I am sorry that you are going through that. I take a bunch of supplements including making my own. If you get vegan you still have the effect, but whey is the milk part, and it can make your digestion worse. I am allergic to milk, but I know many people with all sorts of digestive diseases. I have several. I have UC, gastroparesis, dumping syndrome (two contradicting disorders), diverticulitis, GERD, and several other things. I am malnutrition all the time and take more vitamins than anyone. I am on a prenatal vitamin, (4) 1 mg folic acid, 600mg of calcium twice a day, three 1000 IU of D3 a day, B complex, 400 mg magnesium, 200MCG Selenium, 2 Vitamin C gummies, two 500mg turmeric Curcumin with ginger powder twice a day, BIO-360 once a day, three protein shakes a day, and I think that is all my supplements. However, I would never suggest you take any supplements without talking to your doctor first, unlike me that does not listen to doctors and claim I know myself better.
Hello and welcome to MedHelp forums. We can't advise anything that is contradictory to six doctor's opinions. In general, whey is a protein that comes from cow's milk. Manufactured whey will have the same impact and unfortunately, isn't suitable for all based on digestive issues. With whey protein, there can be an increased incidence of diarrhea. As ulcerative colitis also has issues with diarrhea, you can probably see how that would possibly impact you negatively. Has it been suggested that you limit dairy products because of your GI diagnosis? That's a pretty common recommendation.
Here's some dietary information regarding ulcerative colitis to stay healthy. https://www.webmd.com/ibd-crohns-disease/ulcerative-colitis/vitamins-and-supplements-for-ulcerative-colitis#1
Now, this is your life and your body. You could try small samples of whey protein and see how your body reacts. But in general, it's not well tolerated by those with your condition.