ADHD & gut health

The gut-brain axis has had extensive research into it in the last few years and the understanding that gut health is critical for brain health is no longer disputed. 

The power of the gut microbiome

The human gut contains around 100 trillion microorganisms, including bacteria, viruses, fungi, archaea and single-celled eukaryotes such as yeasts. It is estimated that there are as many as 5,000 different species – collectively weighing approximately 2 kilograms in our gut - and we are quite literally their host!¹¹ Although scientists have unravelled some of the mysteries of our gut microbial community, there remains much to discover.

The microbiome plays a significant role in neurotransmitter functions.⁹ Increasing research is concerned with the links - either directly or indirectly - between gut microbiota and neurodevelopment conditions such as ADHD and Autism. Aside from its function within the gut, the microbiome impacts brain development and the neuroendocrine system. The critical stage of gut microbiome development occurs in utero and at the time of birth and the microbiome of the infant is based on the microbiome of the mother.    

Image source: Current Evidence on the Role of the Gut Microbiome in ADHD Pathophysiology and Therapeutic Implications

The nervous system connection to the gut is through the hepatic and coeliac branches of the vagus nerve, a nerve with a huge role in mood regulation and anxiety and depression. The nerves are stimulated by bacterial by-products, gut hormones and neurotransmitter, mostly of which are created in the gut. A study looking at campylobacter jejuni and citrobacter amalonaticus found that these two bacteria induced anxiety. Conversely, research has found that supplementation with lactobacillus rhamnosus and bifidobacterium longum alleviated anxiety and depression. The gut microbiome also influences dopamine levels in the frontal cortex (the part of the brain responsible for executive functions) and can influence neuropsychiatric disorders like ADHD.²⁶  

The kynurenine pathway is another big factor in the link between gut health and neuropsychology. Kynurenine is produced from tryptophan by some pathogenic bacteria, preventing it from being converted into serotonin. Kynurenine is neuroinflammatory and implicated in neurodiversity. The conversion of tryptophan into kynurenine is associated with a reduced level of lactobacillus reuteri. Inflammatory interleukins are elevated in those with ADHD and is associated with hyperactivity and inattention.²⁶ 

Low levels of Faecalibacterium are also associated with ADHD. They are also associated with atopic diseases such as eczema, asthma and allergies which are also independently associated with ADHD.²⁶ Faecalibacterium are incredibly oxygen sensitive and as a result are not available in supplemental form. Inulin rich foods serve as a food source to encourage colony numbers. 

Most children are conditioned to eat sugary, salty, fatty food from an early age by their parents, often unbeknownst to the consequences and potential impacts on gut health. Children very quickly preferentially crave and select the types of foods they want to eat, often from a young age. There has been a great deal of attention on ultra-processed foods (UPFs), especially following the recent publication of the House of Lords report titled Recipe for Health: a plan to fix our broken food system. In this report, authors acknowledged that the UK is facing an epidemic of unhealthy diets, obesity and diet-related disease. 

So, what is going wrong?

Food scientists are paid large amounts of money to research the science behind food cravings and food addiction. The American market researcher and psychophysics expert Howard Moskowitz coined the expression 'bliss point' to describe just the right amount of saltiness, sweetness, and richness.

There is now no doubt that certain foods, namely ultra-processed foods (UPFs), fill the shelves of our supermarkets with thousands of purposefully manufactured craveable foods designed to increase profits among consumers, and arguably children are the most vulnerable – often rewarded for good behaviour with sweet treats and UPF's. 

Children with ADHD or other neurodivergent conditions often present with food intolerances and gravitate to the very foods they are intolerant to. These types of behaviours arguably reflect gut cravings and the complex symbiotic interactions between the 'host' and gut microbiome.^13,16 


It is important to recognise that processed foods are deliberately developed with mood-enhancing properties (e.g., chocolate and sweets), creating sugar and stimulant cravings. Both wheat (e.g., refined pasta, bread) and some dairy products, when incompletely digested, can transform into opioid-like peptides activating opioid receptors in the brain in a similar way to prescription medication or illegal opiate drugs. Notwithstanding, many processed foods and popular fruit-flavoured drinks (aimed at children) contain artificial sweeteners which have recently been found in several scientific studies to create alterations in the gut microbiome, increasing the risk for leaky gut syndrome.^6,16,23  
 
Researchers in the field of nutritional psychiatry are exploring how leaky gut syndrome, along with sugars, artificial sweeteners, food additives, and yeasty foods, can alter the balance between healthy and dysbiotic gut bacteria in neurodevelopmental conditions. 

Autism and ADHD

Around 1% of the UK population have Autism Spectrum Disorder (ASD) which has been diagnosed, and another 1-2% are likely undiagnosed. Autism is not one thing; it exists upon a spectrum. The British psychiatrist Lorna Wing was one of the leading figures in the field of autism and coined the word 'spectrum.' She was famous for the quote "nature never draws a line without smudging it."

Studies suggest that between 30-80% of individuals with autism also have ADHD.⁶ Professor Jan Buitelaar proposed that autism and ADHD are different manifestations of a single condition, with a range of subtypes, each having distinct times of onset, mixtures of traits, and progression.

Symptoms of ASD

According to the Diagnostic and Statistical Manual of Mental Disorders Volume 5 (DSM-5), there are 3 recognised levels of autism from 1-3, with level 3 being classified as severe, requiring very substantial support. Individuals with autism may display higher attention to detail and a tendency for special interests, which promote heightened focus on particular objects or concepts. Other symptoms may include looping, which is the concept of 'perseverative thoughts' - unconsciously getting stuck on certain thoughts; difficult processing / shifting attention; rigidness; struggle for cognitive flexibility; hyper-fixating and displaying intense and narrow interests (e.g., dinosaurs, planets, trains, but can equally be stones or rubber bands). Hyper-fixating can be beneficial or detrimental, depending on the context. Restricted / repetitive behaviours (RRBs) are common to ASD and can include anything from hand-flapping or rocking to ritualistic behaviour (lining up objects in a certain way). Individuals with ASD may have a preference for routines which help provide predictability and perceived security, safety and comfort, which helps regulate stress and anxiety levels.

Children and adults with ASD may display a phenomenon called object personification, which includes attachments to inanimate objects (non-living things). People with ASD may also be preferring to be alone, be with animals or engage intensely in specific interests. Masking or camouflaging are behaviours and / or social strategies employed to mask the presentation of ASD. Another type of behaviour common to autism is stimming. This is a type of self-regulatory (self-soothing) mechanism, often repetitive behaviour which is linked to movement. It can range from simple hair twirling to flicking/flapping hands, repeating words or phrases, repetitive blinking, foot tapping, finger flicking, humming, to rocking and spinning. Some people with ASD may also experience difficulty reading social cues, social communication, forming or maintaining personal relationships, processing information (it might take a little longer to grasp concepts), switching attention, and cognitive flexibility.
Masking can be described by females with autism as 'exhausting' - on the one hand - but on the other hand, it has benefits in terms of inclusion, friendships, social mobilisation, employment opportunities and so on.

According to prominent autism researcher Simon Baron-Cohen, individuals with autism spectrum conditions are often over-represented in small populations of individuals working in academia and disciplines such as engineering, mathematics, music, physics and computer sciences. In his studies, Baron-Cohen found that male and female students in mathematics, physical sciences, and engineering scored significantly higher on the Autism Spectrum Quotient (AQ) compared to the general population.¹ 

GI conditions and ASD

The influence of the gut microbiome on emotion-dysregulated-related behaviours, as seen in ADHD and ASD as well as brain biochemistry, is of great interest and ongoing investigation by studies such as the Human Microbiome Project.²⁵  

Children with autism have higher reported GI (gastrointestinal) symptoms, including abdominal pain, gaseousness, diarrhoea, constipation and flatulence, with constipation as the most common. 

The prevalence of GI complaints in children with ASD, according to paediatric gastroenterologists and parents, ranges from 23% - 70%.^5,20 Some preliminary studies have reported positive effects of nutritional interventions designed to improve gut health and balance gut microbiota in neurodivergent-related conditions.^7,10,24
Emerging research has implicated the role of the gut microbiome in ADHD and ASD and increased risk of gut dysbiosis, which can act as a catalyst for poor mental health.⁴ Healthy diets can help modulate gut microbiome, which in turn impacts the gut-brain axis.^3,18

Nutritional supplementation studies to date have provided insights into the influence of specific nutrients, such as pre- and probiotics, in modulating both stress, immune and neuronal function.^7,17,22

What can you do at home?

Here are a few top tips for improving gut health:^12,14

• Follow a healthy dietary pattern, such as the one in the book, Smart Foods for ADHD and Brain Health. Research findings have confirmed that a healthy dietary pattern has a positive effect on gut microbial composition.¹⁵

  
• Fibre, antioxidants/polyphenols and other micronutrients can reduce a process called metabolic endotoxemia, an immune response which develops into persistent, low-grade inflammation due to high levels of circulating endotoxins and neuro-inflammation and is associated with improvements in brain health. ¹⁵ 
  
• Increase fibre intake. Fibre is associated with improved brain health and function in a variety of small-scale observational and interventional studies.²
  
• The production and release of serotonin in the enteric nervous system are substantially influenced by dietary choices, in addition to the amounts of complex carbohydrates and tryptophan contained in the diet being the most important factors.²

  
• Specific dietary micronutrients such as zinc, omega 3 fatty acids, folate, and B vitamins are thought to further positively influence brain development and function.
• Supplement with probiotics to improve dysbiosis and improve colonies of anti-inflammatory bacteria.

Always remember, 'brain-selective' nutrients help keep the brain and gut healthy, so remember to eat smart for brain health!

Written by Dr Rachel Gow, with edits and additions by Ella Kaur Maan.