I'm not a surgeon and therefore I don't perform gastric bypasses, but I sure get that question a lot from folks who are interested in having one.
Came across an interesting report on a paper that was presented by Dr. Daniel Leslie at the 89th meeting of the Endocrine Society. The paper reported on a number of variables that predicted bariatric surgical success and they came up with an "equation" that you simply plug into to answer the question of how much weight a gastric bypass patient can expect to lose.
Ready for it? Here's the equation,
"Weight loss in lbs = [(age in years) x -1.79] + [(preoperative weight in lbs) x 0.36] + [(1 if patient if African-American, 0 if European-American) x -16.56] + [(1 for peripheral weight distribution, 0 for central) x 7.45] + [(1 for type 2 diabetes, 0 for no diabetes) x -7.28] + [(1 for obstructive sleep apnea, 0 for no sleep apnea) x 7.15] + 106.63"Of course there are many other variables.
Basically the surgery provides patients with a tremendous anatomical advantage for weight loss. The average patient's weight loss may be describable on the basis of the equation, but you need not be an average patient.
What the equation doesn't and cannot account for are the personal intangibles. The changes a person makes to their lifestyle will also have a tremendous impact on their weight loss. Some patients tend to take gastric bypass as a fresh start of sorts and get more involved in things like exercise and healthy eating. In my experience, those patients tend to lose more.
Just in case you've forgotten my take on bariatric surgery I'll remind you. I tend to call it, "a fabulous last resort".
UPDATE: My original post was off by a factor of 100, the new equation ought to work