Surgical treatment of obesity, known as bariatric surgery, can result in significant and long-lasting weight loss and also help improve associated health issues such as insulin resistance, hyperlipidemia, and elevated blood pressure. Initially, surgery involved jejunoileal bypass, which involves removing a substantial part of the absorptive small intestine. However, this procedure is linked to malabsorption and subsequent undesirable effects like diarrhea. Various revised surgical approaches for obesity have been developed, including Roux-en-Y gastric bypass and vertical sleeve gastrectomy. The mechanisms through which these procedures are thought to be successful lie in the small size of the residual gastric pouch, which decreases meal size due to early satiety, and the beneficial effect of the bypass on the profiles of GI hormones. Recent data suggests that the effects of surgery on bile acids and the microbiome may also contribute to both weight loss and metabolic benefits.

The majority of nutrients ingested by humans are in the form of macromolecules. However, these molecules are too large to be absorbed across the epithelial cells that line the intestinal tract. Therefore, they need to be broken down into smaller constituents through chemical and enzymatic digestion. The pancreas plays a crucial role in enzymatic digestion of meals as its secretions, quantitatively the largest contributors, contain important enzymes. The pancreas also secretes substances that regulate the function or secretion of other pancreatic products, as well as water and bicarbonate ions necessary for normal digestive function. Bicarbonate ions are particularly important in neutralizing gastric acid, ensuring the small intestinal lumen has a pH close to 7.0. This is crucial because high levels of acidity can inactivate pancreatic enzymes and cause injury to the small intestinal mucosa when combined with pepsin. While the pancreas is the primary source of bicarbonate ions needed to neutralize gastric acid, the biliary ductules and duodenal epithelial cells also contribute.

Similar to salivary glands, the pancreas consists of ducts and acini. The acinar cells line the blind ends of a branching ductular system, which eventually leads to the main pancreatic duct and empties into the small intestine through the sphincter of Oddi. Like salivary glands, the pancreas produces a primary secretion in the acini, which undergoes modification as it passes through the pancreatic ducts. Generally, the acinar cells supply the organic constituents of the pancreatic juice in a primary secretion with an ionic composition comparable to plasma. On the other hand, the ducts dilute and alkalize the pancreatic juice while reabsorbing chloride ions. The major constituents of pancreatic juice, which amounts to approximately 1.5 L/day in adult humans, are listed in Box 30.1, along with the functions of pancreatic secretory products. Many of the digestive enzymes produced by the pancreas, especially proteolytic enzymes, are initially produced in inactive forms. Storing them in these inactive forms is crucial to prevent the pancreas from digesting itself.
Surgicaltreatmentofobesity,so-calledbariatricsurgery,canachievesubstantialandlastingweightlossandalsohelpameliorateassociatedhealthproblemssuchasinsulinresistance,hyperlipidemia,andelevatedbloodpressure.Initially,surgeryinvolvedjejunoileal bypass, theremovalofasubstantialpartoftheabsorptivesmallintestine,butthisprocedureisassociatedwithmalabsorptionandsubsequentundesirablesequelaesuchasdiarrhea.Avarietyofrevisedsurgicalapproachestoobesityhavebeendevised,includingRoux-en-Ygastricbypassandverticalsleevegastrectomy.Themechanismsbywhichtheseproceduresarethoughttobesuccessfullieinthesmallsizeoftheresidualgastricpouch,wherebymealsizeisdecreasedbecauseofearlysatiety,andabeneficialeffectofthebypassontheprofilesofGIhormones.Recentdataimplythateffectsofsurgeryonbileacidsandthemicrobiomemayalsocontributetobothweightlossandmetabolicbenefits.
Most of the nutrients ingested by humans are in the chemical form of macromolecules. However, such molecules are too large to be assimilated across the epithelial cells that line the intestinal tract and must therefore be broken down into their smaller constituents by processes of chemical and enzymatic digestion. Secretions arising from the pancreas are quantitatively the largest contributors to enzymatic digestion of the meal. The pancreas also provides additional important secretory products that are vital for normal digestive function. Such products include substances that regulate the function or secretion (or both) of other pancreatic products, as well as water and bicarbonate ions. The latter are involved in neutralizing gastric acid so that the small intestinal lumen has a pH approaching 7.0. This is important because pancreatic enzymes are inactivated by high levels of acidity and also because neutralization of gastric acid reduces the likelihood that the small intestinal mucosa will be injured by such acid acting in combination with pepsin. Quantitatively the pancreas is the largest contributor to the supply of bicarbonate ions needed to neutralize the gastric acid load, although the biliary ductules and duodenal epithelial cells also contribute.
As in the salivary glands, the pancreas has a structure that consists of ducts and acini. The pancreatic acinar cells line the blind ends of a branching ductular system that eventually empties into the main pancreatic duct and from there into the small intestine under control of the sphincter of Oddi. Also in common with salivary glands, a primary secretion arises in the acini, which is subsequently modified as it passes through the pancreatic ducts. In general the acinar cells supply the organic constituents of the pancreatic juice in a primary secretion whose ionic composition is comparable to that of plasma, whereas the ducts dilute and alkalinize the pancreatic juice while reabsorbing chloride ions (Fig. 30.2 ). The major constituents of pancreatic juice, which amounts to approximately 1.5 L/day in adult humans, are listed in
Box 30.1 . This list also outlines the functions of pancreatic secretory products. Many of the digestive enzymes produced by the pancreas, particularly the proteolytic enzymes, are produced as inactive precursor forms. Storage in these inactive forms is critically important in preventing the pancreas from digesting itself.