I can't stress this enough. If you're experiencing digestive stress from your shakes, whether it's one a day or 8 a day makes no difference, you need to switch to a different type of protein. Try a non-dairy protein like beef, egg or soy. Even with a faster transit and digestion rate, it should not be causing you to have diarrhea. That's the equivalent of saying that you get diarrhea from a drinking a gallon of water a day.
Spend $20 or $30 and buy a jug of beef protein isolate and try it for yourself. Ditch the dairy based proteins and you'll see an improvement. I had massive amounts of issues until I switched to beef protein isolate. Now I can drink as many shakes a day as I want and have no negative reactions. No gas , no bloating, no diarrhea, nothing. And if you want to slow down the digestion rate, add a scoop of a fiber supplement that contains plenty of soluble fiber to your shake. That'll slow down digestion and help you feel fuller, longer.
I'm not advocating only drinking shakes, or saying that whole food isn't better. I'm just saying that there's a lot of bashing of shakes going on here, and most of it is due to misinformation. Guys use shakes and have digestive problems, and want to say protein shakes aren't useful, when they don't really understand why they had the issues they had in the first place. You don't say that all cars are pieces of shit just because yours broke down, do you? Don't do the same with your supplements.