Each human being has a stable weight, which is controlled by hormones, biologic feedbacks, and metabolic rate, among other things. If you gain weight to a moderate obesity of 20 – 40 kg of body fat, the fat cells actually get bigger, which is called hypertrophy. The absolute number of fat cells remains constant, which is called hypertrophic adipose deposition. Now, as you gain weight in excess of 40 kg of body fat, there is a point where the fat cell cannot get bigger and a process is triggered that forms new cells. This is called hyperplasia adipose deposition, which results in an increase of the actual number of fat cells.The hyperplasia phenomenon is seen routinely in many of my patients. We certainly have fat that is very hard to get rid off even with exercises. This diet resistant adipose tissue is due to newly recruited fat cells. As we gain weight and the critical size of the fat cell is reached, more fat cells are formed. Now, when we lose weight, the size of the fat cells get smaller but the total body fat does not go back to its previous form because of the extra cells that were produced. Because you cannot get rid of the cell per se, you will not be able to attain the desired weight reduction of certain areas of your body.From the fat transfer standpoint, hypertrophy appears to be the main factor that surgeons take into consideration for high volume fat transfer. An obese patient most likely will not be a candidate for surgery and most patients that have undergone the famous Brazilian butt lift have less than 40 kg. of body fat.Now, fat deposits have certain distribution depending on the sex of the patient. We all have seen this… This fat distribution gives women the gynoid distribution with fat accumulating preferably in the hips, buttocks, and thighs. On the other hand, men have an android fat distribution, which is characterized by deposition of fat in the abdomen and upper body. This complex mechanism is determined by the genetics of the human body added to the complexity of how fat behaves.Many women that come to my office have a very disproportionate fat distribution. Liposuction performed in those areas and fat transfer to the buttocks transform a woman’s proportion to the ideal form and what is considered the universal shape of sexiness. In summary, understanding how fat behaves has helped me as a plastic surgeon sculpt the human body in ways that were not possible before.