If you are thinking about having a gastric bypass to lose weight, know that life after bypass surgery is very different than before. One year after the surgery, you may have lost up to 85 percent of excess weight. You'll have to watch for nutrient deficiencies because of the change in your body, especially calcium, iron, and B12. You'll have to change what you eat, how you eat and even when you eat because of the changes made in your body with the surgery. You'll also have to adjust to the new person you see in the mirror.