Looking at the COA's, the Isolate and concentrate look to have the same amount of lactose (< 2%). Doesn't isolate usually have zero lactose? Are there any extra ingredients like aminogen in the concentrate?
I'm not extremely sensitive to lactose, but I do know that ON whey gives me a little gas compared to isolates which give me none. Any word on this? I want to give the peanut butter chocolate flavor a try.