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Laparoscopic Sleeve Gastrectomy is a procedure in which about 75% of the stomach is excised or removed leaving a narrow "sleeve" through which food passes. No intestines are removed or bypassed during Sleeve Gastrectomy so intestinal malabsorption syndrome never occurs. This surgery restricts the amount of food one can ingest thereby decreasing the calories one can consume leading to weight loss.
Having said that it is not only the restriction that causes the weight loss but many a metabolic changes also occur due to rapid transit of the food in the intestines, releasing intestinal hormones that cause these effect. Thus Sleeve Gastrectomy is no longer considered purely a restrictive surgery but also a metabolic surgery.