I have a friend who is a retired nurse. She is an unhealthy thin person who has little muscular and who's physician wants her to gain muscle and weight. My friend is vehemently anti-fat as food. She eats very low-fat and now drinks a liquid protein supplement to try to gain muscle. It hasn't been working, but she will not add any fat to her diet. She eats so few calories that I believe that her body is burning the protein for energy instead of building muscle from it.