6. Gut-brain axis as a cause of ADHD

With 100 million neurons, the intestinal nervous system contains about as many as the spinal cord. Both are therefore independent nervous systems.
Most of the neurons of the intestinal nervous system neurons are located in:¹

• Plexus myentericus Auerbach (in the muscle wall)
• Plexus submucosus Meissner (adjacent to the mucosa).

Influence specialized neurons of the intestinal nervous system (promoting or inhibiting depending on the transmitter and receptor):¹

• Motor skills (different movement patterns)
• Secretion (water, electrolytes, hormones)
• Perfusion (vascular tone, stimulating (vasodilation) or inhibiting (vasoconstriction) blood flow)
• Resorption
• Signal substance formation

The gut-brain axis plays a role in brain development, particularly in infancy, early childhood and childhood.² The mother’s microbiome, the type of birth and the environment influence the child’s microbiome. Breastfeeding and a healthy diet provide the child’s gut with important probiotic elements, while antibiotics can disrupt the intestinal flora. The intestinal flora also influences neurogenesis.² Microbiota are necessary for normal stress response, anxiety-like behaviors, social behavior and cognition and regulate the homeostasis of the central nervous system via immune function and the integrity of the blood-brain barrier³
Stress can have a significant impact on the gut-brain axis⁴

• 1. Gut-brain axis
• 2. Gut bacteria as a possible causal cause of ADHD?
• 3. Microbiome and short-chain fatty acids (SCFA) in ADHD
• 4. Gut microbiota in ADHD
  • 4.1. Reduced intestinal bacteria in ADHD
  • 4.2. Increased intestinal bacteria in ADHD
• 5. Gut microbiota similar in ADHD and ASD
• 6. Urinary microbiota in ADHD
• 7. Intestinal flora and stress

1. Gut-brain axis¶

The communication of the gut-brain axis is bidirectional. The brain influences the motor, sensory and secretory functions of the gastrointestinal tract top-down via efferent fibers of the vagus nerve. The gut influences the function of the brain bottom-up, in particular the amygdala and hypothalamus, via the afferent vagal fibers⁵

Intestinal bacteria (intestinal microbiome, intestinal flora) influence the nervous system via various mechanisms:⁶

• Metabolic pathway:⁵
  • through modulation of neurotransmitters⁶ such as GABA, serotonin, dopamine, noradrenaline⁵
    • direct
    • indirectly via biosynthetic pathways of the host organism.
  • by secretion of short-chain fatty acids (SCFAs)⁵. These:
    • activate microglial cells⁷
    • influence the permeability of the blood-brain barrier⁸
• Immune system: circulating cytokines⁵
• by changing the HPA axis activity⁹
• by stimulation of the vagus nerve:⁵¹⁰¹¹
  • The vagus nerve has 80 % afferent fibers, which transmit sensory stimuli from the body to the brain, and 20 % efferent fibers, which transport motor signals from the brain to the body.¹

Bacteria can synthesize neurotransmitters and hormones and react to them:⁵

Bacterium	Dopamine (DA)	Noradrenaline (NE)	Serotonin (5-HT)	GABA	Acetylcholine (ACh)	Histamine (Hist)	Other influences
Bacillus species	produce DA1213	produce NE1213
Bacillus cereus	produce DA6
Bacillus mycoides	produce DA6	produce NE6
Bacillus subtilis	produce DA6	produce NE6
Bifidobacterium species				produce GABA1213
Bifidobacterium adolescentis				produce GABA6
Bifidobacterium angulatum				produce GABA6
Bifidobacterium dentium				produce GABA6
Bifidobacterium infantis				produce GABA6
Candida			produce 5-HT13
Cirobacter freundii					produce Hist6
Enterobacter spp.					produce Hist6
Enterococcus			produce 5-HT1213
Escherichia	produce DA612 13	produce NE136	produce 5-HT1314 6
Hafnia alvei (NCIMB, 11999)	produce DA6		produce 5-HT146			produce Hist6
Klebsiella pneumoniae (NCIMB, 673)	produce DA6		produce 5-HT146			produce Hist6
L. lactis subsp. lactis (IL1403)			produce 5-HT14
Lactobacillus species				produce GABA1213	produce ACh1213
Lactobacillus brevis (DPC6108)				produce GABA6
Lactobacillus buchneri (MS)				produce GABA6
Lactobacillus delbrueckiisubsp. bulgaricus (PR1)				produce GABA6
Lactobacillus hilgardii					produce Hist6
Lactobacillus mali					produce Hist6
Lactobacillus plantarum (FI8595)			produce 5-HT146	(ATCC14917) produce GABA6	produce ACh6	produce Hist6
Lactobacillus reuteri (100-23)				produce GABA6
Lactobacillus rhamnosus (JB-1)				produce GABA6; for GABA receptors see *			see **
Lactococcus lactis subsp. cremoris (MG 1363)			produce 5-HT146			produce Hist6
Lactococcus lactis subsp. lactis (IL1403)						produce Hist6
Monasmus purpureus (CCRC 31615)				produce GABA6
Morganella morganii (NCIMB, 10466)	produce DA6		produce 5-HT146			produce Hist6
Oenococcus oeni					produce Hist6
Pediococcus parvulus					produce Hist6
Proteus vulgaris	produce DA6	produce NE6
Saccharomyces		produce NE13
Serratia	produce DA13
Serratia marcescens	produce DA6	produce NE6
Staphylococcus aureus	produce DA6
Streptococcus			produce 5-HT1213
Streptococcus thermophilus (NCFB2392)			produce 5-HT146			produce Hist6
Streptococcus salivarius subsp. thermophilus (Y2)				produce GABA6

* Altered the expression of GABA receptors in the brain via the vagus nerve¹¹; GABA-B1b receptor mRNA increased in the cortex (cingulate and prelimbic), decreased in the hippocampus, amygdala and locus coeruleus, GABA-Aα2 mRNA reduced in the PFC and amygdala, increased in the hippocampus.
** Reduced stress-related corticosterone secretion¹¹; reduced anxiety- and depression-related behavior¹¹

The production of dopamine, noradrenaline and serotonin in intestinal neurons does not mean that the neurotransmitters transported in this way reach the brain.

• Blood-brain barrier
  Acetylcholine can cross the blood-brain barrier. However, dopamine, noradrenaline, serotonin and GABA cannot, which means that these latter neurotransmitters produced in the gut do not directly change the levels in the brain.

• Axonal transport
  We wonder whether neurotransmitters synthesized via the vagus nerve in the gut could be transported to the brain. So far there is no evidence for this. However, there is evidence that nerve fibers of the vagus nerve contain dopamine.¹⁵ Furthermore, peripheral nerves such as the vagus nerve can transport nanoparticles into the brain.¹⁶ Synuclein can also be transported from the body into the brain via nerves, which may be interesting for the question of the development of Parkinson’s disease.¹⁷ In cohort studies, a truncal vagotomy showed a significant protective effect against Parkinson’s disease.¹⁸¹⁹

• Influencing the prodrug balance
  We assume that even if peripherally synthesized or released dopamine, noradrenaline or serotonin from intestinal bacteria could not be directly introduced into the brain via the blood-brain barrier, intestinal bacteria should at least have an influence on the blood level of the precursors that can cross the blood-brain barrier. Consequences are that the blood level could influence the amount of neurotransmitters synthesized from the precursors in the brain.

If the vagus nerve, which connects the intestinal nervous system with the brain, is surgically interrupted (vagotomy), this causes behavioral changes:⁴

• Increase in psychiatric disorders led to
• neurogenic bowel disorders more common²⁰
• reduced locomotor activity during the dark phase of rodents
• elevated noradrenline blood plasma levels
  • basal
  • after immobilization stress
• reduced proliferation and survival of newborn cells, reduced number of immature neurons
• Activation of microglia in the dentate gyrus of the hippocampus

Influence vagal afferents:⁴

• anxiety-like and anxiety-related behavior
• Left-right discrimination and reverse learning
• sensorimotor gating (pre-pulse inhibition)
• Attention control
  • for associative learning
  • with conditioned taste aversion
• gene expression in the nucleus accumbens
• the effects of L. reuteri on the social behavior of autism mouse models (improvement is inhibited)

Vagus nerve stimulation influences / works for:⁴

• the regulation of mood
• the perception of pain (chronic pain)
• Crohn’s disease
• certain epilepsies
• increases neurogenesis in the hippocampus of adults
• modulates the release of noradrenaline, 5-HT and dopamine in brain regions associated with anxiety and depression
• increases the expression of BDNF in the hippocampus, which improved depression-like behaviors in animals with chronic immobilization stress
• influences the reward behavior of mice

Treatment options for microbiota problems are probiotics and fecal transplants.

2. Gut bacteria as a possible causal cause of ADHD?¶

One study found evidence of a causal relationship between gut bacteria and ADHD.²¹ (Note: Even if causality were confirmed, it should be assumed that this is only one of many different possible ways in which ADHD can develop and would therefore not apply to all people with ADHD)

One study found that mice whose guts were contaminated with gut bacteria from people with ADHD had structural changes in the brain (white matter, gray matter, hippocampus, internal capsule), decreased connectivity between motor and visual cortices right in the resting state, and higher anxiety than mice in which gut bacteria from people without ADHD were used.²²

A single case study reported an improvement in ADHD symptoms in a young woman with gut bacteria replacement related to a recurrent Clostridioides difficile infection.²³

3. Microbiome and short-chain fatty acids (SCFA) in ADHD¶

The primary functions of the microbiota include²⁴

• Protection against pathogens by increasing mucus production and thus stabilizing the intestinal-blood barrier
• Support for the immune system
• Production of vitamins
• Production of short-chain fatty acids (SCFAs) from indigestible carbohydrates (“dietary fiber”).

A low-fiber diet reduces SCFA levels, as do antibiotics.

Short-chain fatty acids are:

Abbreviation of the fatty acid	Trivial name	Systemic name	Trivial name salt/ester	Systemic name salt/ester	Chemical formula	Typical plasma value25
C1:0 (no SCFA)	Formic acid	Methanoic acid	Formates	Methanoates	HCOOH
C2:0	Acetic acid	Ethanoic acid	Acetates	Ethanoates	CH3COOH	64 μM
C3:0	Propionic acid	Propanoic acid	Propionates	Propanoates	CH3CH2COOH	2.2 μM
C4:0	Butyric acid	Butanoic acid	Butyrate	Butanoate	CH3(CH2)2COOH	0.54 μM
C4:0	Isobutyric acid	2-Methylpropanoic acid	Isobutyrate	2-Methylpropanoate	(CH3)2CHCOOH	0.66 μM
C5:0	Valeric acid	Pentanoic acid	Valerate	Pentanoate	CH3(CH2)3COOH	0.18 μM
C5:0	Isovaleric acid	3-Methylbutanoic acid	Isovalerate	3-Methylbutanoate	(CH3)2CHCH2COOH	0.40 μM
C6:0	Caproic acid	Hexanoic acid	Capronate	Hexanoate	CH3(CH2)4COOH	0.34 μM

In humans, acetates, propionates and butyrates make up 95% of SCFA, in a ratio of 3:1:1.⁴ A more recent study found other and more detailed differences, which are shown in the table above²⁵

Studies on short-chain fatty acids found reduced SCFA blood levels in ADHD:²⁶²⁷

• Adults with ADHD
  • Formic acid reduces
  • Acetic acid reduces
  • Propionic acid reduces
  • Succinic acid reduced (C4H6O, an aliphatic dicarboxylic acid; food additive number E 363)
• Children with ADHD
  • Formic acid lower than in adults
  • Propionic acid lower than in adults
  • Isovaleric acid lower than in adults
• Antibiotic medication in the last 2 years caused
  • Formic acid reduces
  • Propionic acid reduces
  • Succinic acid reduces
• current stimulant use in children caused
  • Acetic acid reduces
  • Propionic acid reduces

SCFAs corrected the changes that chronic psychosocial stress caused in the gut-brain axis.²⁸ SCFAs

• mitigated the changes in reward behavior triggered by psychosocial stress
• increased the ability to react to an acute stressor
• increased the in vivo intestinal permeability
• had an antidepressant effect
• had an anxiolytic effect
• did not influence the stress-related increase in body weight

4. Gut microbiota in ADHD¶

Studies found abnormalities in the intestinal flora of children with ADHD²⁴
ADHD correlated with leaky gut, neuroinflammation and overactivated microglial cells. The colonic microbiota exhibits a pro-inflammatory shift and harbors more gram-negative bacteria that contain immune-triggering lipopolysaccharides in their cell walls.²⁹

Adults with ADHD had lower plasma concentrations of formic, acetic, propionic and succinic acid than their healthy family members. When ADHD patients were adjusted for SCFA-influencing factors, children had lower concentrations of formic, propionic, and isovaleric acids than adults, and those who had taken more antibiotic medications in the past two years had lower concentrations of formic, propionic, and succinic acids. After adjusting for antibiotic medication, we found that among children, those currently taking stimulant medication had lower acetic and propionic acid concentrations, and adults with ADHD had lower formic and propionic acid concentrations than adult healthy family members.

Disorders of the developing gut microbiota early in life can affect neurological development and potentially lead to adverse mental health outcomes later in life.³⁰

4.1. Reduced intestinal bacteria in ADHD¶

• Bacteroides coprocola (B. coprocola)³¹
• Butyricicoccus²¹

• Coprococcus
  • Anti-inflammatory²⁹

• Desulfovibrio²¹
• Dial register³²
  • Dialister level increased after ADHD treatment
• Enterococcus³³

• Eubacterium
  • anti-inflammatory²⁹
• Eubacterium rectale
  • anti-inflammatory²⁹

• Enterococcus³³
• Faecalibacterium prausnitzii³³
  • anti-inflammatory²⁹
• Faecalibacterium³⁴³⁵³³
  • Anti-inflammatory²⁹
  • Faecalibacterium (Firmicutes strain)
    • Reduced Faecalibacterium (Firmicutes strain) correlated with increased hyperactivity / impulsivity³⁶
• LachnospiraceaeNC2004group²¹
• Lachnospiraceae bacterium³³

• Lactobacillus
  • anti-inflammatory²⁹

• Oxalobacteraceae²¹

• Peptostreptococcaceae²¹
• Prevotella³⁷
  • produce short-chain fatty acids (SCFAs)³⁸
  • anti-inflammatory²⁹

• Romboutsia²¹
• Ruminococcus gnavus ³³
  • Increased against: RuminococcaceaeUCG013²¹

4.2. Increased intestinal bacteria in ADHD¶

• Acidaminococcus³⁹

• Actinobacteria⁴⁰
  • Collinsella⁴⁰

• Agathobacter³⁹
  • correlated with withdrawal symptoms and depression

• Bacillota (synonym: Firmicutes)⁴⁰
  • Coprococcus⁴⁰
  • Subdoligranulum⁴⁰
• Bacteroidetes⁴⁰
  • Bacteroides⁴⁰
    • Correlated with hyperactivity / impulsivity in ADHD³⁶
    • Bacteroides uniformis (B. uniformis)³¹
    • Bacteroides ovatus (B. ovatus)
      • Increase correlated with ADHD symptoms³¹
    • Bacteroides caccae³³
    • Bacteroides faecis (OR: 1.09)⁴¹
    • Bacteroides eggerthii correlated with PTSD (OR: 1.11), not with ADHD⁴¹
    • Bacteroides thetaiotaomicron correlated with PTSD (OR: 1.11), not with ADHD⁴¹
• Bacteroidota⁴⁰
  • Alistipes⁴⁰
    • Pro-inflammatory²⁹
• Bifidobacterium
  • Anti-inflammatory²⁹
  • Increases⁴²
    • A slight increase in Bifidobacterium in the gut is thought to be associated with increased production of cyclohexadienyl dehydratase, which is a precursor to phenylanaline, which is a precursor to dopamine. At the same time, the increase in Bifidobacterium is thought to be associated with reduced reward anticipation, which may indicate reduced dopamine levels in the striatum. How these two seemingly contradictory pathways fit together is not yet clear to us.
    • Bifidobacterium encodes the enzyme arenate dehydratase (ADT), which is important for the production of phenylalanine. Phenylalanine can cross the blood-brain barrier and is a precursor of tyrosine, which is required for DA and NE synthesis.⁴³ However, a small study found no systematic phenylalanine or tyrosine_abnormalities in children with ADHD.⁴⁴
• Eggerthella³⁵
  • Pro-inflammatory²⁹
• Eubacteriumhalliigroup²¹
• Flavonifractor
  • Pro-inflammatory²⁹
• Odoribacter splanchnicus³³
• Odoribacter³⁵
  • Different a study according to which Odoribacter were reduced³³
• Paraprevotella xylaniphila³³

• Phascolarctobacterium³⁹
• Prevotella_2,³⁹
• Proteobacteria (Phylum)³⁹
• Roseburia²¹³⁹
  • anti-inflammatory²⁹
• Ruminococcus gnavus³⁹
  • correlated with rule-breaking behavior

• Ruminococcustorquesgroup ²¹
  • RuminococcaceaeUCG013²¹
• Sutterella stercoricanis (S. stercoricanis)
  • Increase correlated with intake of dairy products, nuts, seeds, legumes, iron, magnesium³¹
  • Increase correlated with ADHD symptoms³¹
• Veillonella parvula³³
• Veillonellaceae³³

No significant difference was found in the alpha diversity of gut bacteria in ADHD.³³³⁴

75 infants were randomly assigned to receive either Lactobacillus rhamnosus GG or a placebo in the first 6 months of life. After 13 years, ADHD or ASD was found in 17% of the placebo group and none in the probiotic group. Bifidobacteria were significantly reduced in the intestinal microbiome of the children with ADHD in the first 6 months of life.⁴⁵⁴⁶

A study of urine and fecal samples using 1H nuclear magnetic resonance spectroscopy and liquid chromatography-mass spectrometry found gender-specific patterns in the metabolic phenotype in ADHD⁴⁷

• Urine profile
  • Hippurate (a product of microbial host co-metabolism that can cross the blood-brain barrier)
    • increased (men only)
    • correlated negatively with IQ (in men)
    • correlated with fecal metabolites associated with microbial metabolism in the gut.
• Fecal profile (independent of ADHD medication, age and BMI)
  • Stearoyl-linoleoyl-glycerol increased
  • 3,7-Dimethylurate increased
  • FAD increased
  • Glycerol-3-phosphate reduced
  • Thymine reduced
  • 2(1H)-quinolinone reduced
  • Aspartate reduced
  • Xanthine reduces
  • Hypoxanthine reduces
  • Orotate reduced

5. Gut microbiota similar in ADHD and ASD¶

The gut microbiota in ADHD and ASD are quite similar in both alpha and beta diversity and differ significantly from non-affected individuals.
In addition, a subgroup of ADHD and ASD cases had increased levels of lipopolysaccharide-binding protein, which positively correlated with interleukin IL-8, IL-12 and IL-13, compared to non-affected children. This suggests an intestinal barrier disorder and immune system dysregulation in a subgroup of children with ADHD or ASD.⁴⁸

Germ-free mice that received a fecal transplant from persons with ADHD subsequently showed⁴⁹

• characteristic autistic behaviors
• alternative splicing of ASD-relevant genes in the brain

When ASD model mice received appropriate microbial metabolites, this improved the behavioral abnormalities and modulated neuronal excitability in the brain.⁴⁹

6. Urinary microbiota in ADHD¶

A study of the urinary microbiome in ADHD found:⁵⁰

• a lower alpha diversity in the urine bacteria of the ADHD group
  • reduced Shannon and Simpson indices (p < 0.05)
• significant differences in beta diversity
• were common with ADHD:
  • Phyla Firmicutes
  • Actinobacteriota
  • Ralstonia (genus)
  • Afipia (genus)
• less frequently with ADHD:
  • Phylum Proteobacteria
  • Corynebacterium (genus)
  • Peptoniphilus (genus)
• Afipia correlated significantly with the Child Behavior Checklist Attention Problems score and the DSM-oriented ADHD subscale

7. Intestinal flora and stress¶

Microbiota are necessary for normal stress response, anxiety-like behaviors, social behavior and cognition and regulate central nervous system homeostasis via immune function and blood-brain barrier integrity³

The vagus nerve and HPA axis influence each other.
Vagal nerve stimulation⁴

• increased the expression of CRF mRNA in the hypothalamus of rodents
• markedly increased the plasma levels of ACTH and corticosterone

In animal models, psychological stress increased the permeability of the intestine and caused a relocation of intestinal bacteria into the host. The activation of the immune response due to exposure to bacteria and bacterial antigens beyond the epithelial barrier causes proinflammatory cytokine secretion and ultimately activates the HPA axis.⁵¹