When we get asked which environmental factors most influence our physical health, many of us will immediately mention lifestyle, or exercise routine and diet. However, when asked about our mental health, nutrition tends to be an afterthought – despite there being evidence that the human brain uses 20 – 30% of the calories we consume each day.
Like many of us, I struggled to priortise a healthy balance in my diet at the start of lockdown – and was simultaneously intrigued by how changes to our environmental factors – particularly diet – can have such a profound role in our mood, brain health and energy levels. Fortunately, I found a short course online, Food and Mood, that promised to explore exactly that.
Like all other organs, our brain is made up of millions of cells or, more specifically, millions of neurons, all of which need energy. Neurons are cells that carry electrical signals from one area of the brain to another – as well as to the rest of our body – through the transmission of action potentials created by the movement of electrons across the membrane. This process requires energy.
Two key areas of nutritional research explore how our diets impact the extent of low-level inflammation in our brain and why the human gut has become known as our ‘second-brain’. The gut-brain axis (GBA) describes the fundamental link between our diet and brain health with much of the focus being on the role of the microbial population in our gut, our microbiome.
Inflammation is a natural process that occurs when our body fights infections or when we mend damaged tissues (ie. healing a wound or repairing muscle tissue after exercise). One of the key processes that occur during inflammation is an increase in blood flow to the site, bringing with it many immune cells which release chemicals, or cytokines, intended to fight infection and remove damaged cells. Microglia are a specialised type of immune cell that account for 10-15% of all cells in our brain and have been implicated in the development of neurological diseases and neurological and psychological health disorders, such as Alzheimer’s and major depressive disorder (MDD).
Scientists have shown that excess consumption of saturated fats encourages the production of pro-inflammatory chemicals which reduces the formation of new neural pathways, a process known as neurogenesis. Evidence also suggests that saturated fats promote destruction of the existing neuronal networks in our brain . Fortunately, there is some evidence that suggests the benefit of omega-3, a polyunsaturated fatty acid, when used as a dietary supplement alongside medication to treat depression . Omega-3 is known to be a regulator of inflammation and it’s metabolites have been shown to increase serotonin levels, in addition to down-regulating pro-inflammatory cytokines .
Another mechanism by which diet influences brain health is through consumption of antioxidants which reduces a process known as oxidative stress, in our cells. Similar to the principle of general stress, oxidative stress is caused by the overproduction of damaging molecules in our body, called free radicals.
Free radicals are unstable, intermediate chemicals that are produced in both healthy individuals and disease states. However, when the levels of free radicals in our body remain unchecked, they can cause cellular damage due to being highly reactive with other, key molecules in our cells, such as lipids, or fats. As a non-neuroscientist, I was particularly fascinated to learn that 60% of our brain is composed of fat tissue. However, this simply emphasises the impact that accumulation of free radicals on our mental and brain health.
Whilst I’ve not yet managed to sustain the 5-a-day recommendation of fruit and veg, it’s important to note that foods such as blueberries, raspberries, beetroot and aubergine rich in anthocyanins promote good brain health. Anthocyanins are antioxidants, like vitamin A, D and E. Antioxidants are molecules that provide an electron to the free radical, making it stable and therefore less destructive.
Learn more about free radicals and how antioxidants reduce oxidative stress, here (Youtube).
It’s perhaps worth mentioning that our bodies need fat (or cholesterol) from our diet to produce, or synthesise, key hormones and absorb nutrients – including vitamin A, D and E (NHS) – so, cutting out this food group should not be an approach for minimising inflammation in our body. For example, studies suggest that short-chain fatty acids (SCFAs) are key to our brain health in several ways. Firstly, SCFAs have been shown to have anti-inflammatory properties  and secondly, SCFAs are thought to encourage the conversion of tryptophan to serotonin, commonly referred to as the ‘happy hormone’.
As tryptophan is one of the essential amino acids – meaning we can only get it directly from our diet – what we eat is now recognised as a key factor to managing mood disorders and brain health. However, despite the fact that 80% of our body’s serotonin is known to be produced in the gut, studies suggest that this reservoir is unable to pass through the blood-brain barrier. Therefore, our brain must make its own serotonin, produced in serotonergic neurons located in several areas of our cortex and brainstem.
Interestingly, scientists have shown that our microbiome and fibre intake play a vital role in the production of short chain fattyacids, therefore influencing our serotonin levels and brain health. In recent years, medical researchers has become fascinated by the role of metabolites, or by-products produced by our microbiome, and how these impact in development of diseases including cancer, metabolic disorders and brain function (see The Guardian). Studies suggest that the overuse of antibiotics, in healthcare and food industry, can hinder the diversity of an individual’s microbiome and a recent study in germ-free mice suggests that a low-fibre diet may hinder recovery of the gut post-antibiotic treatment, highlighting the importance of nutrition in recovery following illness .
The influence of our microbiome in health and disease has recently become one of the hottest topics across many areas of medical research however, translating new findings into simple, daily interventions remains a challenge. Nevertheless, several clinical trials [6–7] have shown that two species of bacteria, Lactobacillus and Bifidobacterium are beneficial for overall brain health.
Thompson, O. The Second Brain: an exploration of how our gut microbiota and our brain work together (2020) Infectious Biology
Probiotics may help boost mood and cognitive function Harvard Medical School
Studies will undoubtably continue to reveal more about the intrinsic link between our gut and human health. Interestingly, scientists have already shown that our microbiome can predict how we respond to healthcare interventions, such as medication or changes to dietary intake (learn more here).
Furthermore, a recent article in New Scientist discusses the idea of precision nutrition which describes an approach that experts believe may combat the ongoing issue of nutrition advice everchanging. One study  found that “(a) strong(est) predictor of an individual’s glycaemic response (or change in glucose level) to any given meal was… microbiome composition”.
Another important factor in how our body utilises food is our circadian rhythm, but surprisingly, researchers did not see any indicators in our DNA (as found in precision medicine).
Read more about the importance of our circadian rhythm here.
Although it is tempting to encourage a ‘personalised’ approach to nutritional advice, Ayela Spiro – a public health expert – explains that “(nutritional) guidelines are important to combat public health” highlighting the impact of government decisions, relating to food policy on “a population level”. With this view in mind, it remains the case that “eating a diverse diet with fibre-rich foods, fruit, veg(etables), nuts and pulses (or lentils)” is important for all aspects of health [Sarah, Berry, King’s College London, as reported in New Scientist], not excluding brain function.
This article was edited and proofread by Olivia Thompson (@infectiousbiology).
Future Learn Course – Food and Mood: Improving Mental Health Through Diet and Nutrition, Deakin University, Australia
As reported in New Scientist –
Berry, S.E., Valdes, A.M., Drew, D.A. et al. Human postprandial responses to food and potential for precision nutrition. Nat Med 26, 964–973 (2020). https://doi.org/10.1038/s41591-020-0934-0
Davis, N. The human microbiome: why our microbes could be the key to our health (2018) The Guardian
Newman, T. Common food additive may impact gut bacteria, increase anxiety (2019) Medical News Today
Lawton, G. Why there is no such thing as a healthy diet that works for everyone (2020) New Scientist
Personalised Nutrition – Is it all in the gut? (2019) British Nutrition Foundation
The microbes that live with us from cradle to grave (2019) Nature Videos
10% Human: How Your Body’s Microbes Hold the Key to Health and Happiness – by Alanna Collen (Buy)
Please take a minute to read about and sign the National Farmers Union (NFU) petition here.