The latest government statistics, released last week, show that from 2001-2016, the rate of cardiac events (heart attacks or unstable angina) fell by more than half among Australian women.
That’s largely because of greater education about risk factors for heart disease (smoking rates continue to fall), and medical advances in prevention and treatment.
One thing that might reduce rates of heart disease even further is to make sure women, in particular, are asked about their current mental health. This can be a pointer to a hidden risk of developing heart disease in the future.
Mental illness can directly affect heart health by placing extra pressure on the cardiovascular system. Depression has been linked to inflammation, which can clog a person’s arteries. Depression also increases the presence of stress hormones in the body, which dull the response of the heart and arteries to demands for increased blood flow.
Less direct effects on heart health include the impact of depression on a person’s health behaviours, such as diet and exercise, and their connections with other people.
We’ve shown Australian middle-aged women with depression have double the risk of having a heart attack or stroke in the following 18 years compared to women without depression.
Preventing heart disease
Cardiovascular diseases including stroke, coronary heart disease, and heart failure remain the number one killer of Australian women. In 2016, three in ten deaths were due to heart disease. Indigenous women are twice as likely as non-Indigenous women to die from this cause.
While we’re seeing significant reductions in the number of people getting heart disease overall, the latest report shows the opposite is true in young women. The rate of cardiovascular events like stroke is increasing in women aged 35 to 54.
Drinking alcohol, smoking, high cholesterol, type 2 diabetes, overweight/obesity, and a family history of heart disease are some of the important predictors of a person developing heart disease over the next five years.
So if someone is considered to have high risk of a cardiovascular event, this risk can be managed with the help of a medical professional.
April 1 saw the introduction of two new Medicare item numbers allowing eligible patients (those aged 45 and over, or 35 and over for Aboriginal and Torres Strait Islander peoples) to be assessed for their risk of developing cardiovascular disease. This is known as a heart health check.
Using the Australian Risk Calculator, the doctor collects information to assess a patient’s risk of experiencing a cardiovascular event in the next five years.
If a person is identified as being sufficiently at risk, they will be targeted with preventative measures such as assistance with lifestyle modifications, and/or interventions like blood pressure or cholesterol medications.
Women have some unique risk factors
While many of the common risk factors for heart disease are shared between women and men, young and middle aged women have some that men don’t.
Polycystic ovary syndrome and complications during and after pregnancy (such as gestational diabetes and pre-eclampsia) are all important considerations.
We’re only beginning to understand how these factors affect a woman’s risk, but they are likely to be as important as traditional risk factors in the context of heart health checks.
Another common issue in young women that influences heart disease risk is poor mental health. Common mental disorders like depression are more common in women than men until age 75.
Both heart disease and depression are largely socially determined, especially for women and girls. Early life trauma, poverty, and gendered violence and discrimination can accumulate across a woman’s lifespan to shape her risk of heart disease and stroke.
Screening for mental health
We did some statistical modelling to see whether depression should be added to the risk equation that underpins the heart health checks.
For women who reported depression in this context, we were able to more accurately predict whether they’d go on to develop heart disease over the next ten years.
While more research is needed, asking about a woman’s mental health may help GPs better identify risk of heart disease in younger women.
Large population-based studies show reducing the prevalence of depression could have major implications for the prevention of heart disease and stroke. One study found having a poor psychosocial profile (depression, stress, isolation and anxiety) contributes 32% of the risk for heart attacks across the population.
In other words, if these psychosocial issues were eliminated, the incidence of heart attacks would be reduced by one-third.
Given the burden of these psychosocial issues is greater for women than men, women may have even more to gain if depression was targeted as part of preventing heart disease.
How can we address depression as a risk factor?
The heart health checks represent a significant step in the government’s investment in preventive medicine and public health.
While time poor clinicians can’t be expected to capture an infinite number of risk factors in a short consultation, these sessions may present a good opportunity for GPs to ask their patients about their mental health in the context of their heart disease risk. Equally, this may be a good time for patients to flag any concerns about their mental health with their GP.
There are few studies yet that definitively show treating depression will prevent a first heart attack or a recurrent event.
This is especially true for women, who are under-represented in this type of research. One study, where participants were given psychological therapy after a cardiac event like a heart attack, found the intervention benefited future heart health outcomes for “white men, but not other subgroups”.
We need more research to tell us if and how treating depression might prevent heart disease and stroke, especially for women. In the meantime, there are many free or subsidised options for the management of depression available either via your GP (psychologists and counsellors) or online.
Associate Professor O’Neil is Deputy Director of The Food & Mood Centre and Director of the Heart and Mind research program. She is also a National Heart Foundation Future Leader Fellow.
This article was first published in The Conversation at https://theconversation.com/for-womens-sake-lets-screen-for-depression-as-part-of-the-new-heart-health-checks-118910 on June 24th, 2019
By Dr Sarah R Dash
You likely know inflammation as the redness and swelling that appears after you cut your finger – it’s painful, but its job is to help us heal. Systemic (or chronic) inflammation is different. We can’t see this low-level autoimmune response, and it’s usually a prolonged reaction to a persistent problem, like chronic disease or environmental stressors (think pollution or smoking). Instead of a quick response to an obvious injury or illness, systemic inflammation is a slower, continuous burn, like a cindering fire. Chronic inflammation can damage and disrupt biological systems and tissues, and this increases risk of illness in both the body and brain. While we cannot control all factors that influence our health, there are several lifestyle behaviours over which we have a bit more say, such as our diets, that influence levels of inflammation.
There are many foods and nutrients that are particularly good at ‘tidying up’ the inflammation in our bodies – you’ve perhaps heard them advertised as antioxidants or ‘super foods’. While there’s no quick fix as promised on a punnet of blueberries, there are many foods that can and should be incorporated as part of a healthy overall diet. For example, fish, rich in omega 3s, and vegetables and whole grain cereals, packed with fibre, are known for their anti-inflammatory properties. When it comes to inflammation, it’s best to choose foods that come from a farm rather than a package.
On the other hand, unhealthy foods (ie fast foods or lollies) can increase inflammation, possibly through influencing the health of our gut. The gut is an important interface between what we eat and our other biological systems. A junk food diet can contribute to a weakening of the gut barrier lining that prevents food particles from leaking out into the bloodstream where they do not belong. Because these food particles are out of place in the bloodstream, off go the alarm bells of our inflammatory response. When poor quality foods are a part of our daily diet, the body maintains this low level of alarm, and we believe this to be a risk factor for mental health problems such as depression.
Chronic inflammation is linked to brain function and mood. In animals and humans, injection of ‘pro-inflammatory’ molecules gives rise to symptoms and behaviours like fatigue, withdrawal, and depression itself. Further, people with higher levels of inflammatory markers in their blood have been shown to be more likely to develop depression in the future.
We are learning more each day about the connection between the gut, inflammation and brain health. What goes on in the gut, as well as the inflammatory response that might follow, can disrupt the processes in the brain that help regulate our mood. For the health of your belly, body and brain, aim to avoid heavily processed foods, and instead rely on a range of colourful, fibre rich foods in your daily diets.
Diet is now a recognised modifiable factor in the prevention and treatment of mental illness. In fact, the recently updated Australian and New Zealand College of Psychiatrists’ clinical practice guidelines for mood disorders included addressing lifestyle factors, including supporting a healthy diet, as a foundation for the treatment of mood disorders, describing this step as “essentially non-negotiable”. These new clinical guidelines are the first in the world to recognise the importance of diet to mental health and they represent a critical step in translating our Food and Mood Centre research into widespread practice.
A recent paper published by the team at the Food and Mood Centre, with an international group of researchers with expertise in nutritional psychiatry, reviewed the many different mechanisms that may drive the beneficial effect of diet on mental and brain health – depression in particular. These many mechanisms are complex and interacting, and not the result of only one biological pathway. They include immune system dysfunction (inflammation), oxidative stress, physical changes to the brain (“brain plasticity”), alterations to stress hormones, the gut microbiota, cellular changes implicated in depressive symptoms, metabolism of specific proteins, and early life nutrition.
Inflammation, which is an activation of the immune system, can arise as a result of many factors present in our lives. For example, lack of exercise, stress, poor sleep, smoking and unhealthy diet. Short-term inflammation that arises as a result of the body’s response to injury or infection can be protective; however, long-term, low level inflammation can contribute to the development of many health conditions, including depression. Diet is a key factor that can influence inflammation, with anti-inflammatory diets, such as the Mediterranean Diet, being consistently associated with a reduced risk of depression and successfully treating even severe clinical depression.
Studies have also shown those with severe mental illness have higher amounts of pro-inflammatory foods, such as ultra-processed foods, refined grains, saturated and trans fats, and added sugar. At the same time, they also have lower intakes of anti-inflammatory nutrients, including several vitamins and minerals, prebiotic fibres, probiotics and omega-3 fatty acids.
Oxidative stress is the imbalance of free radicals and antioxidants in your body, which can result in damage to our cells. It has been implicated in the development of many diseases, including cancer, heart disease and diabetes, as well as depression and other mental health disorders. Studies have shown that people with depression have elevated oxidative stress markers compared to healthy individuals, and other research has shown these stress markers may decrease after treatment with anti-depressants. Diet can influence oxidative stress through either depriving or increasing the supply of nutrients with antioxidant properties, with healthy diets associated with reduced markers of oxidative stress, and a Western diet linked with increased markers, directly in animal studies and indirectly in human studies.
The hippocampus is a region in the brain central to learning, memory, mood and mental health. This region of the brain can change size, in a process known as brain plasticity. A key factor that determines the growth of new neurons, and therefore brain plasticity, is a protein called brain-derived neurotrophic factor (BDNF). Interestingly, lower levels of BDNF have been found in individuals with depression, and unhealthy diets are linked to a smaller hippocampus. Diet has been shown to affect BDNF, with nutrients such as omega 3s and polyphenols (beneficial plant chemicals) having a beneficial effect, while diets high in fat and sugar impair brain cell growth, reduce BDNF levels and affect memory and learning.
The hypothalamic–pituitary–adrenal (HPA) axis, comprising the brain (hypothalamus), pituitary and adrenal glands, has been implicated in the cause of anxiety and depressive disorders. Inflammation is associated with a disruption to the HPA axis, which can result in higher levels of stress hormones, including cortisol. In fact, more than 60% of people with depression show excessive cortisol production. Diet can influence the HPA axis with certain nutrients, including vitamin C, omega 3 fatty acids and polyphenols, showing positive effects on cortisol levels, while high glycaemic index (GI) diets have been linked with small increases in cortisol.
There is growing evidence that the gut and its resident microbiota (the “microbiota-gut-brain axis”) is critical for cognitive function and mental health. Multiple direct and indirect pathways have been suggested by which the gut can influence brain function and behaviour. Gut bacteria can regulate the production of neurotransmitters important in mood such as serotonin, as well as brain proteins that influence the growth of new brain cells, including BDNF. Beneficial products of fibre fermentation in the gut, known as short-chain fatty acids (SCFAs), can also assist with maintaining the gut lining, and without adequate SCFAs, the lining can become compromised, contributing to inflammation. Our gut microbes are also involved in the functioning of the HPA axis.
Depression is associated with dysfunction of the mitochondria, a component of cells involved in energy production, explaining how symptoms experienced by individuals with depression commonly include fatigue and other cognitive complaints. Evidence suggests that a poor diet may contribute to mitochondrial dysfunction, with dietary components characteristic of a Western-style diet including a high consumption of saturated and trans fats, refined carbohydrates and salt, being linked to abnormal mitochondrial production in animal studies.
Tryptophan is an amino acid (the building blocks of protein) that must be obtained through the diet. You may know that tryptophan is needed to make serotonin, an important neurotransmitter related to mood. The microbes in our gut influence the amount of serotonin in our brains by influencing the metabolisms of tryptophan. However, most tryptophan is metabolised to create a whole set of metabolites that are either neurotoxic or neuroprotective and these metabolites are involved in depression and other mental disorders. Compounds in our diet such as polyphenols, as well as beneficial bacteria found in probiotics and fermented foods, help influence this pathway and steer it towards a neuroprotective set of metabolites.
Epigenetics describes changes to the way that genes function without altering the genetic sequence, resulting from environmental factors such as diet. Nutrition during pregnancy, early life and adolescence can influence epigenetics and is becoming well recognised as having important effects on mental health.
Along with the discussed mechanisms, there are many other potential pathways behind how diet influences depression. As much of the research in this field consists of animal studies, more human research is still needed. This will continue to support the development of dietary interventions for the prevention and treatment of mental disorders.
If you’re wondering what diet is best for mental health, the good news is that there’s no one specific diet to optimise your mental health. While the Mediterranean Diet has been the subject of recent studies showing it can treat even severe clinical depression, research from all across the world, in different countries, cultures and populations, have shown that following a healthy, traditional diet, whether that be Mediterranean, Scandinavian, or Japanese, is associated with a reduced risk of depression and anxiety. Although these diets may seem quite different, they actually have many things in common. They generally include plenty of wholefoods, plant foods (wholegrains, fruits, vegetables and legumes), and healthy fats coming from fish, nuts, seeds and olives. If you’re wanting to learn more, The Food and Mood Centre have a free online course about improving mental health through diet and nutrition, which you can register for here and be notified when it is next available.
Sophie is a Research Assistant at the Food & Mood Centre, working on the TANDIM project. She is an Accredited Practising Dietitian with research interests in Irritable Bowel Syndrome and common mental disorders.
Close to half of the calories eaten by Australians each day come from ultra-processed foods. New research from our team at the Food & Mood Centre has linked these foods to poor health and early death. But what are ultra-processed foods and why should we pay more attention to them for the sake of our health?
What are ultra-processed foods?
In 2009, a food classification system known as ‘NOVA’ defined ultra-processed foods as “industrial formulations generated through compounds extracted, derived or synthesised from food or food substrates, containing five or more ingredients and artificial additives, with no wholefood components”.
Put simply, these foods are made in factories, are highly refined and contain little, if any, intact food.
Ultra-processed foods are also known for containing high amounts of nutrients such as sugar, saturated fat and salt in addition to excess energy or calories. And on the flip side, they’re usually low in protein, fibre and essential vitamins and minerals.
In Australia, 42% of our calories come from ultra-processed foods. The most common culprits are:
- Highly refined industrialised and mass-produced breads, including white bread with many unrecognisable ingredients
- Frozen and shelf-stable ready-to-heat or ready-to-eat meals, including frozen pasta, pies and nuggets
- Fast food dishes, including hamburgers, pizza and French fries from fast food outlets
These are closely followed by pastries, buns and cakes, breakfast cereals, biscuits, fruit drinks and iced teas, and confectionary.
According to NOVA, and to really put things in perspective, ultra-processed foods differ from other terms you may be familiar with (e.g. processed foods) in that they require sophisticated technology, equipment, manufacturing and ingredients in order to create highly palatable, convenient, profitable and mass-produced goods.
On the other hand, processed foods are characterised by NOVA as using salt, oil or sugar and more traditional preserving methods (e.g. bottling and fermentation) to increase the durability of food.
For example, mass-produced bread made from complex ingredients never or rarely used in kitchens including emulsifiers, preservatives and colourings is regarded as ultra-processed, but bread made from simple ingredients such as flour, water, salt and yeast is typically considered processed.
Why should we care about ultra-processed foods?
The consumption of ultra-processed foods has markedly increased in recent years and that’s probably because we’re surrounded by them and exposed to considerable marketing strategies.
Ultra-processed foods may also be perceived as relatively cheap, tasty and convenient as well as having the potential to prevent food waste (because of their extended shelf-life) compared to healthier foods, especially fresh produce. All of these factors could certainly be seen as beneficial attributes.
But concerns arise when ultra-processed foods replace a significant proportion of freshly prepared dishes made from unprocessed or minimally processed wholefoods, which are important for our health and wellbeing.
What can happen when a significant proportion of what we eat comes from ultra-processed foods?
Our recently published systematic review and meta-analysis investigated the link between ultra-processed food consumption and chronic diseases as well as mortality. It included 43 studies and a combined sample size of close to 900,000 children, adolescent and adult populations across the globe.
When we combined or synthesised the results from several studies with the same outcome, we found that eating higher amounts of ultra-processed food was associated with greater likelihood of overweight, obesity, abdominal obesity and metabolic syndrome in adults as well as some respiratory conditions in adolescents. Higher consumption was also linked with a greater risk for the development of depression and all-cause mortality in adults.
When we systematically reviewed the literature, we also found that eating more ultra-processed food was associated with higher risk of type two diabetes mellitus, frailty, irritable bowel syndrome, cardiovascular diseases (and some but not all associated risk factors), breast cancer and overall cancer in adults. Higher consumption was also associated with metabolic syndrome in adolescents and dyslipidaemia in children.
What can and can’t current research tell us?
Although our study showed that the more ultra-processed foods we eat, the greater our risk of various chronic physical diseases, mental health disorders and mortality, it was limited to including only observational studies.
So, for example, we can’t tell whether common mental disorders such as depression cause us to eat more ultra-processed foods, or whether ultra-processed foods cause the development of depression.
There has only been one randomised, controlled trial to date that has assessed the cause-effect relationship between ultra-processed food consumption and health outcomes. It is important to emphasise here that this type of study (i.e. a randomised, controlled trial) is considered the crème de la crème of clinical research, especially in the nutrition space where it is often difficult to fully capture what people are eating versus what they say they are eating.
And although we’ve mentioned ultra-processed foods typically contain adverse levels of certain nutrients (i.e. high sugar, fat and salt, with low protein, fibre, vitamins and minerals), this study very cleverly matched the levels of nutrients across both the ultra-processed diet and its comparator — an unprocessed or minimally processed diet.
The ultra-processed diet in this study also didn’t rely on foods and beverages such as chips, lollies, pizza and sodas etc. (as you might’ve imagined), it was mostly made-up of inconspicuous foods that we as Australians eat every day (e.g. industrialised and mass-produced packaged breads as well as frozen and shelf-stable ready-to-heat or ready-to-eat meals, such as beef ravioli pasta).
While the possible underlying mechanisms for how and why ultra-processed foods impact us in this way are still uncertain, the matching of nutrients across both diet types suggests that something other than the nutrient profiles of ultra-processed foods may be driving their effects.
More research is undoubtably needed before conclusions are drawn, but the current state of play suggests that limiting our intake of ultra-processed foods and replacing them with unprocessed or minimally processed foods (including, for example, foods from traditional diets such as the Mediterranean diet) may reduce our risk for chronic diseases and mortality.
So how can we identify ultra-processed foods when shopping?
Ultra-processed foods can be tricky to pick, but briefly, they usually come in packages and contain ingredients that you wouldn’t typically use in home-cooking. And as previously mentioned, many are high in sugar, saturated fat, salt and calories, while being low in protein, fibre and essential vitamins and minerals.
For these reasons, looking at the ingredients list on packages can be useful. A few simple label-reading tips to choose healthier foods and drinks can be located here.
If you find five or more ingredients – as well as ingredients you don’t recognise (e.g. high-fructose corn syrup, hydrogenated or interesterified oils and hydrolysed proteins as well as artificial food additives expressed as numbers, alphanumeric codes or chemical compounds) – chances are you’re dealing with an ultra-processed food.
Ultra-processed foods may also be tricky to pick because depending on how they’ve been made, the same type of food could be considered 1) unprocessed / minimally processed, 2) processed or 3) ultra-processed.
The transformation of whole oranges into homemade juice versus dried or dehydrated preserved orange slices versus store-bought orange juice may provide a clear example of applying these categories to same type of food (see below).
1) unprocessed / minimally processed = whole oranges are peeled and separated into segments or freshly squeezed for consumption
2) processed = whole oranges are sliced, excess juice is removed, and slices are placed in an oven or dehydrator to dry for preservation and later consumption
3) ultra-processed = whole oranges are washed and squeezed by a machine where pulp, oils and oxygen are removed (removal of oxygen helps to prevent oxidative damage to vitamin C during storage). The juice is then heated or pasteurised to inactivate enzymes and kill potentially harmful microbes, and then artificial food additives including preservatives, colours and sweeteners are added. This is the type of orange juice we can buy from stores. And this kind of intensive manufacturing means store-bought juice can be kept in large tanks for up to a year before packaging and reaching our shopping trolleys.
What can I do to reduce my intake of UPFs?
Here are our top simple tips to help reduce your intake of ultra-processed foods:
- Where possible, opt for wholefood items and ingredients in place of ultra-processed foods by shopping in the vegetable/fruit/meat/dairy sections rather than the middle isles of the supermarket where most ultra-processed foods are located.
- Accessing cheap and sustainable healthy foods doesn’t always come easy! If fresh food is not available or affordable, even looking for less processed alternatives with a higher nutrient content can be beneficial. For example, instead of selecting white bread, select whole-wheat bread with minimal ingredients. Comparing the salt content of various breads in the supermarket and checking for the presence of emulsifiers can be helpful tricks. You could also swap flavoured yoghurt containing additives for plain or natural yoghurt. And for an extra boost of nutrients and sweetness, top with your favourite fresh or frozen fruits.
- In many cases, frozen and canned vegetables are nutritious and affordable options. If opting for canned or tinned varieties (e.g. canned tomatoes, tinned corn), take care to select products that are ‘reduced salt’ or have minimal or no salt and sugar added.
- Another way to increase your intake of healthier foods and nutrients, with the added benefit of decreased prices (and even tastier fresh foods!), is to buy and eat fruits and vegetables that are in season.
- For some recipe ideas from the Food & Mood Centre team and for extra information about our research and up-to-date findings in the nutrition space, click the link here.
Melissa is a PhD candidate within Deakin University’s Food & Mood Centre. She holds a Bachelor of Nursing (Pre-registration), Graduate Diploma in Psychology and Bachelor of Psychological Science (Honours). Meghan Hockey is an Accredited Practising Dietitian and a PhD candidate with the Food and Mood Centre. Dr Wolf Marx is an Accredited Practising Dietitian and Alfred Deakin Postdoctoral Research Fellow at the Food and Mood Centre.
Dietary Research Trial
for individuals with irritable bowel syndrome (IBS)
who also have depression or anxiety
This study aims to evaluate the impact of a specific diet in people with IBS and current symptoms of anxiety or depression.
This diet is associated with a number of health benefits, but it has never been tested in irritable bowel syndrome (IBS). We are wanting to see if it is feasible and acceptable for people with IBS.
AM I ELIGIBLE?
You may be eligible to participate if you fit the following criteria:
✓ 18-65 years old
✓ Have irritable bowel syndrome
✓ Current anxiety or depressive symptoms
✓ Not pregnant or lactating
✓ Willing to follow the study diet for 6 weeks
✓ Colonoscopy within the last 3 years ONLY if aged over 40 years
WHAT IS INVOLVED
• You will be asked to follow the study diet or your usual diet for 6 weeks
• If you are allocated to the intervention group, you will be provided with 1:1 advice by a dietitian and given a small food hamper. If you are allocated to the control group, dietary advice and a food hamper will be offered at study completion.
• You will need to provide a stool sample on two occasions
• This study will be conducted entirely online with a video call at week 1 and week 6, along with weekly support phone calls to answer any questions or concerns you may have.
WANT TO KNOW MORE?
To find out more information please contact the research team on (03) 5227 8622 or email: email@example.com
This study is a joint venture undertaken by researchers at Deakin University and Barwon Health.
The Barwon Health Human Research Ethics Committee has approved this study (project number: 65566).
The Deakin University Human Research Ethics Committee has approved this study (project number: 2020-336).
When it comes to health, we’re all after a quick fix, but new research shows neither popping a prebiotic or probiotic pill, or consuming fermented foods, has an effect on boosting our brain power. So, does this mean you should throw out those pills and kefir for good? Not quite.
Cognition – the scientific term for brain power – refers to the mental processes of gaining knowledge and understanding, including thinking, knowing, remembering, judging and problem solving. Research into the role of diet on cognition is rapidly growing with studies showing that certain diets (i.e. the Mediterranean diet) could have positive benefits for cognition, in particular for the elderly. Although it’s unclear how foods may benefit cognition, researchers have a gut feeling the answer may lie in the microbiota-gut-brain axis – the communication pathway between the bacteria (or bugs) that live within our gut and the brain.
For this reason, it’s thought that supplements and foods that have the potential to change the composition of the gut microbiota, may in-turn have benefits for cognition. This includes probiotics, which deliver beneficial strains of bacteria to influence the gut microbiota composition and activity, and prebiotics, which can be thought of as food or substrates used by the microbiota to grow. Both prebiotics and probiotics can be consumed in supplement form or via foods such as fermented foods (i.e. probiotic milks, kefir and kimchi). As suggested, the consumption of these products can alter the composition and function of the gut microbiota. In turn, this can lead to changes in how the gut interacts with the brain – via the immune system, vagus nerve and neurotransmitters – which could lead to potential changes in how we think and feel.
But findings from our new meta-analysis, suggest there was no significant effect for prebiotic, probiotic or fermented food interventions for global cognition. The meta-analysis published in Neuroscience and Biobehavioural Reviews included 22 randomised controlled trials that investigated probiotic (11 studies), prebiotic (5 studies) and fermented foods (6 studies) interventions. Fourteen (64%) of the included studies within this meta-analysis reported that a prebiotic, probiotic or fermented food intervention improved at least one cognitive measure. But when the results of the included studies were pooled (or combined), these findings were no longer significant which meant there was no apparent evidence of an effect of prebiotics, probiotics or fermented foods on overall cognition.
So why were the overall results no longer significant? Many of the included studies were in healthy populations where cognitive changes may be unlikely to be seen: Put simply, if your petrol tank is already full, adding in more petrol is not going to make your car travel any further. For probiotic interventions, studies also varied in the strain and dose of bacteria used (known as colony forming units). Different strains have different properties and we don’t yet know what the optimal strain for cognition is (if indeed these probiotics are able to colonise the gut in the first place – but that’s for another time!). Studies were also relatively small in sample size, which means there may not have been enough participants to detect a statistically significant difference, if one existed. And lastly, adding to the complexity, studies were also relatively short in duration which begs the question: Does it just take time for prebiotics, probiotics or fermented foods to work their magic?
In short, these results may not reflect a lack of an effect, but rather a lack of robustly designed studies investigating the effects of prebiotics, probiotics or fermented foods on cognition. Before we write them off as a health fad, there are many more areas of research we need to explore including what strains are best, what dose may be optimal and who (or which populations) may benefit from these interventions. Interestingly, other meta-analyses do show promise for other brain outcomes like depression. But from what we currently know, feeding your gut bugs through prebiotics, probiotics and fermented foods, is unlikely to be a magic fix to boost your cognition.