Is it okay to loose 15-20 pounds after a BBL? I'm currently 170 pounds and 5"6.

It's okay to lose weight after a BBL. Keep in mind that after a Brazilian butt lift your new shape is locked in. With weight loss your buttock is likely to get smaller but would maintain the same curves. #StayBeautiful @RealDrWorldWide

Ideal candidates are at a stable weight,  in good health , and have excess fat in some parts of the body and too little in other regions.  Gaining weight before the procedure is to no advantage and when you lose weight again the fat cells will shrink just like the donor site fat cells.  Typically, the transferred fat results in a significant increase in volume of the body site being treated.

General rule of thumb is to be within 10 pounds of your weight goal and maintain a stable weight prior to surgery.  I transfer as much fat as possible for each and every patient, so that the result is impressive even with small fluctuations in weight.
In most cases of weight loss, the buttocks and other areas with stored fat will become smaller.  About 50% of the fat that is transferred lives and becomes a part of you.  This will hold stable as long as there is no weight fluctuation.  The fat will not move, but losses in weight will distribute differently. The fat that lives in the buttocks from the procedure will get larger with weight gain and smaller with weight loss.
Kenneth Hughes, MD, Board Certified Plastic Surgeon
Los Angeles, CA

Thank you for your question. I think it is important to be an ideal weight before elective surgery as you do not want fluctuations in your body to affect your surgical outcomes. You can certainly lose weight (that is up to you) and being healthier should always be a goal. However, you should be aware that if you lose weight, you may lose weight in areas sensitive to weight loss. Good luck! 
Best wishes, 
Dr. Suzette Miranda 

Dear dwinie,loss of weight after a BBL will have a negative impact on your results. You can expect some loss of your transferred fat. This is one of the main reasons why it is the best to reach goal body weight before the surgery.  Daniel Barrett, MD, MHA, MS 
Certified, American Board of Plastic Surgery 
Member, Am. Society of Plastic Surgery

Thank you for your question. It is never a good idea to have weight fluctuations after surgery as this may change the results. And especially after a fat transfer procedure, loss of adipose tissue can change the result. Make sure you discuss your weight loss plans with the surgeon who performed the procedure.Best of luck!

This question of weight gain/loss as it relates to BBL is fast becoming one of the most commonly posted questions in the RealSelf community. While it is true BBL is a weight sensitive procedure (in the sense that you need fat to achieve your result), it is never a good idea to pursue short term changes for surgery.
While there is no absolute number (lbs.) in terms of weight for undergoing liposuction with autologous fat transfer, the optimal candidate has reached a healthy and stable weight (preferably with a BMI less than 30, which is associated with a lower incidence of complications).
Potential candidates should also have adequate fat reserves for transfer. This becomes especially important when one considers the anticipated rate of fat resorption and graft loss. I typically recommened patients aim for at least 500 cc transferred per cheek. This would amount to 1 liter of fat (not lipoaspirate) after processing. Candidates with inadequate fat reserves are likely to be disappointed with their ultimate volume once healing is complete. 
With regards to your specific question, I have found that patients who lose weight post-operatively potentially compromise their result (if the weight loss manifests in the grafted area). Remember grafted fat will behave just as if it had never been grafted. If you gain weight for transfer, and lose the weight post-op, you are likely to lose your result. Thus maintaining the pre-op weight post op will be the safest with regards to maintaining your results.
That being said, I would not change your weight for the sake of surgery. If you have reached a healthy stable weight (preferably with a BMI less than 30...your BMI is 27), the focus should be on maintaining.
As always, discuss your concerns with a board certified plastic surgeon (ABPS).

Although weight loss or gain after BBL can alter volume/ size of the buttocks, the change in proportions with respect to the rest of your trunk are permanent. You may want to look at before and after pictures of the fourth patient on my BBL web page at memphisplasticsurgery.com.  she lost 20 pounds after surgery, but the improvement in buttocks shape was maintained. 

Thank you for your question. It is okay to loose weight after a BBL. Just keep in mind that you may loose some volume to the buttocks. The same also goes for gaining weight, you could gain weight in the buttocks. 

Even if you loose weight , your body shape and proportions will still be preserved.  So don't be afraid , since you wont loose your results . Best wishes