According to the American Society for Bariatric Surgery and the National Institutes of Health, Roux-en-Y gastric bypass is the current gold standard procedure for weight loss surgery. It is also one of the most frequently performed weight loss procedures in the United States. In gastric bypass surgery, surgeons staple off a large section of the stomach, leaving a tiny pouch. The small space holds only one ounce of fluid. Patients simply can't eat as much as they did before surgery, because this small pouch can only accommodate a few ounces of food at a time. This results in weight loss.
Additionally, because most of the stomach and some of the small intestine is bypassed, some of the nutrients and calories in foods are not absorbed. One year after surgery, weight loss can average 77 percent of excess body weight. After 10 to 14 years, some patients have maintained 50 percent to 60 percent of excess body weight loss.
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Gastric bypass
The standard procedure for weight loss surgery poses a health risk when nutrients are no longer absorbed.
 
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The International Bariatric Surgery Registry estimates one in 1,000 patients will die within four weeks of the surgery and three in 1,000 will die within three months. Some surgeons in the field put the fatality rate as high as one in 100 for those who have the surgery. Other, less-extreme risks still exist.
These are related to the fact that nutrients bypass a major part of the intestine and are not absorbed by the body.
They include:
 | | Iron deficiency anemia. Because the duodenum is bypassed in this procedure, the body doesn't absorb iron and calcium very well after surgery, which can lead to iron deficiency anemia. This is a particular concern for patients who experience chronic blood loss during menstruation or from bleeding hemorrhoids. |
| | Osteoporosis. Because the body doesn't absorb calcium properly after surgery, there is a greater risk of developing osteoporosis. |
| | Metabolic bone disease. Also caused by bypassing the duodenum, some patients experience bone pain, loss of height, and fractures of the ribs and hip bones. |
| | Chronic anemia. This is a type of anemia caused by a deficiency of vitamin B12 and can usually be managed with pills or injections. |
"It is the matter of being able to absorb vitamin D from your vegetables and your nutrients and if that part of your bowel is being shunted or moved so that food passes quickly and you do not absorb, you are going to lose calories, you are going to lose weight … and if you bypass that area, you do not have the ability to absorb vitamin D," Tom O'Dorisio, M.D., from the University of Iowa, said.
He said patients should be educated about these risks and should potentially supplement their diets with a very highly, well-absorbed form of vitamin D, which is the biologically active vitamin D that most people."
