When protein foods are broken down (digested) by the body, amino acids are produced.  Amino acids are the building blocks for our entire body.  Amino acids and the new proteins made from them are essential to build, maintain and repair the body and all its symptoms.  Protein comes from a number of sources including meat, chicken, fish, textured vegetable protein (TVP), eggs, dairy foods, nuts (many people have adverse reactions to peanuts, best to avoid), seeds, soya beans (tofu and tempeh) and other legumes.  Protein foods will also provide other macro and micronutrients (e.g. from meat we get iron, zinc and B-group vitamins; from dairy foods we also get calcium, vitamin A and D; legumes, pulses, nuts and seeds are an excellent source of fibre).  A number of people have described feeling heavy or bloated for quite a while after eating red meat such as beef.  If you have similar problems reduce your intake or cut it out completely.