Depression affects up to half a million people in Ireland and causes untold problems for sufferers and their loved ones - but suppose it has actually more to do with your bowels than your brain?
Just imagine that, instead of being prescribed anti-depressants by your GP you were instead advised to go away and consume some bacteria!
Because down the road at least, that's a strong possibility.
According to Ted Dinan, Professor of Psychiatry at UCC, Neuroscience expert and author of more than 200 research papers and books, the whole problem of depression could well start in our stomachs.
It's now increasingly clear that there are certain gut bacteria which can positively influence our mood and behaviour - in other words, we are what we eat.
It's an issue which is of interest to Dinan, who previously held the Chair of Clinical Neurosciences and Professor of Psychological Medicine at St. Bartholomew's Hospital, London - prior to that, he was a Senior Lecturer in Psychiatry at Trinity College Dublin.
Dinan, who has worked in research laboratories on both sides of the Atlantic and has a PhD in Pharmacology from the University of London, is deeply interested in the connection between the gut and the brain and the implications which it could have for mental health problems.
"There are a variety of ways that the gut signals to the brain, and the brain can signal back to the gut," he says, pointing out that the gut holds an average of one kilo of bacteria - the same weight as the brain.
Here's where it all gets really interesting:
"What's important from the point of view of human health, is that these bacteria produce a lot of the chemicals that our brain needs."
In fact, these bacteria can produce hundreds, if not thousands of chemicals, a large number of which can influence the function of the brain - such as neuro-transmitters like dopamine, which is released by nerve cells to send signals to other nerve cells.
"They produce substances that we must have - but which we cannot produce ourselves, such as serotonin or 5-HT.
"These are the chemicals that anti-depressants work on," he explains.
"Basically the serotonin in our brain, which regulates our mood, is made from an amino-acid called Tryptophan," explains Dinan.
"Tryptophan is produced by bacteria in the gut - we need tryptophan in the bloodstream so that it will get to the brain and enable the production of serotonin," Dinan explains.
And we're dependent on these bacteria to produce it - because we don't make it ourselves and we don't store it either.
"This means we need a constant supply of it to monitor serotonin levels in the brain and maintain a normal mood."
The specific bacteria in the gut which are regarded as extremely good from a mental health perspective are bifidobacteria and lactobacillus .
"Babies have a lot of these bacteria, but as we get older the levels tend to drop off and that does have health implications ."
Research has found he says, that older people who are ageing in a healthy way - as opposed to an unhealthy way - will have a wide variety of bacteria in the gut, including bifidobacteria and lactobacillus .
In order to have a diverse range of bacteria, we need a diverse diet, he warns - such a diet, he explains, would regularly include everything from protein, fruit and, vegetables to good fats and carbohydrates.
Research has backed up this theory - in his laboratory Dinan has found that the consumption of certain probiotic bacteria can actually alter the chemistry of the brain.
"We have the facilities here to raise animals entirely without bacteria," he says, referring to a series of experiments he has recently carried out with mice.
"These mice experiments took place in the last year or so.
"We raised the mice with no bacteria in their gut in a germ-free facility and fed them on food that is germ-free."
"They literally have no exposure to bacteria.
"They grow up normally, but have interesting patterns of behaviour."
Mice are normally highly sociable little creatures, he points out - but their bacteria-free counterparts are very different:"If you take a germ-free mouse and give it the opportunity to interact with another mouse or with an object, the germ-free mouse will spend as much time with the object as with the mouse.
"Mice are normally very sociable, but germ-free mice don't display these sociable characteristics."
Yet, he says, if you then give these germ-free rodents bacteria, some of these abnormal behaviours will actually disappear.
"The behaviour does seem to be driven by the absence of bacteria in the intestine. Therefore, I think that what is very important is that there is nutritional diversity." Down the road Dinan believes, it's possible that, in the case of mild depression, instead of anti-depressants, doctors could be prescribing psychobiotics - probiotics which have positive mental health benefits. As yet the full scale of the connection is difficult to quantify, he says, but it's increasingly clear that the brain-gut axis is very important to mental health.
Whether psychobiotics will be capable of acting like - and in some conditions, even replacing traditional antidepressants - remains to be seen.
However, Dinan believes that in the future, psychobiotics may have a significant role to play in the ongoing battle against depression.
"It's not inconceivable in the future that in milder forms of depression and anxiety that probiotics and psychobiotics would be used and that instead of taking chemicals and anti- depressants, we would be taking bacteria," highlights Dinan, pointing out that there is already strong evidence that at least some forms of IBS can respond extremely well to treatment with probiotics.
"About 50pc of patients with IBS have anxiety and depression. There is a big link between the two, and there is very good evidence that IBS can respond well to probiotics."
Depression is a very common condition which affects more than 450,000 people of all ages, genders or backgrounds in Ireland - or one in 10 people - at any time.
According to the depression support group Aware, there is depression "with a little 'd'" which we all get as a result of hearing sad news or having a bad day.
However depression with a capital 'D' is when your energy levels are lowered and your concentration is poor as a result of this mental health condition which influences your feelings, thinking, energy and behaviour.
There are several major symptoms which include feeling sad, anxious or bored or experiencing low energy or feelings of fatigue.
Under-sleeping or over-sleeping or waking during the night is a symptom while sufferers also present with poor levels of concentration, a loss of interest in hobbies or family life and low self-esteem, or feelings of guilt. The condition can also cause aches and pains which do not have any physical basis and a loss of interest in living as well as suicidal thoughts and thinking about death.
Depression can be caused in a variety of ways - bereavement, relationship breakdown, money troubles or bullying, though some people may have a predisposition to the condition.
If you feel you may be depressed, speak to a doctor or mental health professional. For more information contact Aware, the depression support group which offers a variety of online information booklets and talks, as well as free information packs.
Contact AWARE on 01 6617211, or its support line on 1890 303 302, which is open Monday - Sunday, 10am to 10pm. Alternatively email firstname.lastname@example.org or visit www.aware.ie.
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