Throughout the evolution of bariatric and metabolic surgery, two primary mechanisms have been identified as generating the loss of excess body weight. While the specific surgical manipulations of the gastrointestinal tract and surgeons performing them are many and varied, the two underlying principles at work are restriction of ingested food and malabsorption of ingested food. Modern metabolic and bariatric operations all employ one or both of these principles.

Restrictive surgeries entail physically limiting the quantity of food a patient is able to ingest. This is typically accomplished by limiting the size and capacity of the stomach while leaving the remainder of the gastrointestinal tract intact. The two most common procedures performed today of a restrictive nature are the sleeve gastrectomy and adjustable gastric band. While the majority of the stomach is surgically and permanently excised during a laparoscopic sleeve gastrectomy, the organ is partially constricted using an inflatable band during a laparoscopic adjustable gastric band application. Following either surgery, patients reach a feeling of satiety much sooner while eating and consume a substantially smaller portion of food. With sleeve gastrectomy procedures, hormonal changes occur as well due to excision of the aspect of the stomach responsible for the production of several systemic hormones [1]. This will be discussed in a later chapter.

Malabsorption of calories and nutrients occurs when a portion of the gastrointestinal tract is bypassed or removed. The gut is exposed to ingested foods for a shorter distance, and therefore less calories and nutrients are able to be absorbed. Typically, there is also a diversion of digestive enzymes from the liver, pancreas, and gallbladder such that they meet with ingested food at a later portion of the GI tract and further decrease the digestion and availability of food and nutrients for absorption. There are no purely malabsorptive bariatric surgeries being performed in modern practice. The duodeno-ileal bypass was performed from the 1950s to 1970s and represented a purely malabsorptive surgery but was fraught with complication [1]. Roux-en-Y gastric bypass and biliopancreatic diversion with duodenal switch are two contemporary surgeries that are considered to be malabsorptive in nature but also have restrictive aspects. After the stomach is divided in either operation, the ileum is also separated and anastomosed such that ingested food and digestive enzymes remain on separate tracks until a final, shortened common channel where digestion and absorption of nutrients can occur. These efforts to cause both restriction and malabsorption typically yield a higher percentage of excess weight loss but also can lead to additional and separate complications. Patients having a gastric bypass or duodenal switch must be advised to supplement their diets with vitamins and nutrients to avoid deficiency [2].