I can't eat lettuce at all, terrible D after about half an hour of consumption. It may sound weird, but when it comes to salad food i ask myself if i would give it to my rabbits. Thay can not eat many green leafy foods due to the acid content, tho apparently (i have not tried them on it for fear of being wrong) rabbits can tolerate Romain lettuce. Iceberg lettuce, or "normal" lettuce is very acidic so no wonder it causes us problems when we already have a sensetive tummy. For the same reason i can not also eat any fruit. Shame as i love it so!
I am very glad that you have found something that works for you and i sincerely hope it continues to work for you!
Indigo