A Comprehensive Review of Autism Spectrum Disorder: Mechanisms, Modifiable Factors, and Clinical Implications — A Focus on Vitamin D

Journal of Complementary Therapies in Health ISSN 2975-9323 |eISSN 2975-9552 Journal of Complementary Therapies in Health 2025:3(3). doi:10.5281/zenodo.17882641 institutoptc.com/journal-complementary-therapies Review A Comprehensive Review of Autism Spectrum Disorder: Mechanisms, Modifiable Factors, and Clinical Implications — A Focus on Vitamin D. Teresa Esteves1, Maria Carreira1* , Rui Afonso2, Tânia Almeida1, Cíntia Nunes1, and João Veiga1. 1 IPN – Portuguese Institute of Naturology, Porto, Portugal; 2 Independent researcher. * Correspondence: [email protected] Abstract Autism Spectrum Disorder (ASD) is a multifactorial neurodevelopmental condition influenced by complex genetic, epigenetic, and environmental factors. Among the modifiable biological factors under investigation, Vitamin D has gained increasing attention for its potential role in neurodevelopment and immune regulation. Evidence indicates that Vitamin D deficiency is highly prevalent in individuals with ASD and may contribute to altered neuroimmune responses, impaired neurotransmission, oxidative stress, and atypical brain development. This review synthesises current findings on the association between Vitamin D status and ASD, highlighting observational studies, mechanistic evidence, and interventional trials. Several clinical studies report that correcting Vitamin D deficiency, particularly during pregnancy or early childhood, may lead to improvements in behavioural, cognitive, and emotional symptoms, although results vary depending on dosage, baseline levels, and duration of supplementation. Given its safety profile and biological plausibility, optimising Vitamin D levels represents a low-risk, potentially beneficial strategy within comprehensive ASD care. Nonetheless, further high-quality randomised controlled trials are required to clarify causality, determine effective supplementation protocols, and better understand the underlying mechanisms linking Vitamin D to ASD neurobiology. Keywords: Vitamin D; Autism Spectrum Disorder; Neurodevelopment; Immunity; Supplementation; Deficiency; Behavioural Outcomes. Citation: Esteves T., Carreira M., Afonso R., Almeida T., Nunes C., Veiga J. A Comprehensive Review of Autism Spectrum Disorder: Mechanisms, Modifiable Factors, and Clinical Implications — A Focus on Vitamin D. Journal of Complementary Therapies in Health. 2025;3(3) 10.5281/zenodo.17882641 Academic Editor: Jorge Rodrigues Received: 10 August 2025 Reviewed: 17 September 2025 Revised: 20 September 2025 Accepted: 21 September 2025 Published: 25 September 2025 Publisher’s Note: IPTC stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: ©2025 by the authors. Submitted for open access publication under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/). Journal of Complementary Therapies in Health 2025: 3(3). 2 of 13 Esteves T., Carreira M., Afonso R., Almeida T., Nunes C., Veiga J. A Comprehensive Review of Autism Spectrum Disorder: Mechanisms, Modifiable Factors, and Clinical Implications — A Focus on Vitamin D doi:10.5281/zenodo.17882641 1. Background ASD is a chronic neurodevelopmental condition characterised by deficits in social communication and social interaction across multiple contexts, and the manifestation of restricted, repetitive patterns of behaviour, interests, and/or activities 1. ASD symptoms can be detected early during a child’s development or may be masked by strategies acquired throughout life. ASD symptoms can be very detrimental to an individual’s social, occupational, and other important areas of life 1-5. It is defined in 3 levels, with Level 1 having milder symptoms requiring support (Asperger's Syndrome – high-functioning autism) and Level 3 having more severe symptoms requiring very substantial support 1. ASD was classified as one of the top ten causes of non-fatal health problems for people under 20 years old. The number of autism cases has been growing over the years. Recently, it was estimated that 1 in every 127 individuals were autistic, or 0.80%, almost 1% of the world's population 6. In young people aged 5 to 18 residing in Europe, the estimated prevalence rate of ASD is 1.4% of the population (between 2015 and 2020). The prevalence among primary school children was four times higher than among secondary school children. A male:female ratio of 3.5:1 was obtained 7. About 70% of people with ASD have co-occurring medical, developmental, or psychiatric conditions. Recent studies indicate that the most common comorbidity is anxiety, present in 60% of individuals with ASD, followed by depression (45%) and AttentionDeficit/Hyperactivity Disorder (ADHD). Individuals with autism and psychiatric comorbidities showed a significant reduction in quality of life compared to those without comorbidities 8. Diagnosis should be made as early as possible so that appropriate intervention can be initiated, thus improving the prognosis. The diagnosis of ASD can be made between 16 and 30 months, as ASD can begin in intrauterine life 9. To date, a single cause for ASD has not been defined. ASD is considered to have a multifactorial aetiology, which culminates in alterations in cellular signalling pathways, neurotransmitters, and brain development. These factors can be neurobiological, genetic, or environmental, among which nutritional deficiencies stand out, with several studies reporting the importance of vitamin D in the aetiology of the autism spectrum 9,10. Several studies have demonstrated an association between the risk of ASD and the amount of UVB exposure, D3 levels during pregnancy, and low cholecalciferol levels in autistic children 10. The objective of this study is to investigate scientific articles from the last ten years that studied the relationship between ASD and Vitamin D, as well as the improvements observed with supplementation. 2. Autism Spectrum Disorder 2.1. Aetiology of Autism Spectrum Disorder The aetiology of ASD is multifactorial with environmental, neuro-hormonal, and genetic risk factors. The main environmental risk factors are related to the perinatal period. Maternal and paternal ages over 35 and 40 years, respectively, gestational diabetes, arterial hypertension, and maternal obesity seem to contribute to the development of ASD. In the intrauterine environment, exposure to valproic acid, environmental pollution, pesticide exposure, and maternal infection are known risk factors. In the neonatal period, low birth weight, lack of oxygen during labour, and prematurity are the most relevant factors 11. In turn, the levels and mechanisms of action of hormones such as oxytocin, cortisol, estradiol, and testosterone seem to influence the risk of developing this disorder, through their interaction with neural pathways 12. Journal of Complementary Therapies in Health 2025: 3(3). 3 of 13 Esteves T., Carreira M., Afonso R., Almeida T., Nunes C., Veiga J. A Comprehensive Review of Autism Spectrum Disorder: Mechanisms, Modifiable Factors, and Clinical Implications — A Focus on Vitamin D doi:10.5281/zenodo.17882641 The high genetic load of autism is consensual, with a heritability estimated at 40% to 90%, with about 400 to 1000 genes identified as being associated with increased susceptibility to ASD. Although not sufficient for the development of the disorder, point genetic mutations affect about 10% to 20% of individuals with autism. Chromosomal abnormalities (deletions, duplications, inversions, and translocations) and some medical conditions such as Turner's, Klinefelter's, Fragile X, Rett, Angelman syndromes, and Trisomy 21, among other genetic entities, are described in about 20% of individuals with ASD 13. 2.2. Genetic and Epigenetic Etiology of ASD Several studies suggest that ASD has a high heritability. However, this heritability depends on environmental risk factors, the relationship between genes and the environment, and epigenetic mechanisms. Currently, more than 900 implicated genes are identified 14. Rare genetic mutations can manifest as Mendelian genetic syndromes, chromosome abnormalities, de novo mutations, transmitted single nucleotide variants, and copy number variation. However, only 1% of individuals with ASD manifest genetic mutations alone 9. Most genes associated with ASD have their peak expression during the first and second trimesters of gestation and affect brain development processes 14. Genetic mutations that are associated only with ASD are not easily identified clinically because the somatic manifestations of a specific genetic mutation vary significantly and because the number of reported cases with sufficient clinical information is often reduced. Thus, variants are identified by more comprehensive genomic tests, rather than derived from defined clinical hypotheses 15. 2.3. Modifications in Cellular Signalling Pathways The cellular signalling pathways strongly associated with ASD can have their function controlled by various genes. The respective modifications impact several phases of brain development 14. There are specific genetic mutations that partially inhibit signalling pathways, culminating in decreased protein synthesis and cognitive alterations. On the other hand, there are gene mutations that result in increased activity and, consequently, in deregulated proliferation, increased cortical growth, and synapse amplification 16. There are also mutations in genes that increase synaptogenesis and neuronal connectivity, while other gene mutations suppress synaptogenesis, leading to delayed spinal cord development and reduced synaptic amplitude. Certain somatic gene mutations cause excessive activation, resulting in hemimegalencephaly and focal cortical dysplasia, with neuronal dysmorphia and loss of radial neuronal orientation 17. Other genes, on the other hand, affect more than one pathway, causing, among other things, a reduction in the G1/S transition and an increase in brain volume, modification of synaptic transmission, or resulting in alterations in neuronal migration due to modification of the polarity of precursor cells 18. 2.4. Variations in Neurotransmitters Several neurotransmitters are implicated in the pathophysiology of ASD, as the disruption of genes encoding multiple proteins can affect the respective neurotransmitters and early brain development 19. Acetylcholine Decreased levels of choline, the agonist for the nicotinic-cholinergic receptor (nAChR), and muscarinic M1 receptors, as well as anatomical irregularities in the number and structure of cholinergic neurons, play a crucial role in processes such as learning, memory, and mood regulation, in addition to influencing gastric secretions and bronchoconstriction – these are common alterations described in ASD. The severity of symptoms is Journal of Complementary Therapies in Health 2025: 3(3). 4 of 13 Esteves T., Carreira M., Afonso R., Almeida T., Nunes C., Veiga J. A Comprehensive Review of Autism Spectrum Disorder: Mechanisms, Modifiable Factors, and Clinical Implications — A Focus on Vitamin D doi:10.5281/zenodo.17882641 inversely related to acetylcholine values and manifests through social deficits, repetitive behaviours, reduced attention, and decreased cognitive flexibility 12. Dopamine Dopamine is involved in higher brain functions (such as emotions, the reward system, motivation, and cognition). Considering the functions of the dopaminergic system, its alterations have a relevant impact on social behaviour, attention skills, perception, and motor activity 20. Histamine Increased histamine impacts cognition, learning, memory, and behaviours in ASD. The histaminergic system may also be a potential therapeutic target 21. Melatonin In individuals with ASD, sleep problems are common, and the reasons are diverse: reduced melatonin concentration during the night, as well as a delayed melatonin peak and reduced amplitude. In some cases, there may also be a tendency for it to be elevated during the day. These alterations correlate with the greater severity of communication and behavioural symptoms in ASD. Furthermore, they are associated with sleep and circadian rhythm disorders, namely insomnia and daytime sleepiness. Currently, melatonin is already used as a drug in the context of ASD, being a target that has beneficial effects on sleep and autistic symptoms 22. Serotonin ASD is associated with elevated plasma serotonin values in a portion of affected individuals. These values tend to significantly reduce motivation for social interest due to the inhibition of separation anxiety. Furthermore, hyperserotonaemia is known to lead to hypersensitivity of cerebral receptors associated with social deficits and repetitive behaviours. Serotonin is a relevant therapeutic study target, and selective serotonin reuptake inhibitors are already recommended in situations of comorbidity with anxiety, depression, and obsessive-compulsive disorder 23. GABAergic System and Glutamate Increased expression of glutamate-related genes and/or decreased expression of GABA genes can be associated with several clinical phenotypes of ASD, including cognitive deficits, hyperactivity, and epilepsy 24. 2.5. Impact of Immunological Processes Immunological dysfunction constitutes an important contributing risk factor for neurodevelopmental deficits in ASD. We can differentiate two periods: prenatal and postnatal 19. In the prenatal period, several studies indicate a higher incidence of ASD in children with congenital infection by rubella, measles, mumps, cytomegalovirus, and polyomavirus. Maternal bacterial infections can also constitute an increased risk for altered neurodevelopment. Research suggests that the maternal response to infection is more relevant to the emergence of ASD than the specific pathogen. Maternal immune activation (MIA), which occurs during pregnancy following infections, is one of the most relevant models and is associated with socio-behavioural and neurodevelopmental modifications. Regarding the mechanisms that mediate brain alterations by the MIA model, studies report different cytokines involved, implicated throughout the development of the central nervous system 14. Inflammatory cytokines related to ASD can act through a maternal mechanism (maternal cytokines cross the placenta), a placental mechanism (MIA leads to inflammation Journal of Complementary Therapies in Health 2025: 3(3). 5 of 13 Esteves T., Carreira M., Afonso R., Almeida T., Nunes C., Veiga J. A Comprehensive Review of Autism Spectrum Disorder: Mechanisms, Modifiable Factors, and Clinical Implications — A Focus on Vitamin D doi:10.5281/zenodo.17882641 and cytokine production in the placenta), and a foetal mechanism (MIA leads to immune and genetic dysregulation in the foetus itself) 19. Still in the prenatal period, the passage of maternal autoantibodies to the foetal brain acts against it. The presence of these antibodies can persist for several years in the baby's brain, causing an increase in brain volume, larger neuronal size, and socio-behavioural symptoms characteristic of ASD 14. In turn, in the postnatal period, the role of autoantibodies against the child's own brain, distinct from maternal antibodies, must be considered. Antibodies in the child's brain can affect different brain areas, resulting in greater severity of ASD symptoms and greater intellectual deficit. Immunogenic factors are also important vulnerability factors for this pathology, and the network of genetic interactions likely leads to persistent immunological dysregulation 15,19. 2.6. Variability of Brain Growth Throughout Development Brain volume in ASD varies substantially during development. Studies on foetal brain growth indicate that there are no significant differences in foetal head circumference immediately after birth in individuals later diagnosed with ASD compared to the rest of the population. After 6-12 months of life, there is an early increase in head circumference and weight. Modifications in total brain volume between 12 and 24 months are associated with greater severity of autism-related social symptoms at 24 months. However, there are individuals with ASD who have no changes or even a decrease in brain growth 25. 2.7. Anomalies in Brain Connectivity and Development One of the alterations in ASD is the reduction of inter-hemispheric connectivity. These changes occur from 6 months of age and affect cognitive functions, including attention, language processing, inhibitory control, and visuospatial processing, as well as the processing and modulation of visual information, important for action control, face recognition, and perception of facial expressions, which are related to social symptoms, processing of basic emotions, and emotional and social functions related to the typical ASD symptomatology 26. The frontal cortex is one of the most frequently altered cortical areas. Atypical cortical growth patterns, anomalies in cortical thickness, disorganisation of neurons in the cortical layers, and in their connections with other brain regions are observed. Specifically, there is an early increased growth in volume or cell number during childhood, followed by a decline in growth and cessation of development later, persisting into adolescence and adulthood. In the latter phase, cortical thickness may be normal, which does not mean structural or functional normalisation, or it may be decreased. Some studies suggest that differences may exist depending on the age of the studied population. These alterations will, in turn, influence the social symptoms of ASD 27. 2.8. Diagnosis of ASD Diagnosis is based on the behavioural observation of the child in different environments, where some questionnaires are also administered 28. Due to its complexity and lack of objectivity in diagnosing this pathology, mechanisms have been developed over the years that can assist in increasing the accuracy of this diagnosis, reducing waiting time, and accelerating the start of intervention 28. 3. Vitamin D Vitamin D plays a crucial role in brain health, providing antioxidant, anti-inflammatory, and neuroprotective benefits, and regulating neurotransmitters and neurotrophins essential for the development, maintenance, and functioning of the nervous system. Vitamin D deficiency during pregnancy and early childhood can impair neurodevelopment, potentially contributing to ASD 29. Journal of Complementary Therapies in Health 2025: 3(3). 6 of 13 Esteves T., Carreira M., Afonso R., Almeida T., Nunes C., Veiga J. A Comprehensive Review of Autism Spectrum Disorder: Mechanisms, Modifiable Factors, and Clinical Implications — A Focus on Vitamin D doi:10.5281/zenodo.17882641 3.1. Vitamin D Metabolism Vitamin D belongs to the group of fat-soluble vitamins and is currently considered a steroid hormone derived from cholesterol, essential for regulating bone metabolism 30. Vitamin D is produced in the skin tissues from exposure to sunlight with ultraviolet B (UVB) radiation and can also be obtained by ingesting foods in the form of vitamin D2 (ergocalciferol) through plant-based foods, and D3 (cholecalciferol) through animal-based foods, such as fatty fish and egg yolk. If necessary, vitamin D can be obtained through supplementation. These forms of Vitamin D are inactive compounds, meaning that to become active and perform their functions in the body, they must be metabolised 31. The first hydroxylation of vitamin D3 and/or D2 occurs in the liver and produces 25hydroxyvitamin D (25(OH)D). The second hydroxylation occurs in the kidneys and originates 1,25-dihydroxyvitamin D (1,25(OH2)D). These vitamin D compounds will facilitate the active absorption of calcium and phosphorus in the small intestine, which increases serum calcium and phosphate levels, thus allowing bone mineralisation. On the other hand, these vitamin D compounds also aid in the mobilisation of calcium and phosphate from the bone and increase the reabsorption of calcium and phosphate through the renal tubules 32. The active form of vitamin D, 1,25(OH2)D, and Parathyroid Hormone (PTH) are the main hormones related to bone metabolism. The binding of 1,25(OH2)D to the vitamin D receptor (VDR) in the intestine promotes greater absorption of calcium and phosphorus. In the bone, PTH and 1,25(OH2)D participate in the regulation of bone resorption and formation, being able to mobilise bone calcium reserves when blood calcium levels are very low (hypocalcaemia) 31. Magnesium is an essential cofactor for the hepatic and renal enzymes that convert vitamin D into 25(OH)D and 1,25(OH2)D. Its deficiency is common in individuals with ASD (due to their inadequate diet, resulting from food selectivity) and compromises this activation, reducing the effectiveness of vitamin D3 30. The largest source of Vitamin D comes from the conversion of 7-dehydrocholesterol to pre-vitamin D3 in the skin through UVB solar radiation. In turn, pre-vitamin D3 undergoes thermal isomerisation, which leads to the formation of vitamin D3 (cholecalciferol). Subsequently, vitamin D3 (obtained by sun exposure or intestinal absorption) is transported to the liver where it undergoes its first side chain reaction at position 25, producing calcidiol or 25-hydroxycholecalciferol. Calcidiol returns to circulation and is then taken to the kidney where it undergoes a second hydroxylation at carbon 1, producing calcitriol or 1,25-dihydroxycholecalciferol 31. The 25(OH)D formed is largely stored in adipose tissue and has a half-life of 21 to 30 days. It is these reserves of 25(OH)D that are measured through blood tests and whose values are referenced 32. 3.2. Vitamin D Deficiency and ASD Vitamin D is essential for overall health, being important for reducing the occurrence of respiratory infections, preventing rickets, weak bones, and skeletal deformities, and helping in the formation of adequate bone mass, reducing the risk of osteoporosis in adult life 31. In addition to bone health, vitamin D significantly interferes with the development of the central nervous system. Vitamin D deficiency during pregnancy and early childhood can impair brain development and result in ASD. Vitamin D deficiency may be a risk factor for ASD or influence the aetiology of ASD 33. Several studies have shown that individuals with ASD have low levels of Vitamin D, which may underlie various behavioural disorders and structural and functional abnormalities of the brain 34. The Endocrine Society of the United States of America defines Vitamin D deficiency for serum values below 20 ng/mL. According to the Portuguese Society of Endocrinology, Diabetes and Metabolism, it is estimated that more than 60% of the Portuguese population Journal of Complementary Therapies in Health 2025: 3(3). 7 of 13 Esteves T., Carreira M., Afonso R., Almeida T., Nunes C., Veiga J. A Comprehensive Review of Autism Spectrum Disorder: Mechanisms, Modifiable Factors, and Clinical Implications — A Focus on Vitamin D doi:10.5281/zenodo.17882641 has Vitamin D deficiency, with values below 20 ng/mL, and considers it a public health problem 35. In a 2024 study, it was found that 63.8% of children with ASD had Vitamin D insufficiency with values between 20/30 ng/mL, while 63.8% of individuals with ASD had deficiency with values below 20 ng/mL. Only 7.4% of individuals with ASD had normal values 36. These values are partly due to the fact that individuals with ASD have a more sedentary lifestyle, with little sun exposure or outdoor activities, and also due to food selectivity, often consuming repetitive meals with limited food diversity compared to other individuals. However, the exact mechanism of Vitamin D in individuals with ASD is still considered unknown, but because it is a common characteristic in all ASD carriers, vitamin D deficiency should be evaluated and corrected 37. Low Vitamin D concentration in pregnant women is also associated with an increase in ASD phenotypes. Vitamin D deficiency mid-gestation was associated with a 2.4 times greater risk of ASD development in children 29. 3.3. Vitamin D Supplementation and ASD Many studies point to a significant improvement in ASD symptoms when children are supplemented with Vitamin D. Improvements in irritability, hyperactivity, repetitive and stereotyped behaviour, low social interaction, attention deficit, anxiety, emotional intelligence quotient, among others, stand out 38. According to the Institute of Medicine, the average necessary intake levels of vitamin D for men range from 272 to 396 IU/day, while for women, the average vitamin D intake varies between 160 and 260 IU/day. When supplementation is considered as total intake (excluding food sources or sun exposure), a slight increase in needs is noted across all groups and age ranges. The Endocrine Society of the United States defined clinical practice guidelines that indicate that the values recommended by the Institute of Medicine may be insufficient, especially for patients who have conditions that put them at risk of vitamin D deficiency. Thus, the Endocrine Society of the United States 39 suggests new dosages for patients at risk of hypovitaminosis D: 0 to 1 year need 400\1000 IU/day potentially up to 2000 IU, patients 1 to 18 years 600\1000 IU/day potentially up to 4000 IU/day. In general, Vitamin D is considered safe, economically accessible, and can have beneficial effects on individuals with ASD, especially when the final serum level is greater than 40 ng/ml. Supplementation should be adjusted to each patient to reach these values 40. Various studies present scientific evidence that proves that Vitamin D supplementation in children with ASD was beneficial. A reduction in ASD symptoms is observed through mechanisms of action that allow Vitamin D to reduce the production of inflammatory cytokines in the brain, potentially alleviating neuroinflammation caused mainly by oxidants and toxins. Furthermore, calcitriol increases DNA repair mechanisms, has anti-autoimmune effects, increases the seizure threshold, increases regulatory T cells, protects neural mitochondria, and increases glutathione levels, the antioxidant that eliminates oxidative by-products 40,41. Another mechanism of action of Vitamin D is based on the identification of Vitamin D response elements, through two different tryptophan hydroxylase enzymes, these enzymes being functionally opposite to each other: one induces the transcriptional activation of tryptophan hydroxylase 2 (TPH2) by vitamin D in the brain and the other induces the repression of tryptophan hydroxylase 1 (TPH1) in tissues outside the blood-brain barrier or peripheral to the brain (peripheral tissues). This mechanism explains the serotonin paradox in ASD: excess serotonin in peripheral tissues and reduction at the cerebral level 42. Studies describe Vitamin D as an active neurosteroid, which plays active neuroprotective roles, crucial for the developing brain, such as cell proliferation and differentiation, immunomodulation, neurotransmission regulation, and steroidogenesis 43. Journal of Complementary Therapies in Health 2025: 3(3). 8 of 13 Esteves T., Carreira M., Afonso R., Almeida T., Nunes C., Veiga J. A Comprehensive Review of Autism Spectrum Disorder: Mechanisms, Modifiable Factors, and Clinical Implications — A Focus on Vitamin D doi:10.5281/zenodo.17882641 Despite the preand postnatal immunological alterations being present in all patients with ASD, it is of extreme importance to continue research in order to understand the various subgroups of alterations and outline possible interventions 11. Vitamin D deficiency may represent only one environmental factor contributing to the development of ASD, but it is known that other metabolic and genetic pathways may be involved. However, the possibility of intervening with a simple, safe, and accessible factor during pregnancy makes Vitamin D deficiency an important Public Health concern 29. 4. Final Remarks It is concluded that ASD has a complex diagnosis that is difficult to conclude, and it can be performed through clinical behavioural observation and investigation. The vast genetic heterogeneity associated with ASD is not sufficient for diagnosis. It is a very complex syndrome, so there may be medical diagnoses covering different behavioural patterns. Diagnosis should be made as early as possible in order to start follow-up and correct interventions so that the patient can minimise the behaviours that ASD can trigger, which hinder their social interaction and intellectual development. Vitamin D deficiency is frequent in children with ASD. Vitamin D is considered safe and accessible, which makes its supplementation simple and extremely important in controlling symptoms in individuals with autism. The prevention of the onset of ASD can be done through vitamin D supplementation in pregnant women, but it is necessary to reinforce the benefits it brings to individuals with autism, where the improvement of symptoms means a better quality of life for the patients, but also for the entire family, caregivers, and therapists. Supplementation must be adjusted to each patient individually. In addition to Vitamin D supplementation, it is important to implement an outdoor exercise routine for individuals with ASD. This routine allows for an improvement in physical condition, improvement in stereotyped behaviours, improvement in motor coordination, but also frequent sun exposure to activate internal Vitamin D production. For the treatment of ASD, there are multiple methods or so-called therapies, which can minimise some of the more severe effects or behaviours and which can also be applied according to the ASD level. Therapies use images for the functional establishment of communication in order to promote and improve the patient's language use, something very important for communication and often compromised. Other therapies with very good results exist, such as music therapy, animal-assisted therapies (dogs, cats, rabbits can be used), and equine therapy for biopsychological and emotional development 44-48. According to naturopathy, each individual should be looked at and treated with care and respect, seeking to balance diet, sleep, physical exercise, personal and family routines, therapies, the environment they are in, and their well-being. The food selectivity of individuals with ASD can be challenging 49,50, and naturopathy can play a fundamental role in assisting and understanding caregivers, providing strategies for the introduction of new foods to be done in a calm and respectful environment, with time and patience. Gradually, the diet is moulded, the digestive system is recovered, and water consumption is increased. Aromatherapy has also shown results and may be used to increase concentration, reduce anxiety, and improve sleep quality 51,52. Traditional Chinese Medicine has also shown positive results at the behavioural level in individuals with ASD 53,54. Bio-energetic strategies such as Reiki or Access Bars can be used in individuals with ASD and their families to lower anxiety levels, work on acceptance, and face reality more positively and clearly 55,56. Journal of Complementary Therapies in Health 2025: 3(3). 9 of 13 Esteves T., Carreira M., Afonso R., Almeida T., Nunes C., Veiga J. A Comprehensive Review of Autism Spectrum Disorder: Mechanisms, Modifiable Factors, and Clinical Implications — A Focus on Vitamin D doi:10.5281/zenodo.17882641 Many individuals with ASD have great potential, and time is needed to understand their interests and enhance their abilities. This process sometimes allows them to find an occupation and can bring a lot of serenity to the whole family. It is necessary to look at these individuals and understand that they are neither right nor wrong; they have a different way of seeing the world, which is why they seem so out of place. Empathy and respect are needed for them to be naturally integrated into society, as they deserve. It is important to have awareness in schools, in companies, and inclusive education for differences. It is fundamental to take care of family members and caregivers, who, in the most severe cases, are often isolated, on the verge of exhaustion, depression, and familial and friendly abandonment. It is necessary to empower these parents, giving them information, strategies, support, and oxygen so they can move forward with all the challenges that life brings. 5. Conclusion Autism Spectrum Disorder is a multifactorial neurodevelopmental condition influenced by complex interactions among genetic, epigenetic, environmental, immunological, and neurobiological factors. Despite advances in characterising these mechanisms, ASD remains challenging to diagnose and manage, reinforcing the need for early identification and comprehensive intervention strategies. Across the literature, Vitamin D has emerged as a potentially relevant modifiable factor. Deficiency is highly prevalent among individuals with ASD and appears to contribute to neuroimmune dysregulation, impaired neurotransmission, and altered neurodevelopment. Evidence from multiple studies suggests that correcting Vitamin D deficiency may lead to improvements in behavioural, cognitive, and emotional symptoms, particularly when adequate serum levels are achieved. Although Vitamin D deficiency represents only one of many environmental contributors to ASD, its correction is safe, accessible, and potentially impactful. For these reasons, assessing and optimising Vitamin D levels (both during pregnancy and throughout childhood) should be considered an important component of clinical care and public health strategies aimed at supporting neurodevelopment and improving quality of life for individuals with ASD and their families. Further high-quality research is essential to clarify the extent of these benefits and to refine supplementation protocols tailored to individual needs. Credit author statement: Conceptualization: T.E.; Investigation: T.E.; Writing, reviewing and editing: T.E., M.C., R.A., T.A., C.N., and J.V. All authors have read and agreed to the published version of the manuscript. Funding: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Conflict of Interest: The authors declare that there are no conflicts of interest. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.