Foods in the milk group are rich in calcium, vitamin d, potassium, and protein. Milk is a highly nutritious liquid formed in the mammary glands of mammals. Milk contains almost every single nutrient that the human body needs. Milk is a rich source of protein, calcium, vitamin d, and potassium. Other constituents of milk include carbohydrate, sugar, water, fat, fiber, vitamin B12, riboflavin, phosphorus and many others.