Comfort eating – the notion that certain foods will cheer us up when we are unhappy or make us better when we are ill – is familiar the world over. Popular favorites that stock our fridges and cupboards range from chicken soup to boiled eggs, and from chocolate to sweet desserts.

Now there is a growing body of scientific evidence to support the idea that what we eat – and how that alters our gut bacteria – may actually be linked to how stressed or depressed we are and how we react to the signals our brains send us.

The main subject of this research is the microbiome – a vast ecosystem of organisms such as bacteria, yeasts, fungi, viruses and protozoans that live in the human gut. Together these can weigh up to 2kg in each human and many scientists now argue the microbiome should be treated as an organ in its own right.

A gut-feeling

The elements that make up the microbiome are best known for breaking down food and extracting from it the nourishing elements our bodies need. But over the past decade research has suggested there is a connection between the gut and the brain, and that the microbiome influences a huge range of health issues, including weight, inflammation, allergies, metabolism and stress.

One of the biggest influencers of mood in the human body is a chemical called serotonin, a hormone that affects mood, anxiety and happiness. Between 80%-90% of it is created in the intestines.

There is also some evidence that gut bacteria even encourages the production of serotonin, according to Erica Sonnenburg, senior research scientist at the Stanford University School of Medicine.

Children, in particular, should not live in an overly sterile environment, and regular contact with animals, mud and everyday germs helps to ensure a healthier microbiome.

Even the very first day of a child’s life can influence its microbiome, with breastfeeding and the process of passing through the birth canal during a natural birth both making significant contributions to the gathering of gut bacteria. 

Unfortunately, drugs and antibiotics – although sometimes necessary for our health – can weaken the microbiome and decrease the amount of it in our intestines.

Transferring anxiety

Several experiments have tried to establish whether there is a link between happiness and the microbiome.

In a small study published in a peer-reviewed paper, scientists examined faecal bacteria as a proxy for the gut environment of 37 patients with major depression and compared it to people without depression.  They discovered those suffering with depression had fewer kinds of some bacteria and significantly more of others.

A separate study showed that the depression and anxiety in mice can be influenced by altering the balance of bacteria in their guts.

A further study took the gut microbiota from people with serious depression and inserted it into bacteria-free rats. These rats then went on to show behavioural changes related to depression.

But, although such research is intriguing, scientists are still far from finding sufficient clinical data to conclusively establish a connection between what we eat and mental health.

The diversity of mental illnesses, together with constantly changing baselines such as food intake and sleep, make conducting controlled experiments complicated.

The cost of depression

However, mental health disorders are estimated to cost the global economy $1 trillion a year, according to a World Health Organization study.

The same report also suggested that scaling up treatment – mainly through counselling and antidepressant medication – would cost a total of $147 billion.

Research into the links between depression and diet is still in its infancy, but early indications suggest that governments would do well to invest in exploring the connections.