What happens to a facial fat transfer if you loose or gain weight?

Facial fat injections will get bigger with weight gain! I would advise you to have a reversible filler like Voluma instead. 

I can tell you that even those of us surgeons with many years of experience with fat transfer (for me it's 20) are still just beginning to understand exactly what is happening when we transfer "fat" to another part of the body - face or otherwise.  Even though we in the medical profession have been injecting or transferring fat in one form or another for over 100 years, and even with the advent of the "modern era" of fat transfer since the early 1990's our techniques and results have improved, it has only been since 2002, after the work of Zuk who first identied mesenchymal stem cells in our fat, or adipose tissue, that our discovery and understanding of the phenomena involved has been exploding.  It is not so simple as to expect that these plump mature fat cells, called adipocytes, get moved from one place to another and just behave the way they always did in your tummy, thighs, hips, flanks, or wherever they came from.  In fact, recent evidence suggests that NONE of those cells survive, and what really happens is that the stem cells and adipocyte progenitor cells, those cells further along the line toward becoming mature adipocytes, in the transferred adipose tissue actually differentiate, along with stem cells and progenitor cells in the recipient site where the fat is injected, in response to various "chemical messages" being sent by a variety of cells in the mix, along with platelets, and a whole host of transformations occurs, including new blood vessel formation, new cell growth and differentiation, and new collagen and "tissue matrix" creation.  We also have evidence to show that these stem and progenitor cells, being multipotent, or capable of becoming many different types of cells depending upon what type of chemical signaling they receive, can differentiate into a wide variety of cell types that they are injected into, including muscle, tendon, ligament, and probably bone.  Thus, transferred fat behaves in a very complex manner, and in fact, this is why I would prefer that we all start using the terms "adipose tissue" and adipose transfer, because it is more than just "fat" that we are transferring, and more is required to produce the results that we see from "fat transfer," including even the "non-living" or non-cellular components of the adipose tissue, like the so-called matrix, or supporting framework and fibers.
Having said that, we also can't escape our actual observations over the years of what we have witnessed with fat transfer procedures gone wrong.  Things like lumpiness and irregularity, disproportionate volumes, cyst and fibroma formation, and the like have been seen to be sure.  Also, adipose, being a living metabolic tissue, will gain and lose volume with overall body weight change.  Those things have been witnessed and recorded.  But we must keep all of this in perspective, and as I alluded to above also, we must recognize that the fat transfer being done now is not your "dad's fat transfer!"  Much has changed in how we harvest, process, and inject the fat, even in the last 5 years, and the results are improving all the time.  This is not to say it is perfect; clearly it is not.  But it is far from warranting abandonment, for as many horror stories as you hear people telling about fat transfer, I can tell about twice as many stories of gratifying results.
The last thing to address is this issue of increase or decrease in volume with weight gain.  You can look at this in two ways:  in a well-done fat transfer procedure in which volumes are proportionate and contours are even, any change in weight will be proportionate and match that of the surrounding areas.  The key here is "well-done."  Nobody can protect you from bad technique, over-enthusiastic filling with volumes too great for the body part in question, and the like.  Bad results are bad results, period.  They may get worse with further weight gain, but I would posit that they probably weren't that great even before the weight gain!  Furthermore, on the other side of the coin, I'm not sure I want a filler in my face that WON'T change at all if I lose weight or gain weight.  If I lose, will the filler now stand out as the surrounding fat and tissue volume becomes less leaving my filled area now disproportionately fuller, or will weight gain in surrounding tissues obscure the effects of the filler, diluting it out and essentially making it worthless.  I'm not so sure that in the context of weight gain or loss, fillers are any panacea either.  I'm not here to bash fillers, because I love fillers and use a lot of them, but I also want fat transfer to get a fair shake in this discussion because in my opinion, physiologically for the right person, it offers a lot more than fillers do.  I think the take-away messages here are:  1) bad technique is bad technique - filler or fat, and 2) I don't think there is anything a plastic surgeon can do to you - fillers, fat transfer, breast augmentation, liposuction, tummy tuck, you name it - that can predictably withstand the changes of weight gain or loss.  You are simply shooting at a moving target when you get procedures done and you are not, or do not stay, at a stable target weight.  This is more of an issue with body modification procedures in general in the context of weight changes than it is an indictment of fat transfer in my opinion.  The best thing for you to do if you are interested in the benefits of fat transfer is to find a board certified surgeon who is up on the latest and greatest in the world of fat transfer and explore all of your options.  You may wind up liking fillers best, but at least you will have truly explored all options and made an educated choice.  Best of luck!

Fat transfers to the face (or any other part of the body) always have partial take: approximately 30-40% of the transplanted fat absorbs, but the part that stays, stays permanently and behaves as the original fat cells from whatever part of your body they were harvested. With minor fluctuations in weight, the kind that we all go through, this ends up having minimal impact. If however, you were to gain a significant amount of weight, the fat may grow somewhat in the new transplanted area. It generally does not become lumpy but with significant weight gain, it can become slightly more prominent. In my practice, I've only seen one such case from somebody who had fat grafting 20 years ago, and it was a minor problem.This can be dissolved with either Kybella or with nano and micro liposuction techniques. Bottom line, I wouldn't be concerned given that routine weight loss and gain don't create a significant impact in a reasonable time frame.