Diarrhea is a possible side effect of using soy protein powder. It is not common but can occur in some people. If you experience diarrhea while using soy protein powder, you should consider reducing your daily intake, switching to a different protein powder, or speaking to your doctor or a registered dietitian for advice. It is important to note that soy protein powder may also cause other digestive issues like bloating, gas and nausea for some people, especially if they have a sensitivity to soy.