Introduction

Whether or not, it can be proved definitively that actual prevalence has increased, an urgent sense of curiosity about possible contributory factors is essential. As we have seen, ASD has a complex etiology involving both hereditary and environmental factors. A number of recent research studies have shown that certain environmental factors can impair the proper development of the brain and nervous system through the gut and immune system, all of which have been proven to be interrelated (Min Zou, et al., 2025). 

It is of concern that some of these issues are known to have a greater impact on poorer sections of society and racial minority groups (Roman-Urrestarazu, 2021). This indicates how essential it is that no stone should be left unturned if there is any suspicion at all that certain environmental factors in modern day living, are putting at risk the healthy development of our most vulnerable children and future generations.

Answers can only be found if we are genuinely open to the full range of possible scientific exploration and evidence without bias or prejudice. It is of great concern to those who genuinely wish to find answers, about how society can best support vulnerable children and adults, that these questions are addressed with an open mind in whatever direction research may lead us.

In Part 2 of this article, I consider the implications of the link current research makes between the gut micro-biome, the nervous system and neurological development with reference to the rise in ASD.

Gut health and its relevance to ASD

ASD is a neurodevelopmental disorder which is understood to be affected by many potential root causes, making it etiologically heterogeneous. It is therefore necessary to look at a range of factors which may be interacting and contributing to its rise in prevalence. In recent years, some of the most significant research studies, have focused on how gut microbiota is closely linked to overall health (Parker et al., 2019; Yang, 2023).  An imbalance in the composition of the microbiota can contribute to blood-brain barrier disruption with implications for the development of neurodevelopmental disorders such as ASD (Mehra et al., 2023). The following discussion explains why this link is important in relation to ASD both from a causal and effect point of view.

ASD, Gut microbiota, the immune system

The microbiota in our bodies which consist of a vast number of microorganisms, including bacteria, yeasts, and viruses, coexist in various places such as the gut, skin, lung, oral cavity. They are intrinsically linked to our health and immune system and contribute extensively to our genetic information, (Hou, 2022).

 Importantly, the gut microbiota has been shown to regulate the immune system, and the function of central nervous system immune cells, (Zhou, 2025); Michaudel & Sokol, 2020). The implication is that the gut microbiota can influence brain function and behavior through the gut-brain axis via the nervous system, immune system, and metabolic pathways. For this reason, it is thought there is a plausible link between‘microbiota dysbiosis’ (imbalance of gut microbiota) and ASD through the gut, microbiota brain axis.  A 2018 study at UC Davis MIND Institute found that children with autism spectrum disorder (ASD) have reduced immune system regulation, as well as shifts in their gut microbiota (Yang, R. 2023).

Many factors contribute to the health or otherwise of our gut microbiota, one of the most crucial being our diet. What we eat therefore, shapes the structure, composition, and function of the gut microbiome, which interacts with the gut immune system to maintain it in a balanced healthy state (Zang, 2022). 

Animal studies have shown the direct effects of different types of diet such as those high in sugar, fat and fish, fibre or fermented food on the composition of gut microbiota. They have also shown how travel and migration to societies with different food sources, can have a profound impact on gut microbiome composition and health (Ross, 2024). 

This can be seen in studies of societies with exposure to pre-industrial diets, with a higher diversity and richness including high-fiber diets, low-fat and sugar, exhibiting an abundance of microbial species lost in the western population. It’s been found that autoimmune and other chronic Western digenerative diseases are rare or absent in agropastoral and hunter-gatherer populations (Sing et al., 2024). The way in which the human gut microbiome changes over time as it transitions from a rural or non-industrialized to a post industrialised lifestyle is crucial to our understanding of how the gut microbiome interacts with the environment (Schaan et al., 2024). 

The changing health of the gut microbiome may, over evolutionary time, have impacted on our immune system and neurological health. Dietary factors can impact the inflammatory signalling between the gut and the brain, with specific microorganisms and metabolites either promoting or inhibiting immune activation, potentially contributing to neuro-inflammatory processes (Zang, 2022). For example, protein malnutrition has long been known to be associated with immune defects and intestinal inflammation (Zang, 2022). Different food components can have an either beneficial or negative effect on gut microbial composition.  For example, additives, sugar and alcohol  can have a detrimental effect on the intestinal barrier and lead to pathogen infection, and subsequent gut inflammation. On the other hand, a more nutritionally balanced traditional Mediterranean diet involving whole foods and a fiber-rich diet, not only provides necessary nutrients, but also nourishes a healthy gut microbiome with high diversity and well-balanced composition. (Ross, 2024; Zang, 2022).

Throughout this century, the business industry of producing high calorically dense ultra-processed food has expanded exponentially, meeting the demands of pressures on our pace of life, reduced food preparation time and limited budgets. It has incrementally impacted on our choice of food and overall diet that is typically low in fiber, high in saturated fats, salt, and refined carbohydrates, (Perler, 2023).

Current studies are also exploring a potential link between prenatal alcohol exposure and gut microbial dysbiosis (Bodnar, 2024; Kitchin, 2019; Upreti, 2023). It is well established that prenatal alcohol exposure can have detrimental effects on the developing brain and central nervous system, leading to abnormalities. It can disrupt synaptic connections, and impair the development of regions of the brain involved with social communication and behaviour regulation. This can lead to many symptoms looking like ASD as they have a significant overlap with Foetal Alcohol syndrome. One study found that approximately 33% to 50% of individuals with Foetal Alcohol Syndrome share similar symptoms to ASD (Moller, 2024) making accurate diagnosis complex but essential.

Some studies have found that maternal alcohol consumption during pregnancy influenced the gut microbiota composition in both mothers and newborns although the way and degree to which it does so has not yet been fully established. It is nonetheless a particular concern as many women may consume alcohol prior to confirmation of an unintended pregnancy (Wang, 2021).  

Implications for Autism

The bidirectional connection between our gut and brain, or the ‘microbiota–gut–brain axis’, has important implications for Autism and other neurological and developmental disorders (Taniya, 2022). The gut–brain axis connects an extensive network of nerve cells called the enteric nervous system (ENS) (or the ‘second brain’) to the central nervous system via the vagus nerve. Our emotions are therefore regulated via neurotransmitters such as serotonin, through the gut–brain axis (Taniya, 2022).

The composition of the gut microbiota is shaped from the moment of conception and prenatal development which means the maternal gut microbiota health, affected by the quality and balance of her diet, will in turn affect that of the unborn child. This maternal influence continues through the birth process as microbes are transmitted via the birth canal and after birth through breast milk (Yao, 2020). After 1 year in which a unique and complex microbiota  develops, and then stabilizes around the age of 2–3 years. An infant’s brain also grows from 36% to about 90% of its future adult volume in the first two years of life. 

Significantly, stress has been found to also be a key factor influencing a baby’s gut micro-biome.  An animal study demonstrated that maternal separation from newborns from day to 2 to 12 daily for 3 hours led to microbial dysbiosis and an increase in pro-inflammatory immune cells, (Taniya, 2022). Prenatal stress, including multiple aspects of psychological, physiological, and environmental stress experienced by the mother during pregnancy, can have a long-term impact on fetal gut microbiota.  Research points to concerns about how stress factors may lead to changes in maternal hormone levels, which may turn affect the development of the fetal gut and the colonization of the microbiota (Matsunaga, 2024). It is therefore possible that stress could impact on the development of an  infant’s immune development, metabolic functions, and neurobehavioral functions and increase the risk of neurodevelopmental disorders such as autism (Woo, et al., 2023).  

Studies have also shown that colic in early infancy is linked with gut inflammation and dysbiosis, independent of mode of feeding, with fewer Bifidobacilli. (Rhoads, et al., 2018) which suggests the disruption of the healthy microbiota occurred before birth. Interestingly, some recent animal studies have shown that maternal colonization with gut Bifidobacterium breve during pregnancy, can play a crucial role in positively modifying nutrient transporters in fetal brain and as well as fetal brain metabolism and growth (Lopez-Tello, 2024).

The infant stage, is a critical period for establishing a balanced gut microbiota able to resist pathogen invasion, boost immunity, and prevent disease (Nunez, 2025; Wong et al., 2021). The widespread use of antibiotics is another major contributory cause of gut microbiota imbalance. While antibiotics eliminate harmful bacteria, they may also disrupt the balance of beneficial bacteria at the susceptible infant stage, thereby increasing the risk of disorders such as autism (Ahrens et al., 2024).

In a 20 year Swedish study, significant associations were made with Neuro Divergent diagnosis in general and for specific subtypes, spanning intellectual disability, speech disorder, attention-deficit/hyperactivity disorder, and autism. It revealed microbiome connections to future diagnosis as well as early emerging mood and gastrointestinal problems. It emphasised how immuno-dysregulation and metabolism could be compounded by stress, early-life infection and antibiotics (Ahrens, 2024).

These factors eventually lead to gut microbiome dysbiosis and colonization of pathogenic microbes, which impact the functioning of the Central Nervous System function by the production of neurotoxins. Early infancy is a critical time for the colonization of beneficial so the use of antibiotics during that period can increase gut microbiome dysbiosis inside the infant gastrointestinal tract. In turn these may induce epigenetic changes over successive generations (Eshraghi et al., 2018).

 One common characteristic of ASD is highly selective eating patterns, often with strong dietary preferences associated with linked sensory difficulties which can lead to nutrient deficiencies and contribute further to an imbalanced microbiota composition (Pérez-Cabral et al., 2024). Some studies have shown that a gluten-free, casein-free diet improved some of the symptoms of autism spectrum disorder, such as hyperactivity, communication and behavioural disorders (Ross, 2024). Current knowledge about the bidirectional relationship between the gut-microbiotal-brain axis leaves researchers with the questions about how such eating disorders may be provoked by microbial dysbiosis (Terry, 2022). 

Demographics of those most likely to be affected by effects of poor diet

If our typical Western diet has had an impact on our gut and neurological health, the corollary is to consider whether certain demographics are more impacted than others. Some research points to the way in which social disadvantage impacts on the propensity towards Autism and also the likelihood of a diagnosis. The findings of (Roman-Urrestarazu et al., 2021) showed that the interaction between ASD status and social disadvantage is an important factor to consider. 

Heavy reliance on highly processed foods, poor maternal gut health and stress factors may characterise a greater proportion of children from socially disadvantaged families and in turn increase their susceptibility of developing neurological disorders such as ASD (Taniya et al.,2022). Similarly, the association between a disrupted microbiome and stress would suggest that children from a minority ethnic background experiencing economic hardship, severe stress from geographic dislocation, separation from familiar family, language, society and culture may be significantly more likely to receive an autism diagnosis than their peers.

As we consider research relating to the way in which a typical Western diet may be affecting our neurological health through our gut microbiome, there are a number of other environmental influences that can have a positive or negative impact on the gut microbiome. Research shows that a healthy microbiome cannot be defined by this single issue and that many environmental configurations are linked to neurological health and wellbeing (Ross, et al. 2024). This wide range of environmental issues, along with how they may disproportionately impact disadvantaged groups of society, will be discussed in part 3 of this article.

….…………………………………………………………………………………………………………………

In Part 3, I will reflect on research into various environmental factors impacting neurological development. I weigh up how hereditary and environmental factors may be interacting to impact disproportionately on disadvantaged socioeconomic groups.

References: 

• Ahrens, A.P., et al., (2024). ‘Infant microbes and metabolites point to childhood         neurodevelopmental disorders.’ Vol. 187, Issue 8p1853-1873.e15April 11, 2024 Cell. 2024 Apr 11;187(8):1853-1873.e15. doi: 10.1016/j.cell.2024.02.035
• Eshraghi et al., (2018). ‘Epigenetics and Autism Spectrum Disorder: Is There a Correlation?’ Front. Cell. Neurosci., 27 March 2018. Sec. Cellular Neuropathology. Vol 12 - 2018 https://doi.org/10.3389/fncel.2018.00078
• Hou K, et al., (2022).  ‘Microbiota in health and diseases.’ Signal Transduct Target Ther. 2022 Apr 23;7(1):135. doi: 10.1038/s41392-022-00974-4.
• Lopez, Jensen de, K.M.  Thirup Møller, H. (2024). ‘Prevalence of Autism in Scandinavian Countries (Denmark, Norway, Sweden), and Nordic Countries (Finland, Iceland, the Faroe Islands, and Greenland).’ Neuropsychiatr Dis Treat. 2024 Aug 19;20:1597-1612. doi: 10.2147/NDT.S466081.
• Lopez-Tello, J.(2024). ‘Maternal gut Bifidobacterium breve modifies fetal brain metabolism in germ-free mice.’https://doi.org/10.1016/j.molmet.2024.102004
• Mehra, A., et al., (2023). Gut microbiota and Autism Spectrum Disorder: From pathogenesis to potential therapeutic perspectives. Journal of Traditional and Complementary Medicine. Vol. 13, Issue 2, March 2023, Pages 135-149 https://doi.org/10.1016/j.jtcme.2022.03.001
• Matsunaga M. et al., (2024). ‘Intestinal microbiome and maternal mental health: preventing parental stress and enhancing resilience in mothers.’ Commun Biol. 2024 Feb 29;7(1):235. doi: 10.1038/s42003-024-05884-5.
• Min Zou et al., (2025). Intervention and research progress of gut microbiota-immune-nervous system in autism spectrum disorders among students. Front Microbiol. 2025 Mar 12:16:1535455.DOI: 10.3389/fmicb.2025.1535455
• Michaudel, C. & Sokol, H., (2020). ‘The Gut Microbiota at the Service of Immunometabolism.’  Vol 32, Issue 4, 6 October 2020, pp 514-523. https://doi.org/10.1016/j.cmet.2020.09.004.
• Moller, R. (2024). ‘The Connection Between Autism & Fetal Alcohol Syndrome (FAS)’ https://www.abtaba.com/blog/fetal-alcohol-syndrome-fas
• Mulligan et al., (2025). ‘Epigenetic signatures of intergenerational exposure to violence in three generations of Syrian refugees.’ Sci Rep 15, 5945 (2025). https://doi.org/10.1038/s41598-025-89818-z 
• Nunez, H.et al., (2025). ‘Early life gut microbiome and its impact on childhood health and chronic conditions.’ https://doi.org/10.1080/19490976.2025.2463567.
• Parker A., Fonseca, A. & Carding, S.R. ‘Gut microbes and metabolites as modulators of blood-brain barrier integrity and brain health.’ Gut Microbes. 2019 Aug 1;11(2):135–157. doi: 10.1080/19490976.2019.1638722 
• Pérez-Cabral, I.D. (2024). ‘Exploring Dietary Interventions in Autism Spectrum Disorder.’ Foods 2024, 13(18), 3010; https://doi.org/10.3390/foods13183010
• Perler, B. K. (2023). ‘The Role of the Gut Microbiota in the Relationship Between Diet and Human Health.’ Annual Review of Physiology  Volume 85, 2023 Article Annual Review of Physiology Volume 85, 2023. Vol. 85:449-468 (Volume publication date February 2023)                 https://doi.org/10.1146/annurev-physiol-031522-092054
• Rhoads, J. M. et al. (2018). ‘Infant Colic Represents Gut Inflammation and Dysbiosis.’ J Pediatr. Pediatr. 2018 Aug 31;203:55–61.e3. doi: 10.1016/j.jpeds.2018.07.042
• Roman-Urrestarazu A, van Kessel R, Allison C, Matthews FE, Brayne C, Baron-Cohen S. (2021), Association of Race/Ethnicity and Social Disadvantage With Autism Prevalence in 7 Million School Children in England. JAMA Pediatr. 2021 Jun   1;175(6):e210054. doi: 10.1001/jamapediatrics.2021.0054. Epub 2021 Jun 7.
• Ross, F. C. et al. (2024).  ‘The interplay between diet and the gut microbiome: implications for health  and disease.’ Nat Rev Microbiol 22, 671–686 (2024).    https://doi.org/10.1038/s41579-024-01068-4
• Schaan, et al. (2024). ’Temporal dynamics of gut microbiomes in non-industrialized urban    Amazonia.’mSystems. 2024 Mar 19;9(3):e0070723. doi: 10.1128/msystems.00707-23. Epub 2024 Feb 20.
• Taniya, M.A. et al. (2022).  ‘Role of Gut Microbiome in Autism Spectrum Disorder and Its Therapeutic Regulation.’ doi: 10.3389/fcimb.2022.915701
• Terry, S.M. (2022). ‘A critical analysis of eating disorders and the gut microbiome.’ Terry et al. Journal of Eating Disorders 2022, 10(1):154 https://doi.org/10.1186/s40337-022-00681-z
• Upreti et al. (2023). ‘Microbiota and nutrition as risk and resiliency factors following prenatal alcohol exposure.’ Front. Neurosci. 17:1182635. doi: 10.3389/fnins.2023.1182635
• Wang, Y. et al. (2021). ‘Impacts of Maternal Diet and Alcohol Consumption during Pregnancy on Maternal and Infant Gut Microbiota.’ Biomolecules 2021, 11, 369. https:// doi.org/10.3390/biom11030369
• Watts, Renata  (February 22, 2023). ‘Nowhere to go: The lack of provision for young     people with complex needs’. Special Needs Jungle. https://www.specialneedsjungle.com/lack-provision-young-people-complex-needs/
• Wong, Giselle C. et al., (2021). ‘The Gut-Microbiota-Brain Axis in Autism Spectrum Disorder.’ In: Grabrucker AM, ed. Autism Spectrum Disorders [Internet] Brisbane (AU): Exon Publications; 2021 Aug 20. Chapter 8. Available from:https://www.ncbi.nlm.nih.gov/books/NBK573606/. doi:10.36255/exonpublications.autismspectrumdisorders.2021.gutmicrobiota
• Woo T, et al., (2023). ‘MicroRNA as a Maternal Marker for Prenatal Stress-Associated ASD, Evidence from a Murine Model.’  J Pers Med. 2023 Sep 20;13(9):1412. doi: 10.3390/jpm13091412.
• Yang, R. (2023). Fecal microbiota transplatation: Emerging applications in                             autoimmune diseases.  https://doi.org/10.1016/j.jaut.2023.103038
• Yao Yao et al. (2020). ‘The Role of Microbiomes in Pregnant Women and Offspring: Research Progress of Recent Years.’ Front Pharmacol. 2020 May 8;11:643. doi: 10.3389/fphar.2020.00643
• Zang, P. (2022). ‘Influence of Foods and Nutrition on the Gut Microbiome and                        Implications for Intestinal Health.’ Int. J. Mol. Sci. 2022, 23, 9588.                        https://doi.org/10.3390/ijms23179588
• Zhou, M. (2025) ‘Intervention and research progress of gut microbiota-immune-  nervous system in autism spectrum disorders among students.’ Front. Microbiol. 16:1535455. doi: 10.3389/fmicb.2025.1535455.