Your plastic surgeon will intentionally over-inject fat because some will be naturally reabsorbed. However, the fat stem cells which establish themselves in the new location are expected to last for decades. Because grafted fat cells are living tissue, they stay where healthy blood supply is established. Natural aging can change the buttocks shape and weight loss shrinks fat cell size. That means patients must maintain a healthy weight and supportive muscles in the buttocks to maintain the look they love.
