MPM: Most protein shakes give me gastric problems. I'm allergic to the whey protein. It didn't happen immediately but after a while (a shake a-day, not continually but maybe for 2 weeks, a 2 week break, and then another week or 2), Then I had diarrheamost days. It took a while to work out the connection. I stopped the shakes and quickly recovered.
Many recipes for sweet things (pancakes, cakes, cookies) use whey protein and they do the same thing. I've never lost the problem. My immune system remembers!