What is The Importance of The Gut-Brain-Microbiota Link in Early Adulthood?

It could be said that the same essential elements are needed for brain health as for physical health, and these include good exercise, nutrition, adequate amino acids from protein, good gut health, quality sleep, adequate water, meditation, stress reduction, creative expression and a sense of community and belonging. (1)

Early adulthood (18-25) is a key time for neurobiological development within the HPA axis, which starts at the hypothalamus in the brain. This connects to the pituitary gland in the brain and the adrenals, (which are above the kidneys), which align with certain changes also happening within the gut microbiota. This, coupled with rising mental health issues across Western societies, links to a possible gut brain-microbiota axis. In particular, studies have looked at the diversity and abundance of the gut microbiota in relation to an individual’s diet, physical activity, exercise, substance use and sleep. (1)

During puberty there are inevitable fluctuations in hormones which can influence the stress responses within the HPA axis and the development within the brain’s neurotransmitters. During this time, the brain is developing a sense of identity, self-consciousness and cognitive flexibility, all of which interlink to an individual’s state of mental wellbeing. In fact, hormones themselves can greatly activate the stress response via the HPA axis, which can impact mental health.
(2)

Linked to this is the gut microbiota, which is influenced by genetics and early-life influences such as antibiotic use, breast-feeding and birth itself. When this is coupled with an adolescent with poor dietary habits, lack of physical activity and/or substance use, lack of sleep and/or trauma, many chronic illnesses can start to form. (3)

The World Health Organisation states that mental health illnesses cause 1 in every 5 years lived with a disability and that suicide is the leading cause of death among 15-29 year olds. Because of this, and many other factors, emerging adulthood is a critical time to make long-lasting changes for better mental and physical health. (6)

Research shows that there is a strong link between what we call the gut-brain-microbiota axis, the gut-brain-neural network, the HPA axis, the gut-immune system and the neurotransmitters that help to regulate hormones and gut bacteria. Communication occurs across all of these pathways, for example, information travels across the intestinal mucosal barrier and the blood-brain barrier.
(7)

The health of the gut microbiota is important as it regulates metabolites such as beneficial short-chain fatty acids produced by gut bacteria, as well as hormones such as serotonin and gamma-aminobutyric acid. SCFA’s, for example, can be anti-inflammatory to the intestines. Stress has been shown to create changes in the gut microbiota as well behavioural and psychological processes, so increasing beneficial gut bacteria for increased levels of SCFA’s may help to rebalance some of these alterations.
(7)

In functional medicine we refer to an imbalance of the gut microbiota as a ‘dysbiosis’, and increasing evidence suggests that there is a link between gut microbiota health and brain/mental health. It has also been shown that antibiotics which can disturb the microbial balance can also cause emotional behaviours of anxiety and depression. In addition to this, dysregulation of the HPA axis, via vagus nerve communication, as well as metabolites from SCFA’s and hormone levels, can all play a role in altering intestinal motility, mucus production, gut barrier integrity and immune signalling. In fact, there is even a link between immune and inflammatory pathways that stem from gut health and emotional wellbeing.
(7)

Inflammation from the immune system that may occur due to a gut dysbiosis and intestinal permeability may even be implicated in other neurological diseases such as Parkinson’s disease, Alzheimer’s, MS, and autism. This occurs due to bi-directional signalling from the central nervous system, vagus nerve and immune/inflammatory cytokines, and so it’s important to consider both gut health and mental health when aiming to future-proof an individual’s health in the long-term. So a gut dysbiosis occurring during early adulthood or earlier could result in a cascade of future negative health effects, and the main factors at play do appear to include diet, physical exercise, substance use and sleep.
(7)

It is well known that the Western diet is associated with the excessive consumption of sugar, processed and refined carbohydrates, saturated fats and a reduction in fruits and complex carbohydrates. This Western diet is associated with poor mental health, obesity, dental issues and cardio-metabolic risks for young adults. This may be partly due to the link between diet, and a decrease in certain beneficial gut bacteria as well as an increase in certain un-beneficial gut bacteria, which are linked to metabolic diseases and neurological imbalances such as anxiety. For example, diets high in fats can trigger microbial dysbiosis, inflammation and leaky gut, which may predispose an individual to infection and further inflammation.
(8)

The Mediterranean diet, on the other hand, has been well documented as helping to increase beneficial microbes due to a higher increase in fruits, vegetables, nuts, seeds, some fish, low saturated fat, meat and dairy. The beneficial microbes that prefer this diet may in-turn create SCFA’s within the large intestine which are beneficial to colon and brain health. This is also partly due to complex fibres and probiotics which are more readily available in whole/natural foods.
(9)

Gut-brain-microbiome axis health may be correlated with poor diet, mental illness and hyperactivation of the HPA axis, which includes the increased secretion of stress hormones such as cortisol, a reduction in happy hormones such as serotonin and dopamine and the release of inflammatory cytokines. There are certain foods which may help to re-balance this complex system, and these include polyphenol-rich foods such as dark chocolate which may decrease cortisol levels, and antioxidants from omega-3 fatty acids which may help to dampen inflammation. In contrast to this, high sugar and saturated and polyunsaturated fats may impair neurogenesis, reduce BDNF levels and impact cognitive ability.
(10)

In addition to this, prebiotic and probiotic foods and supplements may improve overall health outcomes. There is increasing research around the role of Akkermansia, for example, which may help with metabolic health, mental health, metabolic-inflammatory diseases and weight gain. For a general rule of thumb, an individual at any stage of life, but particularly early adulthood should be increasing vegetables, fibre, fermented foods, omega-3 and possibly supplementing with probiotics and antioxidants such as vitamin C and D. They should also be looking to decrease high sugar, high fat and processed foods which can all lead to inflammation.
(10)

As well as looking at diet, physical exercise is well known to improve both physical and mental health. The health benefits of physical exercise in young adulthood are far- reaching and wide and to name but a few, they are: improved neuro-hormonal connections, reduced oxidative stress and autoimmune conditions, better HPA axis regulation, better lipoprotein levels, less cardiovascular risk, better mental health, less anxiety and depression and improved long-term mortality,
(9)

But what is the connection to the benefits of exercise and the gut microbiota? Well, it can increase diversity of beneficial bacteria and also increase metabolites which are the by-products of these beneficial bacteria which can help to increase our colon health. Exercise can also increase gut motility which can increase absorption of beneficial compounds, which further increases energy and overall health. That being said, intense exercise can actually be inflammatory and therefore detrimental to gut diversity, and exercise is very personalised, based on age, gender, BMI and genetics,
(9)

Also of consideration is the use of nicotine, vaping, alcohol use and cannabis use in early adulthood, which may all have effects on the permeability of the gut lining, lowering bacterial diversity and possibly increasing impulsivity, addiction and a loss of cognitive processing and attention. In particular, binge drinking may cause changes in the parts of the brain associated with learning, memory, attention and executive function. Not only this, but the use of substances may be pro-inflammatory, leading to a cycle of systemic inflammation.
(9)

And life in general in the ‘Western’ world is often correlated with high-fat diets, shift work, inconsistent eating times, jet lag, eating late at night and staying up later than is ideal. Disrupting the circadian rhythm may increase symptoms of depression and anxiety, coupled with a tendency towards seasonal affective disorder (SAD) in the winter months. SAD can have an impact on the uptake of serotonin and dysregulate the HPA axis, which when coupled with hormonal changes during puberty, shifts in the sleep/wake cycle, and lower melatonin may cause changes in mood and behaviour. This is also correlated to the gut microbiome, which has been shown to respond in different ways depending not only on type of food ingested, but times eaten,
(9)

So it can be seen that there is a complex and intrinsic link between the functions of the body, which is particularly sensitive to change during early adulthood. There also seems to be a direct link between mental health and gut health, and if an individual was able to adhere to positive lifestyle factors in early adulthood it may create more chances of a positive outcome for both mental and physical health later on in life.

So here is a list of some of the most nutritious and beneficial foods to include for better brain health:

⭐️Ensure you are eating plenty of oily fish as these contain EPA and DHA which are vital for brain function and can help to boost your mood. At least two portions per week is ideal,
(11)

⭐️Alongside this, increase your intake of green leafy vegetables such as spinach, kale, broccoli and watercress. Dark greens contain magnesium which is important for many functions of the brain, and these nutrient-dense foods also help to beat anxiety and depression due to their wide complex of nourishing nutrients and fibre which feed the good bugs for better gut health,
(11)

⭐️Don’t only eat green, eat the rainbow! This includes things such as strawberries, blueberries, sweet potatoes, and apples. Vibrant colours mean that these foods contain phytonutrients which are high in antioxidants such as vitamin C, E and K, which help to fight free radical build up in the body and in turn have beneficial effects on our brain,
(11)

⭐️Inflammation is a big driver for brain-related issues and so it makes sense to include anti-inflammatory foods such as fish, vegetables, berries and turmeric,
(11)

⭐️While I don’t advocate snacking, I would recommend the occasional handful of nuts, either with your food, or as a snack on the side. These contain beneficial nutrients such as zinc and selenium. Zinc can support our natural defence system and low zinc can lead to low serotonin, which is a risk factor for increased depression and anxiety, (11)

⭐️Whether you’re vegan or carnivore, include some beans and pulses! One cup of chickpeas, for example, provides 85% of your daily B6, which is required to make serotonin, and helps to create melatonin which regulates our body clocks. Low B6 can cause trouble concentrating, and can cause irritability, nervousness and sadness. So, adding mixed beans to a chilli will give you a boost of vital nutrients,
(11)

⭐️Animal protein does have its benefits and so I would recommend (for those that eat it) beef because it contains omega 3, iron, folate and B12. B12 is very important for producing crucial brain chemicals which regulate our mood and iron supplies the brain with red blood cells which helps with better overall function. In addition to this, eggs are a brilliant powerhouse of nutrients, for example, eggs contain choline which is needed to make neurotransmitters and cell membranes which are key for learning and memory,
(11)

⭐️Finally, enjoy a green tea and a square of dark chocolate! And green tea may improve brain function, reduce anxiety and improve memory as it contains a collection of bioactive compounds such as L thianine which works together to increase GABA, which positively influences our mood. And dark chocolate contains potassium, fibre, magnesium, zinc, iron and flavonoids. The flavonoids can increase energy, focus, mood and memory. A square or two of dark chocolate a day can increase feelings of calm and contentment and help to beat depression,
(11)

1 – Lee, J., Walton, D., O’Connor,
C., Wammes, M., Burton, J. and Osuch, E., 2022. Drugs, Guts, Brains, but Not Rock and Roll: The Need to Consider the Role of Gut Microbiota in Contemporary Mental Health and Wellness of Emerging Adults.
International
Journal of Molecular Sciences,
23(12), p.6643.

2 – Romeo, R.D.; Bellani, R.;
Karatsoreos, I.N.; Chhua, N.; Vernov, M.; Conrad, C.D.; McEwen, B.S. Stress history and pubertaldevelopment interact to shape hypothalamic-pituitary-adrenal axis plasticity.Endocrinology2006,147, 1664–1674

3 – McVey Neufeld, K.A.; Luczynski,
P.; Seira Oriach, C.; Dinan, T.G.; Cryan, J.F. What’s bugging your teen?-The microbiota andadolescent mental health.Neurosci. Biobehav. Rev.2016,70, 300–312.

4 – Cresci, G.A.; Bawden, E.
Gut Microbiome: What We Do and Don’t Know.Nutr. Clin. Pract.2015,30, 734–746

5 – Fulkerson, J.A.; Sherwood,
N.E.; Perry, C.L.; Neumark-Sztainer, D.; Story, M. Depressive symptoms and adolescent eating andhealth behaviors: A multifaceted view in a population-based sample.Prev. Med.2004,38, 865–875

6 – World Health Organization.
Mental Health. Available online: (accessed on 1 December 2021).

7 – Wang, H.X.; Wang, Y.P. Gut
Microbiota-brain Axis.Chin. Med. J. (Engl.)2016,129, 2373–2380

8 – Centre for Disease Control
and Prevention (CDC). Youth and Tobacco Use. Available online: (accessed on 6 April 2022).

9 – Teichman, E.M.; O’Riordan,
K.J.; Gahan, C.G.M.; Dinan, T.G.; Cryan, J.F. When Rhythms Meet the Blues: Circadian Interactionswith the Microbiota-Gut-Brain Axis.Cell Metab.2020,31, 448–471

10 – Galima, S.V.; Vogel, S.R.;
Kowalski, A.W. Seasonal Affective Disorder: Common Questions and Answers.Am. Fam. Physician2020,102, 668–672

11 – Ramsey, D., 2021. Eat
to Beat Depression and Anxiety : Nourish Your Way to Better Mental Health in Six Weeks.
HarperCollins Publishers.