Many folks with a milk protein intolerance are also unable to tolerate soy. In fact, a common shorthand for milk protein intolerance is MPSI where the S stands for soy.
In the beginning, Davy couldn't handle soy and I had to eliminate it. This meant no easy dairy substitution to soy milk and soy cheese. No easy solution of eating off vegan menus (which are heavy in soy). No Wheat Thins or many other processed foods which contain soybean oil or soy lecithin. Any "cheating" and he had a full blown GI reaction.
While I was still nursing, I was instructed to add baked in soy back into my diet and see how Davy did. And he was fine! Hallelujah! So we kept experimenting. He still reacted to large amounts of soy like soy sauce or edamame or soy cheese but at least I could eat crackers again.
Once he started solids, we allowed him to have crackers with soy lecithin in them and he was fine. He also does okay with soybean oil. This means he can eat Graham Crackers, puffs (select brands are dairy free - we like Happy Family), animal crackers, etc. We then tried letting him self ingest edamame (soy beans) and it didn't go well. Edamame is considered an easier soy to process since it is pure soy so it is a good first try. But even that was too much for my poor little guy. His first exposure to soy sauce was also a bad experience.
This was an accidental exposure. We were at the mall and the boys had already finished the HUGE lunch I had packed for them so I gave them each a few carrots out of my Hibachi. And Davy turned bright red. Hives all over his chin and face. Luckily, I carry Benadryl!
We stay away from "raw" soy but do allow soy as an added ingredient (like a small amount of soybean oil or soy lecithin). He does occasionally gets a few red marks around his mouth after eating a large amount of soy, but it doesn't move beyond those very few hives. As long as it is small number, we don't even offer Benadryl for this as it doesn't seem to bother Davy at all. This is a huge relief because SOY IS IN EVERYTHING.
We are highly highly fortunate that Davy outgrew the more severe reaction to soy. And are hopeful this means he will someday stop reacting to soy at all!