I've never done shakes...
Some of the artificial sugars are known to cause diarrhea, so... I stay away from those.
Is what you mean, by shakes "causing a flare," is that they send you running to the bathroom? If I understand that to be what you mean, it's just that the activity of the high end of your GI tract (mouth, swallowing) starts activity which continues through your whole GI tract (run to bathroom) - and in a flare I find that aggravated by any drink that's icy cold or very hot - like tea. Broth, eaten slowly with a spoon, doesn't quite do the same thing to me.
Did.... I guess right at your meaning; could you say more? Thanks!