Yes, and Yes. God, yes! A good diet is a godsend!! ! Trouble is what works for one may not work for another. The key is listening to your body, and using a bit of common sense.
I think people with ASDs are more senstive to diet, so they need first to get checked out for food allergies. Then they ought eliminate as much processed food as possible, in particular anything with:
1 - MSG, or monosodium glutamate, which may or may not be listed. It is only listed if it's been added in pure form. But it occurs in many additives as the result of industrial processing, which often employ high heat or enzymes that create MSG from the common amino acid L-glutamine. "Hidden" MSG comes in many forms, but mainly in forms of processed vegetable additives (mostly from soy or corn) or industrial byproducts (like hydrolyzed additives). If you see an ingredient listed that has been hydrolyzed, autolyzed, "malted," or "fermented" (which means treated with industrial enzymes, and not naturally fermented in most cases), it likey too is a source of hidden MSG.
2 - Processed sugars, like corn syrup, maltodextrin, dextrose, etc.
3 - Aritifical sweeteners, especially aspartamine, which like MSG, is an "excitotoxin" that can effect people with sensitive neuro-makeups more. I recently kicked my aspartamine habit, which wasn't easy, but worth it.
4 - Refined carbs, like white rice, processed potatos, and bleached flour. Because of the low fiber content of these carbs, these refined carbs can upset your blood sugar level and PH balance of your GI tract, making your metabolism more inefficiant and puts you at risk of many health problems. They are also very low in nutrition (even if "enriched") yet high in calories, so eating a lot of these carbs is like straving yourself while making yourself fat and unhealthy.
I am also of the opinion that people with ASDs should avoid soy (which contains hormones that mimic estrogen) and beef that comes from hormone-injected cattle, because the hormonal affect of these foods may be more severe in people who are more sensitive neurologically. However, I know this is a controversial viewpoint, especially with all the hype about soy and all the claims from the beef industry that these hormone injections are safe for humans. Whatever - my take is, these people aren't looking out for us ASD types. So consume at your own risk!
As for the much touted casein and gluten free diet, I honestly think avoiding what I list above should a proirity. If that doesn't work, then cut out gluten and casien. Honestly, I think people who do go casein/gluten free (as with many other diets) often end up cutting out so much proicecssed foods, that much of the benefits comes from that anyhow.
For myself, what I find I need is a diet that includes minimally processed meat and egg proteins (I'm allergic to certain vegetable proteins, like soy, corn and tree nuts), lots of whole fruits and veggies, varied sources of fats (oils, dairy, eggs, almonds, peanuts, meat), and occasional grains/legumes. I cook or prepare most of my food, so I know what I'm eating.
I eat about 6-8 serving of veggies daily, which is the "core" of my diet. I'll eat either raw or minimally cooked veggies with every meal. I find I function so much better if I get my veggies. One thing I have always struggled with is getting minerals I need - iron, calcium, potassium, etc. Supplements help but only so much. But eating leafy greens twice a day does the job (this includes kale, parsley, turnip greens, chicories, lettuces). I also find I need to eat at least one source of anthocyanins a day, the antioxidant found mostly in blue-red veggies and fruits like purple cabbage, radicchio (a personal favorite), red-leaf lettuces, blood oranges, blackberries, red grapes, and so forth.
Since veggies tend to be low in calories, I get most my calories from proteins, dairy, fats and fruits. On most days, I eat three small meals with three snacks inbetween, and a couple days a week, I limit my intake to under 1000 calories (I seem to respond well to occasional fasting/calorie restriction). If I eat any grains I try to eat whole grain and not very much at a time. If I snack on processed foods, I try to avoid eating too much at once or in a day (I seem to tolerate processed foods if I don't eat them regularly or gorge myself on them).
I have found that fats are very important to my diet. While saturated fats are said to be very bad and must be avoided, I find I need to eat some saturated fats along with other healthier fats. If not, I feel weird and get insane cravings butter. LOL. I usually eat a bit of cheese or have a pat butter with something, but not much more than that. That seems to be enough. I also take an Omega 3 supplement as well as cook with healthy oils like olive and canola oils.