The reasons why can be complex and equally treating these, often crippling and debilitating conditions, can be challenging - often involving a range of interventions including medication and therapies like CBT.
But one thing that is often overlooked is how what you eat and drink can significantly affect your mood and mental health.
We all accept that what we put in our body can shape our physical health yet strangely we seem less attuned to what it is doing to us psychologically.
But now an increasing body of exciting research is suggesting that cultivating more of a balance of beneficial bacteria in our gut can help lead to a boost in mood, a reduction in brain fog, a greater ability to handle stress and a reduction in anxiety and depression.
The brain-gut connection
So how are these bugs in your belly and your mind connected?
Psychologist on behalf of Healthspan and co-author of Irritable Bowel Syndrome: Navigating Your Way To Recovery Dr Megan Arroll explains: "The brain-gut axis (BGA) is a bi-directional communication system between your gut microbiome and your brain.
"When we talk about brain-gut interactions the actual brain is often referred to as 'the big brain' and the enteric nervous system in the gut as 'the small brain'.
"The big brain is the most important for balancing the digestive system, appetite and weight control but research has shown that the trillions of organisms that live in our gut (the microbiota) also have a key role to play in health.
"This is an exciting development as we can actively influence the gut environment offering potential new avenues for a range of treatments."
Hungry for more
So could the increasing numbers of us experiencing stress, anxiety and possibly depression be helped by investing in probiotic foods and supplements? Possibly.
"Although research is booming," Dr Arroll says, "we are still in the early stages. There are numerous studies using animals but still few human research studies. One involving patients with IBS and another with those living with Chronic Fatigue Syndrome showed taking probiotics did improve their psychological health."
In fact, the small 2017 study from McMaster University in Canada involving 44 adults who had IBS and moderate anxiety and depression showed that after six weeks 64 per cent of the group taking probiotics revealed reduced anxiety and depression symptoms compared to 32 per cent given a placebo.
Another trial from 2016 on 40 patients with major depressive disorder showed those given probiotic supplements over eight weeks showed a significant reduction in their depressive symptoms compared to those given a placebo.
A 2016 systematic review of five randomised controlled trials concluded it "supports the role of probiotics in reducing the risk of depression".
Various studies on healthy adults given probiotics have also shown they can boost mood and help make us better able to handle stress and anxiety.
One theory is that healthy bacteria produces key neurotransmitters, like serotonin, chemicals that regulate mood and which are said to have similar effects to some anti-depressants like selective serotonin reuptake inhibitors (SSRIs).
Serotonin is known to create feelings of optimism, confidence and general well-being.
As Dr Arroll points out additional research and larger trials are needed but what there is, is compelling and making us hungry for more.
Good mood food
What complicates the issue is that your gut microbiome is affected not just by diet but also by your external environment, your age, gender, any medications you are on, lifestyle choices (drinking, smoking), recurrent stress, lack of sleep and so on.
Gut bacteria also varies widely from one person to another so one kind of prebiotic or probiotic food or supplement that has a beneficial effect on one person may not on another.
That said, it seems trying to improve your gut bacteria through certain foods or supplements should have beneficial effects on your health generally.
Certainly research has shown that those experiencing depression who are eating too much processed food have a less diverse microbiome than those who eat a more balanced and healthy diet and as Rob Hobson, head of Nutrition for Healthspan points out, "We need a diverse diet to create diverse gut bacteria."
Nutritional diversity
Rob says a traditional Mediterranean style diet with its focus on a wide range of fruit, vegetables, nuts, seeds, dairy produce like natural Greek yogurt and some cheeses like feta, legumes, seafood and extra virgin olive oil and olives is a model of nutritional diversity.
Studies have also shown eating in this way increases beneficial gut bacteria and reduces your risk of becoming depressed.
Two separate studies both found those who ate a typical Mediterranean diet were 30 per cent less likely to experience depressive symptoms.
It is thought the combined effects of this largely plant-based diet containing anti-inflammatory omega 3 fatty acids (found in fish, nut and seeds), vitamin and mineral-rich fruit and vegetables and anti-inflammatory fatty acids and antioxidants in olive oil offer some protection against depressive illnesses.
It is suggested that chronically elevated levels of inflammation in the body and brain are one of the underlying causes of depression and other cognitive disorders.
Probiotic foods and supplements (like Healthspan Super 20 Pro) are also thought to help reduce inflammation.
Mind-enhancing probiotics
While there is, as yet, no conclusive proof that probiotic foods or supplements can target specific mental health conditions Dr Arroll says the first line of treatment should be to get medical help: "If someone is having symptoms of anxiety or stress which start to impact on their ability to do everyday activities they should speak to their GP in the first instance. However, probiotics and probiotic foods may be used as an adjunct to these treatments."
Regularly eating probiotic foods like natural live unsweetened Greek yogurt, kefir (a fermented yogurt drink, available from health food shops), some cheeses, miso, sauerkraut, sourdough bread or taking supplements (particularly after a course of antibiotics or stomach bug which can wipe out all your healthy bacteria) have been shown to help improve health generally.
Dr Arroll points out: "Research looking at students during stressful exams period found that drinking fermented milk helped prevent symptoms of stress.
"Another study looking at brain activity demonstrated reduced activation in the part of the brain that controls emotional processes in participants who consumed a fermented probiotic drink."
Supplements are also a simple way of getting more beneficial bacteria into your gut.
Significantly, probiotic use also appears safe and although, as yet, they are not a substitute for recommended or prescribed treatments for stress, anxiety and/ or depression overall the studies show the "benefits of probiotics for mental health are promising", says Dr Arroll.
