Food & Mood – Part 2: Physiology

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This guest post was written by Katie O Callaghan – a final year dietetics student based in London with a passion for evidence-based nutrition and holistic healthcare.

Diet and nutrition influence normal physiological processes including inflammation, oxidative processes and brain function. Such processes may influence our mental health, as well as our physical health. 


The inflammatory response occurs when the body increases production of immune cells, and cytokines in order to fight infection. This is a vital process, but long-term or ‘chronic’ inflammation can be harmful to the body. 

Higher levels of inflammation may be involved in the pathogenesis of depression1.

Although more research is warranted, the Western’ diet is thought to be associated with a higher risk of systemic inflammation.

Check out part 1 of this series for more information about dietary patterns and mental health.

Diets high in refined sugar and saturated fat and low in antioxidants, fibre and poly-unsaturated fats can trigger the activation of the immune system.

This happens in response to an overproduction on pro-inflammatory cytokines, which can raise the levels of inflammation within the body.

The consumption of omega-3 fatty acids, present in oily fish and walnuts, can lead to a reduced inflammatory activity within the body2.

Cytokines increase inflammatory signalling which can alter the metabolism of neurotransmitters such as dopamine, glutamate and serotonin. This can significantly affect our mood considering these chemicals are known to be actively involved in both the development and the treatment of depression.

Cytokines send signals via neurons, these can be thought of as inflammatory ‘messages’, and can impact areas of the brain involved in mood regulation. Enzymes involved in metabolism of the amino acid tryptophan, can be negatively influenced by excessive cytokine activity. Reduced tryptophan production can in turn reduce the rate of serotonin production. 3 This well-researched pathway has been seen to promote the development of depressive disorders. More will be discussed about the effect of tryptophan on mood below.

Oxidative stress is an imbalance between antioxidants and free radicals in the body.

A low level of oxidative stress is necessary for health, for example it is needed for building muscle and maintaining a steady rate of energy production for the body.

However, an imbalance in this process can lead to a disturbance in the production of free radicals and reactive oxygen species which can fuel irregular inflammation and attack mitochondria, the energy producing components of our cells.

When this happens, the brain can be impacted and fatigue and mood disturbances can occur.

Poor quality diet and excess alcohol can contribute to the disturbance of such processes and therefore, can lead to neuropsychiatric symptoms4&5.

It is important to remember that inflammation and oxidative stress are normal processes necessary for energy production and metabolic pathways. To help keep these within healthy ranges we can eat a varied wholefood diet incorporating, fruits, vegetables and omega-3 rich foods such as oily fish, while limiting refined sugar and saturated fat.

Summary: The typical Western diet is associated with increasing oxidative stress – this can have a negative impact on mental health via changes in  inflammation levels and neurotransmitter function. Whereas, a traditional Mediteranean-style diet may help to balance inflammation levels. 

Brain Chemicals 

Diets which include a high proportion of saturated fat and sugar can have a negative effect on certain molecules which influence brain development. 

Brain-derived neurotrophic factor (BDNF) is one of these. BDNF, a chemical in the brain promoting brain development, is often lower than normal in patients with diagnosed depression and an increase in its production has been seen to reduce depressive symptoms.

As well as maintaining a healthy diet, with a low intake saturated fat and sugar, exercise has also been shown to increase BDNF. Studies show that just 45 minutes of moderate intensity exercise a week, was enough to increase BDNF and improve mood6

Tryptophan is an essential amino acid found in protein based foods such as mozzarella, soy, turkey, tofu and eggs. Unlike some amino acids, it cannot be synthesized by the body and must be obtained from food. 

Tryptophan contributes to the synthesis of the neurotransmitter serotonin- the ‘happy’ chemical which is associated with happiness and well-being7.

It does this in a rate limiting process, meaning the rate of tryptophan hydroxylation effects the rate of serotonin synthesis. Some evidence suggests that the ingested tryptophan, increases its concentration in the central nervous system and in turn stimulates serotonin synthesis in the brain8

In terms of dietary tryptophan intake influencing mood – the evidence is conflicting.

Depleted tryptophan, in those with a history of depressive illness, has been linked to low mood states and increased feelings of depression. Some studies have also shown that increasing dietary tryptophan in healthy individuals is linked to less depressive symptoms. However other research, which examined the effect of dietary tryptophan on both anxiety and depression showed no associations on tryptophan and mood 9.

Studies that do show correlations between the amino acid and mental health disorders have suggested increasing dietary sources of tryptophan as well as tryptophan supplementation could potentially lead to enhanced mood states and possibly improve sleep 9.

However there are many things to consider before commencing this and one should always seek medical advice before doing so. Some side effects of tryptophan supplementation reported in literature include drowsiness, nausea and dizziness. Such side effects are often observed when tryptophan is supplemented alongside drugs such as anti-depressants, which aim to enhance serotonin function, and so extra caution should be taken in these situations. It is important to always consult a doctor or health care professional before commencing supplements which may have contra-indications with other medications8,10 

Consumption of carbohydrates is also thought to increase the availability of tryptophan for the brain, which in turn can increase levels of serotonin11.

Carbohydrate consumption can affect blood amino acid profile and enhance the uptake of tryptophan by the brain. Insulin is released after carbohydrate intake- and while insulin does not directly affect plasma tryptophan levels, it can reduce the concentration of other amino acids which compete with tryptophan to cross the blood brain barrier. This may allow higher levels of tryptophan to enter the brain. This may explain why carbohydrate craving is observed in those with mood disturbances, premenstrual syndrome in women and emotional/stress eaters12,13.

Summary: A healthy diet which includes carbohydrates and sources of tryptophan (like: eggs, tofy, turkey and cheese) may be beneficial for mental health. However, there is no clear benefit of additional tryptophan or carbohydrates in those who already have a balanced diet. 

The Gut-Brain Axis

In the last 10 years there has been extraordinary progress in our understanding of the gut-brain-axis (GBA).

The GBA connects the cognitive and emotional centres of the brain with intestinal functions of the gut.

It is a complex system which facilitates communication between the endocrine (hormones), immune and nervous systems14,15

Our intestinal function is highly influenced by both the brain and by the millions of microorganisms who live in the gastrointestinal (GI) tract. The large intestine is an area of exceptional biodiversity with millions upon millions of microorganisms present in any individual’s gut. The microbiome is the collective term used to describe all of these microorganisms 14,15.

Disruption to this microbiome is thought to be associated with the prevalence of neuropsychiatric disturbances.

This microbiome can be disrupted by many internal and external mechanisms. For example, exposure to social stress for as little as two hours has been associated with altered gut microbiota. Various foods can also have an effect on its diversity and make up 14.

Different bacteria have the ability to produce a variety of neurotransmitters. The gut microbiota has the ability to synthesise or play a part in the synthesis of many neuro-active products. Lactobacillus, one of the most abundant bacterial genus have the ability to synthesise gamma-amino butyrate, likewise Escherichia can produce norepinephrine, serotonin and dopamine, neurotransmitters, all known to effect mood.

It is not yet clear if such metabolites reach brain receptors directly, signal from a distance or if they synthesise adequate levels to elicit a response- however the ability exist and potentially could influence mental health14, 16.

Monoamines are a type of neurotransmitter which are produced in not only neural cells, but also in the gastrointestinal system. In the past it was thought that such chemicals only served the central nervous system directly- but now it is widely believed that they also play a role within the gut which may indirectly effect the brain and cognitive processes17.

The gut microbiome plays a role in regulating serotonin by controlling concentrations of it’s precursor- tryptophan.

Bifidobacterium infantis can raise circulating tryptophan which can then impact serotonin levels18. The many theories undergoing research at present are beyond the scope of this article, but it appears we have just scratched the surface of understanding the vast complex world of these fascinating microbes which exist inside each and every one of us. 

While probiotics can be defined as live microorganisms that can confer a health benefit to the host, psychobiotics are described as the group of bacteria which when consumed in adequate amounts have positive effects on mental health18,19.

Oral probiotic consumption of such bacteria have been seen to reduce anxiety as well as reduce self-reported sad mood in adults.

One randomised controlled trial found that healthy individuals showed less cognitive reactivity to sad mood and had reduced aggressive thoughts when given a multispecies probiotic, when compared with a placebo control, over a 4 week period. Strains incorporated in the probiotic included B.Bifidum, B.Lactis, Lactobacillus Bevis and L.Casei 20

Other pre-clinical research on animals has shown associations between stress reactions and gut microbiome characteristics, however such findings have not been seen to correlate in human studies.

It is likely that gut brain interactions are host specific and preclinical evidence should be carefully interpreted and navigated.

Although still in its very early stages, current evidence suggests that targeting this fascinatingly complex system in the future could assist in the treatment of a wide range of disorders including anxiety and depression14.

Summary: Managing stress and consuming fibre rich foods can help keep our gut bacteria diverse and healthy, and in turn have positive effects on our mental health and mood. Evidence is starting to emerge in relation to the role of probiotics. 


Depression and anxiety are complex disorders with a huge range of confounding factors. However, there is some interesting evidence emerging in relation to the physiological impact of good nutrition on mental health.  

The physiological mechanisms discussed in this article support the link between the traditional Mediterranean/Nordic/Okinawan diet and positive mental health outcomes. 

Click here to read Food & Mood – Part 1: Diet Patterns & Important Nutrients

Click here to read Food & Mood – Part 3: Drinks


1 Today’s Dietitian ‘Nutrition Therapy for Mood Disorders’ [Accessed October 2019 via]

2 Giugliano, Ceriello & Esposito ‘The Effects of Diet on Inflammation: Emphasis on the Metabolic Syndrome’ [Accessed October 2019 via]

3. Glaser, Derry & Fagundes ‘Inflmmation Fans the Flames and Feasts the Heat’  [Accessed October 2019 via]

4.Akbaraly et al (2009) ‘Dietary Pattern and Depressive Symptoms in Middle Age’ [Accessed October 2019 via]

5. O’Neil et al (2014) ‘Relationships Between Diet and Mental Health in Children and Adolexcents: A Systematic Review’ [Accessed October 2019 via]

6. Kerling et al (2017) ‘Exercise increases serum brain-derived neurotrophic factor in patients with major depressive disorder’ [Accessed September 2019 via]

7. Ruhé Mason & Schene (2007) ‘Mood is indirectly related to serotonin, norepinephrine and dopamine levels in humans: a meta-analysis of monoamine depletion studies’  [Accessed October 2019 via ]

8. Fernstrom (2012) ‘Effects and Side Effects Associated with the Non-Nutritional Use of Tryptophan by Humans’ [Accessed October 2019 via]

9. Lindseth, Helland & Caspers (2015) ‘The Effects of Dietary Tryptophan on Affective Disorders’ [Accessed October 2019 via]

10. Chouinard, Young, Annable & Sourkes (1979) ‘Tryptophan‐nicotinamide, imipramine and their combination in depression’ [Accessed October 2019 via ]

11. Benton (2002) Carbohydrate ingestion, blood glucose and mood’ [Accessed September 2019 via]

12. Wurtman & Wurtman (1995) ‘Brain Serotonin, Carbohtdrate-Craving, Obesity and Depression’ [Accessed October 2019 via]

13. Wurtman et al (2003) ‘Effects of normal meals rich in carbohydrates or proteins on plasma tryptophan and tyrosine ratios’ [Accessed October 2019 via]

14. Martin, Osadchiy, Kalani & Mayer (2018) ‘The Brain-Gut-Microbiome Axis’ [Accessed October 2019 via]

15. Psyche Scene Hub (2018) ‘The Simplified Guide to the Gut-Brain Axis – How the Gut and The Brain Talk to Each Other’ [Accessed October 2019 via

16. Galland (2014) ‘The Gut Microbiome and The Brain’ [Accessed October 2019 via]

17. Mittal et al (2017) ‘Neurotransmitters: The critical modulators regulating gut-brain axis’ [Accessed October 2019 via]

18. Dinan & Cryan (2017) ‘Brain-Gut-Microbiota Axis and Mental Health’ [Accessed October 2019 via]

19. Food and Agriculture Organization of the United Nations and World Health Organization (2006) ‘Health and nutritional properties of probiotics in food, including powder milk with live lactic acid bacteria’ [Accessed October 2019 via management/en/probiotics.pdf]

20. Steenbergen et al (2015) ‘A randomized controlled trial to test the effect of multispecies probiotics on cognitive reactivity to sad mood’ [Accessed September 2019 via]


Maeve has been consulting on The Food Medic Educational Hub for 12 months now and has been a huge asset to the team. Her ability to translate some very nuanced topics in nutrition into easy-to-follow, informative articles and infographics is really admirable.

Dr Hazel Wallace

Founder of The Food Medic

Maeve is incredibly talented at sharing scientific information in an easy to understand way. The content she shares with us is always really interesting, clear, and of very high quality. She’s one of our favourite writers to work with!

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Senior UX Writer at Thriva Health

Maeve has written extensively for NHD magazine over the last few years, producing a wealth of dietetic and nutritional articles. Always evidence based and factual, Maeve creates material that is relevant and very readable. She provides high quality work with a professional and friendly approach. Maeve is a beacon of high quality knowledge and work within the nutrition writing community; and someone NHD magazine is proud to work with.

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