Attention Deficit/Hyperactivity Disorder (ADHD) is classified as a neurodevelopmental disorder, meaning that it affects the nervous system and therefore the brain. Researchers speculate that the common symptoms of ADHD are due to genetic alterations in the frontal-striatal circuitry of the brain,1 which are neural pathways that connect the frontal cortex with the striatum, basal ganglia, and thalamus.

What Are The Functions of These Areas of the Brain?

Prefrontal cortex/Frontal Lobe - logic, reasoning, problem solving, voluntary movements, language/speech, memory, personality, categorization/classification, reward and motivation (dopamine), impulse control, empathy, manage attention

Striatum - are a small group of structures that act as the primary input system for the basal ganglia, receiving nerve impulses/information from the frontal lobe/cortex

Basal Ganglia - motor control, eye movement, ability to form thoughts/develop plans/problem solve, spatial and temporal awareness, movement, emotion, reward, personality, learning, and memory.

Thalamus - the great translator. The thalamus receives sensory/motor input and transmits these nerve impulses to various areas of the brain. In return, the thalamus receives input from the brain and transmits these nerve impulses to the peripheries of the body.

How is Information Transmitted in the Nervous System?

Afferent (sensory) neurons: carry input from sensory organs (eye, skin, mouth, nose, ears) as nerve impulses to the Central Nervous System (CNS) to communicate with the brain and spinal cord.

Efferent (motor) neurons: are located in the CNS. They receive input and carry nerve impulses from the CNS to the muscles, glands, and other areas in the body.

Of course, the central nervous system is much more complex! However, for our purposes, we simply need to understand how the body communicates information and how the gut-brain axis communication pathway plays a role in ADHD.

Communication within the nervous system is dependent on chemical messengers that relay signals, called neurotransmitters. Genetic alterations in the transmission of neurotransmitters have been implicated in ADHD, specifically a polymorphism on the Dopamine Active Transporter 1 gene (DAT1) and dopamine D4 receptor (DRD4).1,2

This is important because the frontal-striatal circuits are dependent on dopamine, serotonin, and norepinephrine, all which are monoamines that have been implicated in ADHD symptoms.3 Monoamines contain one amino group and are deactivated by a riboflavin (B2) dependent enzyme group called monoamine oxidases, which are found in the brain and the gastrointestinal tract (GI).

Monoamines are derived from essential (meaning the body cannot make it and we need to consume it) amino acids found in our foods:

Phenylalanine: a precursor to tyrosine, which converts into dopamine and norepinephrine

Meats and Fish
Eggs and Dairy
Legumes
Nuts

Tyrosine: can also be consumed in foods

Parmesan cheese
Meats and Fish
Roasted soy beans
Legumes
Nuts and Seeds
Whole grains

Tryptophan: a precursor to serotonin and niacin (B3)

Poultry and Seafood
Eggs and Dairy
Nuts and Seeds
Legumes
Whole grains
Potato

Learn more about neurotransmitters and how they function here.

So, What Does This Have To Do With The Gut-Brain Axis?

Our digestive tract has its very own nervous system, called the Enteric Nervous System (ENS), made up of neuronal circuits that control peristalsis (muscular movement of food through the GI tract), local blood flow, absorption and secretions, and regulates immune and endocrine functions.4 Although the ENS can function independently from the CNS, they communicate via neurological, hormonal, metabolic, and immunologic pathways.5

The connection between the gut and the brain is a sensitive balance of interdependence. Healthy brain function is dependent on the neurotransmitters and short-chain fatty acids (SCFAs) produced in the GI tract.

Conversely, the GI tract, and the microbiome that inhabits it, are sensitive to changes in the brain, such as stress. Emerging research connects the gut microbiota to CNS disease and disorder expression,6 therefore a healthy microbiome may be required for proper neurotransmitter production and a healthy nervous system. These particular microbiota are referred to as psychobiotics, and they are often used in targeted probiotic therapy. A GI Map (a comprehensive stool test) can be an excellent tool to determine appropriate targeted probiotic therapies.

Bacteria Species Associated with Neurotransmitter Production5

Lactobacilli and Bifidobacteria: can break down glutamate to produce Gamma-aminobutyric acid (GABA) - the primary inhibitor for the CNS

Learn more about the role GABA plays in ADHD here.

Neurotransmitters Produced by Bacteria That Feed Off of Non-Digestible Fiber5

Bacilli: dopamine and norepinephrine

Enterococci and streptococci: serotonin

Escherichia coli: serotonin and norepinephrine

Lactobacilli: acetylcholine

Another product of bacterial digestion of insoluble (non-digestible) fiber are short-chain fatty acids (SCFAs), butyrate, acetate, and propionate, which have been associated with a variety of health benefits. Butyrate has shown the most promise, in mental health, since it can cross the blood-brain-barrier and has neuroprotective effects.5

What Is The Take Away?

The gut-brain axis is an important communication pathway that has clear associations with ADHD behaviors. Nurturing gut health through eating foods that encourage neurotransmitter production, and that feed our microbiome, may help balance out some of the symptoms associated with the genetic alterations found in ADHD.

References

1. Kamradt JM, Nigg JT, Friderici KH, Nikolas MA. Neuropsychological performance measures as intermediate phenotypes for attention-deficit/hyperactivity disorder: A multiple mediation analysis. Dev Psychopathol. 2017;29(1):259-272. doi:10.1017/S0954579416000195

2. Qian A, Wang X, Liu H, et al. Dopamine D4 Receptor Gene Associated with the Frontal-Striatal-Cerebellar Loop in Children with ADHD: A Resting-State fMRI Study. Neurosci Bull. 2018;34(3):497-506. doi:10.1007/s12264-018-0217-7

3. Karmakar A, Maitra S, Chakraborti B, et al. Monoamine oxidase B gene variants associated with attention deficit hyperactivity disorder in the Indo-Caucasoid population from West Bengal. BMC Genet. 2016;17(1):92. Published 2016 Jun 24. doi:10.1186/s12863-016-0401-6

4. Costa M, Brookes SJH, Hennig GW. Anatomy and physiology of the enteric nervous systemGut 2000;47:iv15-iv19.

5. Bermúdez-Humarán LG, Salinas E, Ortiz GG, Ramirez-Jirano LJ, Morales JA, Bitzer-Quintero OK. From Probiotics to Psychobiotics: Live Beneficial Bacteria Which Act on the Brain-Gut Axis. Nutrients. 2019;11(4):890. Published 2019 Apr 20. doi:10.3390/nu11040890

6.Almeida C, Oliveira R, Soares R, Barata P. Influence of gut microbiota dysbiosis on brain function: a systematic review. Porto Biomed J. 2020 Mar 17;5(2):1-8. doi: 10.1097/j.pbj.0000000000000059. PMID: 33299942; PMCID: PMC7722401.

The Gut-Brain Axis and ADHD