Vegan diets are often promoted as an alternative treatment or even as a “cure” for Autism Spectrum Disorder.
As is often the case with promotion of vegan diets, there is often little thought given to how suitable such a restrictive diet pattern may actually be for certain individuals with specialized nutritional needs, such as those with Autism.
To date I’m not entirely convinced that any of the vegan proponents that I have talked with, truly understand the complex etiology of autism and the unique bio-chemical differences these individuals typically exhibit, in order to be able to tailor a “well-planned” vegan diet for those with Autism.
Individuals with Autism often have a myriad of bio-chemical dysfunctions such as impaired detoxification pathways, methylation dysfunction to genetic errors in processing certain vitamins and nutrients.
This makes the need for having a robust, balanced, nutritionally complete diet extremely important, unless you want to further exacerbate these organic dysfunctions in the body such as hypomethylation and transsulfuration function.
Following a sub-optimal diet such as strict vegan, which may be deficient or lacking many conditional essential nutrients such as taurine, may be a recipe for disaster for individuals who are already typically depleted in these nutrients.
Here are five reasons why i believe that strict vegetarian and vegan diets are not bio-chemically suitable diets for individuals with Autism spectrum disorder and many other health conditions such as chronic fatigue syndrome, fibromyalgia and multiple sclerosis.
1. Taurine Deficiency, Transsulfuration Metabolites & Autism Spectrum Disorder
Participants diagnosed with ASDs had significantly decreased plasma reduced glutathione(GSH), plasma cysteine, plasma taurine, plasma sulfate, and plasma free sulfate relative to controls. 
Depletion of these nutrients and antioxidants such as glutathione is thought to correlate with increased oxidative stress in the body, another proven part of the complex etiology of autism.
Studies have found taurine levels to be significantly lower in vegans than in a control group on a standard American diet. Plasma taurine was 78% of control values, and urinary taurine was 29%.
Prolonged absence of dietary taurine intake causes decreased plasma taurine and severely restricted urinary taurine output. 
Vegan diets typically provide no intake of the conditional essential amino acid taurine and low protein vegan diets can often be lacking in precursor amino acids such as cysteine, exacerbating this issue further.
The consequences of following a taurine restricted diet for individuals with Autism are likely to be increased oxidative stress and a further decrease in detoxification capacity, particularly of mercury.
2. Copper & Zinc Trace Element Imbalances
Several studies have suggested a disturbance in the copper (Cu) and zinc (Zn) metabolism in ASDs.
Zinc deficiency, excess Copper levels, and low Zn/Cu ratio are common in children diagnosed with an ASD. 
Vegetarian diets in general are exceptionally rich in total copper, whilst often being relatively low in zinc by balance.
Strict vegan diets are even more prone to this issue, especially fad raw vegan diets, which often severely restrict or remove the few zinc rich raw vegan dietary sources that there are such as nuts, seeds, beans, legumes and pulses.
Most plant-foods which are good sources of zinc, also tend to be equally as rich in the mineral copper. So this can make it very difficult to get an ideal balance of zinc to copper on strict vegetarian diets, especially for individuals who may already be suffering from disturbances in copper/zinc metabolism.
This is why many vegans and vegetarians often have to resort to synthetic zinc supplementation in order to maintain a healthy zinc status as a strict vegan.
Anecdotally I believe that the copper and zinc mineral imbalance issue may be a key factor as to why many individuals become emotionally unstable, aggressive, paranoid, anxious or experience a worsening of mental, emotional and behavioral symptoms when following strict vegan diets.
3. Vitamin B12 Deficiency, Hypomethylation & Autism Spectrum Disorder
What is even more interesting is that the research has found that parents of children with Autism, also share similar metabolic deficits in methylation capacity and glutathione-dependent antioxidant/detoxification capacity. 
It is thought that genetic polymorphisms affecting enzymes in transmethylation and transsulfuration metabolic pathways may contribute, in part, to this metabolic imbalance.
Many studies have now found elevated levels of Hcy(homocysteine) in individuals with Autism Spectrum Disorder. Hyperhomocysteinemia is a consequence of hypomethylation and is also considered to be a proven independent cardiovascular disease risk factor.
Strict vegan diets typically provide no reliable dietary source of cobalamin(Vitamin B12), which is an extremely important nutrient for healthy methylation function and homocysteine metabolism.
As such, it’s not surprising that the majority of the scientific literature has concluded that a high percentage of vegans suffer from sub-optimal Vitamin B12 status, clinical deficiency and higher homocysteine levels than both vegetarian and omnivore counterparts.
Studies have also found decreased brain levels of Vitamin B12 in neurodevelopmental disorders such as autism and mental health condition such as schizophrenia.
Levels of methylcobalamin(MeCbl) and adenosylcobalamin(AdoCbl) were found to be around 3 times lower in those with Autism compared to age-matched controls.
In autistic subjects lower MeCbl was associated with decreased MS(methionine synthase) activity and elevated levels of its substrate homocysteine (HCY). 
Following a strict vegan diet which may not provide any reliable daily, dietary intake of active forms of Vitamin B12 such as methylcobalamin and adenosylcobalamin, is again potentially very risky for those with already diminished methylation capacity and potentially low levels of Vitamin B12 in the brain.
4. Vitamin D Deficiency, Autism Spectrum Disorder & Vegan Diets
Other research indicates that restoring Vitamin D levels may also improve some symptoms of Autism Spectrum Disorder.
I recently compiled an article with the latest scientific research on Vitamin D deficiency & Autism Spectrum Disorder, which can be found at: Vitamin D Deficiency & Autism Spectrum Disorder
Vegan diets typically don’t provide any reliable dietary source of Vitamin D and as a result can further exacerbate deficiency states. Many studies have now found lower Vitamin D levels in vegans compared to omnivores.
“In the EPIC-Oxford study, vegans had the lowest mean intake of vitamin D (0.88 μg/d), a value one-fourth the mean intake of omnivores.” 
Oily fish such as sardines are an excellent and rare natural dietary source of Vitamin D3, providing around 50% of the DRI(daily recommended intake) per typical portion.
Oily fish such as sardines also contain many other important nutrients that vegans and individuals with autism may be low in such as Vitamin B12, long-chain Omega-3 fatty acids(DHA), iodine and Co-Enzyme Q10 for example.
5. Mitochondrial Dysfunction + Vegan Diets Are Lacking In Mitochondrial Nutrients(Taurine, Carnitine & Co-Enzyme Q10)
Cutting edge research has discovered that mitochondrial dysfunction may be the missing link in the puzzle that can connect the diverse medical symptoms and complex etiology of Autism Spectrum Disorder. 
Strict vegan diets can be lacking in many nutrients that are very important for healthy mitochondrial function.
Some of these include taurine as discussed above, carnitine which is another conditional essential amino acid and the antioxidant Co-Enzyme Q10.
Animal foods such as oily fish, eggs, lean poultry and grass-fed meat are typically the most dense sources of the above nutrients.
Now whilst these conditional essential nutrients may not necessarily be of concern for the average healthy individual, it may well prove to be a different story for those who are already deficient and depleted in these nutrients.
It doesn’t make much sense if you are already suffering from plasma Co-Enzyme Q10 or taurine deficiency, to then go follow a diet pattern which is deficient or lacking in these nutrients.
 A prospective study of transsulfuration biomarkers in autistic disorders.
 Plasma and urine taurine levels in vegans.
 The role of zinc and copper in autism spectrum disorders.
 Abnormal transmethylation/transsulfuration metabolism and DNA hypomethylation among parents of children with autism
 Decreased Brain Levels of Vitamin B12 in Aging, Autism and Schizophrenia
 Health effects of vegan diets
 Mitochondrial dysfunction can connect the diverse medical symptoms associated with autism spectrum disorders
 Mitochondrial Dysfunction in Autism
The information in this article has not been evaluated by the FDA and should not be used to diagnose, cure or treat any disease, implied or otherwise.