ecol. 2017 Feb;93(2):fiw233.
  7. Grimaldi et al. Microbiome. 2018 Aug 2;6(1):133.