How or why do some people lose fat after having fat transfer to their buttocks ?

Fat is a graft. When place over the buttock we expect some of it not to take. Also the fat grafted can increase in size if you gain weight or decrease if you loose weigth

 Fat transfer or fat grafting follows the same basic principles as grafting other tissues such as skin grafting, cartilage grafting bone grafting etc. In order for fat to survive it has to reestablish a new blood supply in the area it was grafted into. This is a delicate process and not all fat well reestablish a blood supply in time for it to survive. The grafted tissue will stay alive from nutrients in the surrounding tissues for a few days.After that it must have a blood supply to deliver oxygen and nutrients. During the first phase of tissue grafting the fat survived by a process known as plasmatic inhibition.After this phase the grafted tissue goes through neo vascularization.   This happens between the fourth and seventh day after the procedure. The smallest blood vessels called capillaries slowly grow into the grafted tissue reestablishing a blood supply.During this stage the tissue is susceptible to damage from pressure or movement. The percentage of fat that survives is variable.There are many factors that determine how successful a fat grafting procedure is.The total percentage of fat that survives is estimated between 10 to 70%. Factors that influence fat survival include the size of particles grafted, avoiding post operative movement and pressure, how much tissue is grafted during a single session, the lack of trauma to the grafted tissue, the vascularity of tissue grafted into.There are other variables as well.Surgeons work hard at maximizing fat survival. Most of us follow The basic principles that maximize fat survival.Do overall health of the patient also makes a substantial difference.Cigarette smoking for example will decrease fat survival. The only thing the patient can do to maximize fat survival is avoid excess movement and pressure after the procedure.This includes avoid sitting on the Area treated.  Each surgeon will have their own protocol in regards to aftercare.In my opinion at three weeks activities will have little to no impact on the long-term survival of grafted fat. The fact that does not survive becomes necrotic and has to be absorbed by the body. This process goes on for many months.During this process there will be some inflammation in the tissues which leads to some swelling.As the swelling decreases and non-viable fat is absorbed most patients will experience some decrease in volume. Most plastic surgeons will generally over treat to compensate for loss of fat overtime. If too much fat is grafted during any one single session to percentage of fat that survives decreases.Some patients will do better having multiple procedures if the goal is a drastic enhancement in volume. I hope that helps.Best,Mats Hagstrom M.D.