I have heard the two biggest food eliminators for autism, ADHD, epilepsy, and a few others are gluten and casein (milk protein). Another major one, as someone mentioned, is MSG.
Do some investigating via the Internet, so that you know specifically what gluten is all in (wheat, rye, barley, oats and their byproducts), what casein (found in cow milk products) is all in (it will surprise you), and ALL the different names MSG is manufactured under. MSG is known as an Excitotoxin. I'm allergic to casein, but am able to eat products with whey; your daughter should be able to as well.
Basically, you need to give up most processed foods and go totally organic. My daughter has Sensory Processing Disorder. We are attempting this conversion; yes, easier said than done, especially since my daughter has food issues as well. However, I muself am soy and gluten intolerant and have a casein allergy, so our household is about as clean as it can be of these ingredients.
My husband also has the casein allergy; he enjoys drinking rice milk. I'm not a big fan of rice milk, but I do like goat milk. We also use goat butter and goat and sheep cheese. Sheep cheese tends to be a little sharp; I buy a hard cheese from Spain that I grate like Parmesan. For goat cheese, I have found a raw milk mild cheddar cheese that, when shredded, looks and tastes just like mozzellera. We buy the bulk of our food at Whole Foods. I know. Just swallow hard and pay up. <grin> We've had to adjust other areas of our finances so that we can afford to buy organic groceries. However, I've seen a difference in all of us, and I especially think it's been helping my daughter.
Another thing I'll toss out there is to have your daughter tested by an Integration Doctor or a Nauropathic doctor for heavy metal posioning. My daughter was very, very high; she's on her 3rd chelation right now, and I'm looking forward to getting her urine retested to see how much lower all her concentrations are (they should be at zero). I was also tested and have gone through 3 chelations myself.
So then I asked myself; if I'm 40 years old and have this much heavy metals in my body, how is it that my 9-year-old daughter can have even MORE than me (be more toxic) if she hasn't lived as long as I have? So I went on the Web and started investigating this and discovered that some people are unable to detoxify their own bodies! Yes, it's true, and there is a genetic test you can take that will inform you whether or not your body can detoxify, whether it is a 50/50--either you can or you can't, or if you're a Poor Metabolizer, which means your body is unable to detoxify just about anything, including prescriptions or OTC meds.
My daughter and I had the genetic test done (CYP 2C9), and we both came back as Poor Metabolizers. Since my daughter has the EXACT same mutations as I do, that means my husband also is a Poor Metabolizer, because my daughter can't have my mutations unless she also gets the exact same mutations from her father. This is possibly why, at the age of 41, I am very ill and my daughter has SPD; her body wasn't able to cope with all of her childhood antibodies for earaches and sore throats and her childhood vaccines.
It's a wonderful thing to know how your body metabolizes, because it makes a world of difference as to how you eat and take care of your body. If you know when you're young that your body is a Poor Metabolizer, it's a lot easier to get rid of the things your body can't handle; thus, you have less reason for your body to be fed things it can't handle (like soy, gluten, casein, other food intolerances, seasonal allergies) and you won't end up with chronic conditions, or anti-inflammatory conditions (like me).
If you'd like my info, or just to chat, you can contact me.