Gaining weight for skinny BBL? how can I quickly gain weight if my metabolism is fast & I have a hard time gaining?

Gaining weight is often counterproductive. If you gain weight that is fat, some of the fat will likely be associated with the buttocks.  In almost every case, I do not recommend weight gain prior to a BBL. I think it is better to maintain your stable weight.  
Kenneth Hughes, MD, ABPS Board Certified Plastic Surgeon
Los Angeles, CA

Great question!  Although gaining weight may help with volume in the short term, most patients end up loosing the weight they gained as they don't like how their face or arms look after the weight gain.  Once you loose weight, you will also loose the volume from the grafted areas so in the end, there's really no point in gaining the weight unless you plan on maintaining the higher weight.  If you are looking for other options, please see a board certified plastic surgeon.Best of luck and hope this information helps!- Dr. Roger Tsai  @DrRogerTsaiBoard Certified Plastic Surgeon

I recommend seeing a plastic surgeon for consultation to see if you have enough fat for a BBL.  Gaining weight may not be a great option if you plan to lose the weight after surgery.  Weight loss may cause the buttock augmentation to shrink.