The Biome Depletion Theory (BDT), also referred to as the “lost friends theory,” sets forth the notion that practices that arose with post-industrial societies have dramatically altered our biome [38]. According to the BDT these changes, which include modern sanitation, water treatment, usage of toilets, modern medicine, agriculture, soaps, etc., have contributed to dramatic alterations in the human enteric ecosystem. The BDT suggests that disruptions in the enteric macrobiome (i.e., multicellular organisms such as helminthes, protozoans and other fauna) are the key drivers of autoimmune and inflammatory processes, rather than microbiome disruption. The industrial practices and technological advances listed above are believed to have led to the elimination of the macrobiome organisms which co-evolved with humans over centuries, and, through natural selection, contributed to the stabilization and support of our immune system. This theory proposes that eradication of macrobiome organisms has led to a dramatic increase in hyperimmune associated allergic and autoimmune disease. Thus, “biome depletion” refers to the loss of macrobiome organisms from the biome, which in turn has led to decreased exposure to these organisms in post-industrial societies and that their loss has led to immune de-stabilization. Furthermore, there is evidence that helminthes co-evolved with the microbiota to provide modulatory support to the immune system and that host-microbe-helminth interactions are key factors in immune system regulatory networks [39 - 42].

The BDT grew out of the human hygiene hypothesis [43 - 45] which postulates that increased susceptibility to allergic disease is caused by a lack of exposure to symbiotic microorganisms, infectious agents, and parasites by virtue of our modern “cleanliness”. Bilbo et al. [46] postulate that biome depletion, in and of itself, may not necessarily be causative for disease, but when added to the mix of other factors such as westernized low vegetable fiber/ pro-inflammatory diets, sedentary lifestyles, chronic psychological stress, and/or Vitamin D deficiency, may, in combination, alter the immune system towards a hyperreactive and/or sensitive state (Fig. 1). This could, in turn, increase the likelihood of disease. Furthermore, Bilbo et al. [46] have used an analogy of the ecosystem of the human body as a three-legged stool in which the macrobiome is one leg, the immune system is another, and the microbiome is the third leg. Fauna and protozoans, as constituents of the macrobiome, are not placed in the microbiome because of their mutual and similar effects on the immune system and because they are not considered to be a part of the modern microbiome since modern practices of industrialized nations have made them extinct from the intestinal ecosystem of most western societies. If there is disruption in any of these three areas, the ecosystem becomes destabilized and can lead to proinflammatory, allergic, and/or autoimmune related conditions.

BDT hypothesizes that alterations of the microbiome are a consequence of eradication of helminthes, protozoans and other symbionts that co-evolved with humans to regulate, stabilize, and provide modulatory activity for our immune system; their loss is what is hypothesized to drive the disease processes. Conceptually, the microbiome works with the other legs of the stool to promote stability of the human body’s ecosystem and immune function. The BDT hypothesizes that reconstitution of the human biome via reintroduction of helminthes or other macrobiome constituents is a reasonable and necessary step in order to ward off allergic, autoimmune, and inflammatory disease processes.

Bilbo et al. [38] postulate that ASD may fall into a larger family of hyperimmune-mediated disorders that are a consequence of post-industrial society. In this sense, this may be considered a modern pandemic that may be related, in part, due to biome depletion and associated factors. They propose a model in which ASD may be a result of a “three hit paradigm” (Fig. 2).

The BDT is consistent with the “three hit paradigm”:

• • Hit 1: Biome depletion.
• • Hit 2: Environmental stimulus (e.g. acetaminophen exposure, vitamin D deficiency, antibiotic exposure, etc.) at critical times in development [47, 48].
• • Hit 3: Genetic and/or epigenetic predisposition.

Bilbo et al. [38] suggest that this theory can be tested using simple steps. The idea is to determine if inflammatory and/or cognitive diseases of childhood decline in response to steps which are aimed at enriching the biome. Examples of potential efforts at “biome reconstitution” include probiotics, microbiome transplantations, helminth reintroduction to the human ecosystem [49, 50], and/or minimizing the effects of chronic psychological stress, avoiding inflammatory diets, and decreasing sedentary lifestyles.

The propionic acid (PPA) theory of ASD [51 - 53] suggests that ASDs may be a result of disturbances in the enteric microbiome resulting in the increased production of the short chain fatty acid (SCFA) known as PPA. Microbes which have been reported to be in abundance in certain subgroups of ASD patient cohorts include Clostridia, Bacteroides, and Desulfovibrio species [54]. PPA as well as other SCFAs can alter diverse metabolic and immune pathways, gene expression and synaptic plasticity, in a manner that is consistent with findings of ASD. Particularly interesting is upregulation of cyclic adenosine monophosphate response element binding protein (CREB), which plays a major regulatory role in synaptic plasticity and the neurobiology of seizures, movement, as well as reward systems and memory [55]. MacFabe [3, 36] has proposed that phosphorylation dependent upregulation of CREB activity results in enhancement of memory, causing an “inability to forget” which could explain anxiety, perseverative, and repetitive behaviors which are partially diagnostic for ASD.

MacFabe and colleagues [36, 50, 56] concentrated on PPA as an important enteric SCFA increased in ASD stool, and produced directly or indirectly by many ASD associated bacteria (i.e. clostridial & desulfovibrio species), following the fermentation of refined and wheat based sugars. Furthermore, PPA is a ubiquitous molecule that naturally occurs in many food products (wheat and dairy), and is also added to many refined foods as a preservative. In addition, it is an important intermediate of fatty acid energy metabolism in humans [57], and is known to be associated with particular inborn metabolic errors of metabolism, many of which are underreported (propionic and methylmalonic acidurias, holocarboxylase deficiency, biotinidase deficiency), and show some clinical similarities to ASD [58]. PPA has broad effects in brain, gut and immune systems, including specific activation of free fatty acid G coupled receptors, neurotransmitter synthesis and release, innate immunity, mitochondrial/lipid function, gap/tight junctional gating, and alteration of gene expression, all of which have been linked to ASD [50]. Further, since PPA is a small, lipid-soluble molecule, it can easily pass across membranes by both passive and active means (i.e. monocarboxlylate receptors). It is also rapidly metabolized. Thus, it could have both acute and transient effects on metabolic systems throughout the lifecycle, which could account for clinical reports of waxing and waning of behavior in children with ASD after consumption of certain food products rich in refined dietary carbohydrates.

To test the central effect of PPA, MacFabe et al. [57] infused PPA, related SCFAs (butyrate, acetate) and other control compounds (propanol) into the central nervous system (CNS) of adult rats via brief intracerebroventricular (ICV) pulses. ICV PPA infusions resulted in reversible ASD-like behaviors (repetitive, impaired social, object preference, perseveration) as well as electrographic and metabolic changes (innate neuroinflammation, lipid, redox), similar to those reported in ASD [57]. Further, MacFabe has presented a developmental model of systemic PPA exposure (subcutaneous, intraperitoneal) where prenatal and postnatal exposure to PPA results in enduring ASD-like behaviors in a sexually dimorphic manner [59 - 61]. Furthermore in vitro work using rat pheochromocytoma (PC12) cells, have shown that PPA, and to a lesser extent butyrate, is capable of altering expression of a broad number of genes and genetic pathways implicated in ASD [51]. Although preliminary, these findings comprise a compelling microbiome-gut-brain connection that may, in part, be related to aberrant enteric microbiome species, their metabolic end-products or altered metabolism associated with ASD [8, 19, 32, 34, 62 - 65]. This naturally leads to the speculative notion of whether preservation or manipulation of the enteric microbiome or its metabolic end-products could be a meaningful and worthwhile endeavor to reduce risk of ASD or improve ASD symptoms [9]. To this end, it has also been speculated as to whether abnormal enteric gut species could play a role in the etiology of ASD, particularly regressive ASD. It may also offer a potential explanation of why certain diet manipulations may provide therapeutic benefit for certain children with ASD as modifying the gut ecosystem through dietary changes may influence the taxa or metabolic fermentation products of the microbiome.

Further, the PPA theory of ASD puts forth the compelling notion that autistic regression may be a result of multiple interacting factors that present at critical times in development. MacFabe and others have proposed that factors such as C-section, antibiotic use and hospitalization in early life, may be linked to increased risk of developing ASD through early disruption of the normal development of the infant microbiome. Of note, antibiotics can disrupt the microbiome as well as the gut and metabolic systems in several ways to promote disease. Recurrent antibiotic use is well known to increase the colonization of clostridia species, particularly Clostridium Difficile. However, long term administration of antibiotics (e.g. beta-lactams) for routine pediatric infections (i.e. upper respiratory infections) can also disrupt carnitine metabolism and directly inhibit mitochondrial function and carnitine uptake, resulting in detrimental metabolic effects [19, 66]. Furthermore, antibiotics can have proinflammatory effects in the gut by impairing the activity of tight junctions and increasing the diffusion of immunogenic molecules from the gut lumen into the body. All of these factors could combine to cause immune and metabolic disturbances especially if certain other genetic and environmental factors exist. Fig. (3) outlines the broad physiological effects of PPA.

If the PPA model holds to be valid, then there is the theoretical potential that at least some forms of ASD may comprise a family of inherited and/or acquired metabolic disorders that are potentially treatable and possibly even preventable. In later sections of this review, we will touch on treatments that target certain aspects of the PPA model and their efficacy at addressing certain metabolic and behavioral features of ASD.

The Cell Danger Response (CDR) theory put forth by Naviaux [12, 37, 67] refers to an adaptive response by the cell to a perceived threat. The potential pathological nature of this response occurs when the CDR becomes chronic (i.e. persisting after the threat has dissipated). The CDR presumes that extracellular adenosine triphosphate (ATP) is a cellular purinergic signal for external “danger”. Since, under normal conditions, ATP is predominantly intracellular, the presence of this molecule in the extracellular environment is an indicator of local cellular (tissue) destruction. Thus, when non-damaged cells detect extracellular ATP they shift cellular functions toward a protective cellular response repertoire. These changes include changes in “cellular electron flow, oxygen consumption, redox metabolism, membrane fluidity, lipid dynamics, bioenergetics, carbon and sulfur resource allocation, protein folding and aggravation, vitamin availability, metal homeostasis, pterin, and 1-carbon metabolism, and polymer formation” [12]. If the CDR remains activated, then systemic metabolism may be disrupted, resulting in alterations in the enteric microbiome and the initiation of chronic disease processes.

The proposed triggers for the CDR in contemporary society include changes in human migratory patterns, diet, toxin exposures and cultural activity that have occurred over the past 150-300 years. The CDR proposes an imbalance between cellular physiology and the modern environment, a so-called “metabolic mismatch”. In an evolutionary sense, the CDR posits that the surrounding ecosystems have changed faster than species can adapt. Thus, the CDR predicts that cells are adapting to external changes by alterations in physiology such as upregulation of mitochondrial function and altering epigenetic regulation [68, 69]. If this theory holds true, it could account for a unique pattern of mitochondrial dysfunction that has been associated with ASD by multiple groups [7, 57, 70 - 81].

To test the proposed connection between the CDR and ASD, Naviaux et al. used two well-accepted mouse models of ASD, one environmentally-triggered model of ASD, the poly(I:C) mouse model [37], and a genetic mouse model of ASD, the FMR1 knockout (KO) model. The FMR1 KO is a model of Fragile X syndrome, a genetic disorder with a high rate of ASD [82]. ASD behaviors were reduced in both mouse models of ASD using the antipurinergic, antiparasitic and antifungal drug suramin. Suramin is thought to antagonize extracellular ATP signaling, thereby reducing the key signal which initiates and maintains the CDR [83]. In the Fragile X mouse, suramin not only corrected behavior but also corrected the pathophysiology associated with ASD, leading to improvements in metabolism (e.g. purines, microbiome, s-adenosylmethionine/s-adenosylhomocysteine (SAM/SAH), glutathione, nicotinamide adenine dinucleotide (NAD+), fatty acid oxidation, glycolysis, eicosanoids, gangliosides, cholesterol metabolism, phospholipids) and synaptic structural abnormalities [82]. Both behavioral and metabolic abnormalities were also corrected in the poly(I:C) mouse model. Using these two models Naviaux et al. showed that both environmental and genetic factors that cause ASD-like symptoms in animals have metabolic components consistent with the CDR theory, and that these metabolic abnormalities can be remediated by antagonizing extracellular purinergic signaling (i.e. extracellular ATP). Further, both models showed that one of the primary metabolic shifts related to suramin treatment, which presumably leads to behavioral changes, was through alteration of metabolites from the microbiome. As Naviaux predicts that the CDR shifts full body metabolism and in turn alters the host microbiome. The evidence for suramin altering extracellular purinergic signaling and host microbiome is promising.

It is significant to note that a single dose of suramin also resolved autistic-like behaviors in an adult mouse model of ASD, which may mean that, at least in rodents, if the underlying core biological process can be addressed, then timing related to when the intervention is given may not matter [82]; this challenges the traditional concept in medicine of a “static encephalopathy” which arises from that idea that neurological circuits are irreversibly “hard wired” if critical time windows of development are passed without achieving certain developmental milestones. These are promising findings that need to be confirmed in humans. To this end, a small clinical trial has examined the safety of suramin in patients with ASD (clinicaltrials.gov/ct2/show/NCT02508259). The results of this trial will be highly anticipated.

Many classes of drugs effect mitochondrial function and may be toxic to the mitochondria. These include certain anesthetic agents, antipsychotic medications, certain classes of antibiotics, non-steroidal anti-inflammatory drugs (NSAIDs), anticonvulsants, statins, steroids, proton pump inhibitors (i.e. Omeprazole) and certain cancer medications [66, 84, 85]. Other known mitochondrial toxins include pesticides and herbicides that have been theorized to play a part in ASD etiology and/or pathophysiology [86 - 92]. While this review will not cover the full mechanisms of action and/or a detailed breakdown of each drug, drug class, and their potential mechanisms of action, we will briefly acknowledge that any of these agents, if given during pregnancy and/or early postnatal life, may contribute to the risk of developing ASD.

Both classic and novel types of mitochondrial dysfunction appear to be prominent in ASD. First, there is a high rate of the diagnosis of classic mitochondrial disease in ASD compared to the general population, with a higher rate of biomarkers of mitochondrial disease [93] (See www.nature.com/mp/journal/v17/n3/full/mp2010136a.html). Secondly, studies have reported high rates of electron transport chain dysfunction in children with ASD in immune cells [94]. Third, there are several novel forms of mitochondrial dysfunction associated with ASD that may be acquired as a consequence of environmental influences. Indeed, one type of mitochondrial dysfunction is characterized by unique abnormalities in citric acid cycle, redox and acyl-carnitine profiles consistent with the abnormalities found in the PPA model of ASD [19, 57] (See www.nature.com/tp/journal/v3/n1/full/tp2012143a.html and www.microbecolhealthdis.net/index.php/mehd/article/view/27458). In another type of novel mitochondrial dysfunction, the mitochondrial respiratory rate associated with ATP production is about twice that of controls [72, 74]. Interestingly, these overactive mitochondria are sensitive to increased reactive oxygen species and it is believed this increase in mitochondrial respiration is an adaptive and/or compensatory change of the cell to protect itself from potentially harmful environmental triggers [70, 71, 74] See www.nature.com/tp/journal/v4/n4/full/tp201415a.html and www.journals.plos.org/plosone/article?id=10.1371/journal.pone.0085436).

Whether mitochondrial dysfunction is an etiological risk factor for ASD or whether it is a pathophysiological process that results from the underlying process that causes ASD is unknown, but it is interesting in that the CDR predicts that the mitochondria would be in a hyperactive state. The fact that mitochondrial dysfunction appears to only affect a subset of children with ASD and not the whole ASD population highlights the fact that many subsets of pathophysiological processes are most likely involved in ASD. Indeed, this may be a reason why the CDR may only be partially correct and other theories, such as the BDT, 4-EPS and/or PPA theory, may be complementary. Furthermore, these theories may not be mutually exclusive theories and may overlap.

It has been suggested that mitochondrial toxins and/or modifiers may be potential risk factors for ASD (Table 1). Interestingly, the only two FDA approved medications for ASD are solely indicated for non-core behavioral abnormalities (i.e. irritability) associated with ASD. Both of these new-generation “atypical” antipsychotic medications have detrimental effects on mitochondrial function [95 - 97]. The adverse effect profiles of atypical antipsychotics include weight gain, alterations in lipid and glucose metabolism and increases the risk for obesity, metabolic syndrome, tardive dyskinesias and type II diabetes [98 - 102]. These prominent adverse effects no doubt involve deleterious effects on mitochondrial function, although limited research has examined the role that the mitochondria play in adverse effects of antipsychotic medications. Thus, it is of supreme importance to find biomarkers that may predict which children are more likely to be affected negatively by these medica-tions. Interestingly, a recent study showed that the weight gain resulting from olanzapine, an atypical antipsychotic, was associated with microbiota disturbances and that weight gain was attenuated by antibiotic administration in a rat model [103]. Furthermore Bahr et al. [104] showed that risperidone, another second generation atypical antipsychotic, demonstrated increased weight gain and an altered enteric microbiota in children via increased ratios of Firmicutes:Bacteroidetes when compared to antipsychotic naive children. Interestingly, this ratio has also been correlated to the development of obesity and risk for Type II diabetes and the inverse ratio has been associated with weight loss [105, 106].

Bactericidal antibiotics (quinolones, aminoglycosides, and β-lactams) induce reactive oxygen species (ROS) and cause mitochondrial dysfunction in mammalian cells. This effect can be blunted with pre-treatment with N-acetyl-L-cysteine (NAC) or through the usage of bacteriostatic antibiotics [107]. Interestingly, Rose et al. [70, 71] demonstrated that ROS induced mitochondrial dysfunction in lymphoblastoid cell lines (LCLs) from children with ASD could be prevented with NAC pretreatment. It is possible that ROS produced by bactericidal antibiotics during illness may induce mitochondrial dysfunction and that NAC could be given in combination with the antibiotics to minimize the negative effects on host mitochondria and possibly even prevent antibiotic induced metabolic disturbances.

In addition, the use of both beta lactam antibiotics and omeprazole [66], which are commonly used for routine infections and gastroesophageal reflux, respectfully, impair carnitine metabolism and deserve re-evaluation as potential contributors to mitochondrial dysfunction and ASD, particularly the regressive-subtype.

Acetaminophen is widely used in obstetrics and pediatric medicine. Although it is an approved medication in North America for over a century, its mechanism(s) of action are still not fully elucidated. Recently, its safety and efficacy has come into question. Indeed, its effectiveness as an anti-inflammatory and/or antipyretic has demonstrated questionable efficacy and effectiveness to date. It is important to note that other agents have demonstrated similar and/or better safety and tolerability profiles for chronic use [108]. Acetaminophen has also recently been under scrutiny related to associations between prenatal exposure and hyperkinetic and attention deficit hyperactivity disorders [109, 110]. As emerging evidence is starting to show through multiple empirical pre-clinical studies, case reviews, and theoretical models, acetaminophen may have potentially catastrophic consequences on the development of metabolic and neural systems if used during pregnancy or during the perinatal or postnatal period. If acetaminophen’s risk profile has changed over time or whether we are better at detecting problems now is of considerable debate, but if biome depletion and missing microbes are to play, then there should be evidence that shows its effects on the microbiome and its interconnected metabolic constituents. Interestingly enough, p-Cresol, the human analogue to 4EPS described by Hsiao et al. [25], which has been shown to be elevated in urine of patients with ASD has been linked to acetaminophen toxicity via individual variations in the microbiome [111]. Clostridium difficile metabolizes tyrosine to p-cresol which competes with acetaminophen for sulfuration in the gut. Researchers found that the ratio of sulfunated to glucorinated p-cresol in the urine was predictive of acetaminophen toxicity [112, 113].

Because of the poor glutathione and methylation status in children with ASD and their first-degree relatives [114 - 117], individuals with ASD and their relatives may be especially vulnerable to acetaminophen’s effects on sulfuration as described above.

It is interesting to note that the etiological basis of ASD has been speculated to occur via activation of the endocannabinoid system (ECS) by acetaminophen at critical time points during development [118]. Another interesting finding is that some children with ASD seem to have transient improvements in their ASD symptoms during bouts of fever [119 - 122]. Since activation of the CB₁ receptors appear to be involved in the fever response, it is possible that the ECS may be activated in ASD during fever [123, 124].

Furthermore, the PPA rodent model has also shown impairments in glutathione redox metabolism suggesting that SCFAs may render an individual more sensitive to further glutathione depletion via acetaminophen exposure, and result in further sensitivity to a broad range of xenobiotics [19, 36, 51].

As acetaminophen may be poorly metabolized by a subset of children with ASD [125] and there may be physiological disruptions that have been associated with ASD that would make individuals with ASD poor metabolizers in the first place, it may be wise to limit its use in individuals with ASD.

Furthermore, due to the many possible mechanisms by which acetaminophen may promote the formation of ASD, we would strongly suggest that it be critically re-examined for its use in obstetric and pediatric care due to its potential effects on the developing fetus and child.

Many lines of evidence have pointed to the importance of preconception and prenatal folic acid in the prevention of ASD [103, 126], however there are inconsistent results [127] which may have arisen due to many factors (e.g. abnormalities in genetic, epigenetic, and/or immune system function, etc.) [128]. Folate status is complex and is affected by diet [129] and autoantibodies that may affect its transportation into the brain and across the placenta [130, 131]. Genetic polymorphisms in folate metabolism in children and their mothers modulate the risk of developing ASD [132]. Furthermore, folate is produced by the microbiome (i.e. Bifidobacteria). If there are disruptions in the mother’s microbiome during pregnancy, this may impact folate production and in turn lead to increased risk for ASD and explain why higher doses of folates protect against developing ASD. Thus, the number of factors that may influence folate metabolism to increased ASD risk is complex.

It is important to understand that Folic Acid is an oxidized form of folate that needs to be reduced in order to be utilized by the body. Frye et al. [133] found a high percentage of folate receptor alpha autoantibodies in children with ASD. Many of the mothers of the children also demonstrated these autoantibodies. Interestingly, this autoantibody blocks the transport of folate into the brain, requiring treatment with a reduced form of folate known as folinic acid (leucovorin calcium). Children with ASD treated with folinic acid in an open-label fashion for 3 months at 2mg/kg/day in divided doses demonstrated a significant positive behavioral responses in core ASD symptoms [133]. This data has been further validated in both a double-blind placebo controlled and open-label clinical trials [134, 135]. These data suggest that reduced folates may be advantageous and therapeutic to children with ASD since they may restore perturbations in folate metabolism. Likewise, since many of these same abnormalities have been found in mothers of children with ASD, reduced forms of folate may be advantageous for some women to decrease the risks of having offspring with ASD. In any case, further study on folate derivatives, which may be able to treat abnormalities in folate metabolism in children and mothers are warranted.

One area of research that is ripe for exploration is the empirical examination of folate pathway abnormalities, including folate receptor alpha autoantibodies and/or polymorphisms in folate and folate-related genes in both mother and offspring in order to formulate treatment plans which minimize the effect of dietary and other environmentally triggering factors (e.g. acetaminophen usage [109, 110, 118, 136 - 139], types of prescribed antibiotics [107], avoidance of pro-inflammatory foods [140 - 143], bovine milk [129], etc.). One important strategy would be to prophylactically treat the child and/or mother with a reduced type of folate, such as folinic acid, either to prevent ASD all together or decrease symptom burden and improve outcomes. Further, as mentioned earlier, since the mother’s microbiome may be disrupted and alter folate production, this may be another reason why high dose folates may be advantageous as a prevention strategy during pregnancy. Also since children with ASD have been shown to have abnormal enteric microbiomes, folates may be able to improve biochemical pathways when folate production in the gut is limited. While these points are purely theoretical at this time, it would not be a difficult study from a methodological stance to follow large cohorts longitudinally to see if ASD incidence and prevalence rates go down in response to the proposed strategies listed above.

While folate has been documented to have a significant role during pregnancy in contributing to ASD risk, there are other factors that have been suggested in the literature that are also important. These will be reviewed below.

Other possible factors that could influence the maternal microbiome and affect infant development that could be putatively related to the micobiome include prenatal toxin exposure and maternal sleep hygiene. These will be reviewed below.

An extremely important influence on colonization and the constitution of the enteric microbiome is breast feeding. Breast milk is a primary prebiotic source that provides a rich source of oligosaccharides and is a known major postnatal contributor to the development of the immune system [210]. While the infants are unable to digest these oligosaccharides, important bacteria such as B. infantis utilize these carbohydrates to help establish a diverse infant microbiome. As stated above, the first colonization with B. infantis comes from the vaginal canal, if the baby is vaginally delivered. Insufficient perinatal breastfeeding is correlated with subsequent development of ADHD [211]. There is both positive and negative data linking insufficient breastfeeding to ASD risk [212, 213]. As stated earlier, it’s also important to note that there is probably no single factor, except for in rare events that, in isolation, accounts for ASD. Therapeutically speaking, breastfeeding is a safe practice that may contribute to the formation of a healthy microbiome that may decrease ASD risk.

The development of the human microbiome begins before birth and at age 3 years old appears to resemble the adult microbiome [214]. It may be that the critical timeframe to establish the microbiome may be as early as the first 28 days of life. Since there seems to be a correlation between the development of the microbiome and the development of ASD, the gestational factors listed in this review should also be considered as possible environmental factors that could influence ASD risk in the perinatal and postnatal setting as well. Therefore, caution related to these factors is warranted, although evidence is still needed to firmly associate them with ASD risk.

Above we highlight certain important triggering events in the prenatal, neonatal, and early life related to metabolic alterations that have been linked to ASDs through various interconnected pathways including the enteric microbiome, mitochondrial function, and oxidative stress related interactions. Table 2 outlines possible remediation strategies that may decrease ASD prevalence by minimizing the effects on the developing child’s metabolic systems.