From time to time genuine scientists who are motivated to help people with autism make discoveries of simple interventions of great promise. These are interventions that don't cure autism, but the managing the condition just a little easier. One such discovery has been the relationship of MTHFR mutations to the vitamin treatment of autistic spectrum disorders. 
Autism is a condition that is responsive to brain and bloodstream levels of a chemical called homocysteine . You may have heard of it. High homocysteine is associated with an increased risk of heart disease. It's also in a class of problem chemicals known as excitotoxins . Homocysteine can literally "excite" the autistic brain . If you are the caretaker of an autistic individual, probably you will agree you have enough excitement already.
High homocysteine levels are also associated with sleep problems and gastrointestinal upset . Lower homocysteine levels are associated with better sleep and fewer gastrointestinal problems. Obviously, getting concentrations of this excitotoxin down as much as possible is a worthwhile goal in nutrition for people who have autism.
Unfortunately, that can be the problem. Well-intentioned parents, guardians, nutritionists, and doctors give people with autism too much vitamin B9, which is also known as folate (when it's found in food) or folic acid (when it's the synthetic form found in supplements).
What's the Problem with Folate and Folic Acid in Autism?
Folate in food and folic acid in synthetic vitamin supplements aren't the form of vitamin B9 the body can actually use . These forms of the vitamin can't be used by cells, or even absorbed by cells, at least not in adequate amounts, until they have been transformed by an enzyme known as MTHFR. The acronym stands for methylenetetrahydrofolate reductase.
If your body makes MTHFR properly, then both folate and folic acid do their job as the vital nutrient methylfolate. If an autistic person has one of four MTHFR mutations, however, there are slight defects in the protein that makes up the enzyme that keep folic acid and folate from being transformed into their active state .
The inability to use B9 affects the body's ability to use B6  and B12 . There are other mutations that can affect the enzymes that govern the body's utilization of those vitamins, but it's primarily B9 that is the problem when homocysteine levels are high.
So What's the Proper Vitamin Supplement for Autism When There's an MTHFR Mutation?
Vitamins are vital. Everybody needs vitamin B9. If you have an MTHFR mutation, however, excessive B9 is toxic. Too much folic acid from fortified white flour, or even potentially too many green leafy vegetables (over 12 servings a day) could put so much folic acid and folate into the body that it can't use that cells aren't able to absorb the methylfolate they need.
That's why there are two steps in lowering the homocysteine that can cause sleep problems and diarrhea in people who have autism.
There is a new measurement called a DFE (dietary folate equivalent) that takes into account that it's easier for the body to get B9 from supplements than from food, so you'd really want to make sure any autistic person in your life is not getting more than 700 mcg DFE's per day to avoid toxicity. Take more than that, and folic acid begins to act as an anti-vitamin.
The second step in treating homocysteine problems is to make sure any autistic person does get enough methylfolate. This is a synthetic vitamin, but it's chemically identical to the form of folate or folic acid your body actually uses. A dose of 400 micrograms a day is enough.
It's possible to use the much less expensive homocysteine test as an indicator for treatment, too. If homocysteine is high, then folic acid limitation and methyfolate supplementation are indicated, especially if there is an MTHFR mutation, but even if there isn't. As always, follow your doctor's advice regarding B6 and B12 for high homocysteine.
What Kinds of Changes Can You Expect from Methylfolate Supplementation in Autism?
The science is relatively new, but a clinical trial of giving autistic children methylfolate in China found that this simple, inexpensive supplement increased sociability, improved both preverbal and verbal language, increased attention span, and improved affective (emotional) communication . That's a tall order for a vitamin pill, and you're most likely to get these results when there is an MTHFR mutation that has been causing a problem. But it's such a simple intervention that it is almost certainly worth trying.