Breast fat transfer - does the fat stay?

Fat transfer to the breasts can provide a natural increase in volume, and with proper technique, much of the transferred fat can permanently survive. Typically, about 60–80% of the fat remains long-term. Caffeine consumption does not cause fat loss in the breasts or affect fat graft survival. However, significant changes in weight or hormones can impact long-term results.
Erman AK, Dr. MD, Plastic Reconstructive & Aesthetic Surgeon

Dear Terrific798166,
you will probably lose some fat in the early postoperative period. However, once you are fully healed your breast fat will be the same as fat in the rest of your body. If you are considering surgery, I would suggest you consult a board-certified plastic surgeon. Only after a thorough examination, you will get more information and recommendations.
Daniel Barrett, MDCertified, American Board of Plastic SurgeryMember, American Society of Plastic SurgeryMember, American Society of Aesthetic Plastic Surgery

Not all fat transferred survives and only 60% of the fat survives. The fat that does survives can be adversely effected by weight changes. If you lose weight then the fat can be decreased.

Gary Horndeski, M.D.

Thank you for posting your question here.
Fat grafting to the breast has been gaining traction recently since it uses your own tissue and there has been a trend of people preferring the use of natural materials over synthetic ones. It grows with you as you gain weight and it decreases as you lose weight so it keeps up with your general proportions.
Usually around roughly around 50-80% of fat transferred survives , this is dependent on multiple factors and for the most part is a little unpredictable , so some patients might need multiple sessions to achieve their desired outcome.
There is also a limit to how much fat can be transferred without compromising the transferred fat ( if you put too much , the fat you transfer will be far from the new blood supply so it won't survive.
The fat that doesn't survive for the most part is re-absorbed but in some individuals can form cysts or granulomas.
For someone who wants to change the size of their breast dramatically (a couple of cup sizes etc) I would recommend going for an implant. Fat grafting can be coupled with it if desired to improve contours etc but usually won't offer you as big of a change as what an implant would.
I hope this helps , good Luck!

Yes it is. It is your own fat and once it finds a new blood supply in its new location it will live and be there for ever just like all the fat in the other areas of your body. If you gain weight the fat cells will get bigger and your breasts will get bigger and then if you lose weight the fat cells will get smaller and your breasts will get smaller. The fat behaves just like it does anywhere else in your body.