This is long overdue on my part (since Cale is now a year past outgrowing his infant allergies), to collect my thoughts on why GFCF (gluten-free, casein-free) diets generally need to be GFCFSF (add soy-free) in order to be effective. This is crucial because most people replace casein (the main protein in milk) with soy! So why should soy also be eliminated?
Background: Calories come from three sources: fat, carbohydrate (from simple sugars up to complex carbohydrates of whole grains), and protein. Enzymes are specialized proteins that facilitate a chemical reaction, like digestion. Enzymes end with -ase; fat-digesting enzymes are lipases, carbohydrate-digesting enzymes are amylases, and protein-digesting enzymes are proteases. Sugars end with -ose, so the amylase lactase will digest the milk sugar lactose. A peptide is a piece of either a partially- or a fully-digested protein. Most allergies are reactions to proteins, while most reactions to carbohydrates (like lactose intolerance) are intolerances.
Gluten is, very loosely speaking (ref: scroll down to heading titled What is Gluten?), the protein in wheat including spelt, rye, barley, and sometimes oats. Why sometimes oats? Most oats are grown in the same field as wheat and are contaminated with wheat gluten. Note that spelt is a type of wheat; sometimes people confuse "alternative wheat" (meaning not the usual variety) with "alternative to wheat" which spelt is not.
Casein is the predominant protein in milk; the other protein is whey but it is less often allergenic.
Similar:
soy proteins sometimes cross react with antibodies against casein
(from Casein, Gluten, Soy and DPP IV). So if casein causes an allergic reaction via antibodies, soy can often trigger the same antibodies and hence the same allergic reaction. (I replaced milk with rice drink on my cereal.) Apparently the side effects can take up to a year to clear up, too! (Cale cleared up in two-and-a-half days.) Luckily,
there are only trace amounts of [soy] protein in soy lecithin and soybean oil. Therefore, you may want to avoid all other soybean products, but still allow lecithin and oil
(from GF Meals). If you're strict about soy-free, lecithin (which isn't always from soy, but its origins are not always in the ingredients) and oil ("may contain several oils including soybean oil") are the hardest to eliminate from commercial food, so that's a convenient loophole for less-severe reactions.
Caution: One study has shown that self-diagnosed problem foods are often incorrect, so always talk to your doctor! Find a more sympathetic doctor if you need, but always re-test your assumptions so you don't face the the food-free diet. Instead, consider additions (probiotics and enzymes) instead of removals (presumed allergens). A broad palette of added enzymes may not be as healthy as specific enzymes, so start small and assume less. For the whole class of neurodevelopmental disorders, as many as 92% (reference) improved with enzymes targeted for gluten and casein.
Corn: And if that weren't enough fun, corn may also need to be on the GFCFSF list too! Personally, I'm fine as long as the corn I eat is not highly processed.
