Protein powder has become a smoothie staple for the gym-going set, but whey protein powder can be problematic for certain people. Visser shared that, "Whey can be allergenic for a lot of people, which may stimulate inflammation."
If you have a dairy allergy or lactose intolerance, you'll be better off seeking out a nondairy protein source. Protein powder made from egg whites, collagen, or beef protein isolate (don't worry, it's flavoured) are good alternatives. If you are looking for vegan options, try pea or hemp protein (and then try this plant-based Carrot Cake Smoothie).