My Mom was a big fan of the health readings by Edgar Cayce, who is known as the Father of Holistic Nutrition. He gave many people recommendations for improving their health. One of the recommendations was to avoid certain food combinations. He claimed they are difficult to digest, therefore creating toxic buildup of dark bile in the digestive system.
He claimed that when coffee and milk are combined outside of the body (i.e. in your coffee cup), the chemical reaction of the acids in the coffee and the milk create an indigestible curd.