Doi:10.1016/j.jamcollsurg.2005.03.02

Alterations in Gastrointestinal Physiologyafter Roux-En-Y Gastric Bypass Todd A Ponsky, MD, Fredrick Brody, MD, FACS, Edward Pucci, MD Morbid obesity is increasing exponentially in the US.
Although the Roux-en-Y procedure is an effective Approximately 33.3% of Americans were obese in 1991, therapy for weight loss, it alters normal digestion and versus 25.4% in Currently, 54.5% of Americans nutrient absorption. These alterations can be mild with are overweight, with a body mass index Ͼ Diet and no apparent sequelae or cause life-threatening complica- exercise are implemented initially, they usually fail after tions. After RYGB, severe malnutrition occurs in ap- 2 years. In response to this epidemic, surgical therapies proximately 4.7% of the patients postoperativelyThe for morbid obesity have evolved. There are several dif- most common nutritional deficiencies in these patients ferent weight loss operations available, including Roux- are iron and vitamin There are numerous case en-Y gastric bypass (RYGB), biliopancreatic diversion, reports that document malnutrition that lead to myop- duodenal switch, vertical banded gastroplasty, and the LapBand. The most common procedure performed in To understand the pathophysiologic mechanisms of the US is RYGB, which is both a restrictive and malab- RYGB, a review of normal nutrient absorption is re- sorptive operation. Although there are several variations quired. My colleagues and I describe normal nutrient available for RYGB, the general principles are similar.
absorption followed by the malabsorptive physiology as- The first part of the operation creates a gastric pouch of approximately 20 cm3 to restrict oral intake. The secondcomponent of the operation involves construction of a biliopancreatic limb that diverts bile and pancreatic Nutrients are divided into five groups: protein, car- fluid from the proximal to the distal small intestine. This bohydrate, fat, vitamins, and minerals. Normal ho- distal diversion creates a segment of small bowel that meostasis and weight maintenance depends on the allows food to pass without interacting with bile or pan- digestion and absorption of each of these nutrients.
creatic fluid. This diversion delays the interaction of Conversely, altered pathways of absorption result in pancreatic enzymes and biliary secretions with the food weight loss and potentially substantial deficiencies bolus. Finally, this bypass effectively alters normal nutri- Although this procedure can alter normal physiology, it has the propensity to increase survival for morbidly Amino acids are the basic components of protein. A obese patients. A recent study by Patterson and col- chain of 2 or more amino acids is called a peptide. The suggests that life expectancy improves by almost amino acids in a peptide are linked by the carboxyl group 11% after laparoscopic RYGB, compared with diet and of one amino acid to the amino group of another. A exercise alone. Flum and Dfound that morbidly protein is composed of one or more long peptide chains.
obese patients had a 33% decreased hazard of death after Smaller proteins, chains of less than 30 amino acids, are referred to as oligopeptides and include somatostatinand vasopressin.
Where the peptide-laden food bolus enters the Received November 4, 2004; Revised March 2, 2005; Accepted March 2, mouth, cephalic stimulation through acetylcholine en- hances acid production from gastric parietal cells. The From the Department of General Surgery, The George Washington Univer-sity Medical Center, Washington, DC.
acid, along with acetylcholine, stimulates pepsinogen Correspondence address: Fredrick Brody, MD, Department of General Sur- release from chief cells. The gastric acid then facilitates gery, The George Washington University Medical Center, 2150 PennsylvaniaAve NW, Suite 6B, Washington, DC 20037.
conversion of pepsinogen into pepsin. Cleavage of pep- 2005 by the American College of Surgeons Physiologic Basis of Roux-en-Y Gastric Bypass sinogen into pepsin requires a gastric pH Յ 5. Pepsin drate absorption starts in the duodenum and is com- then initiates the cleavage of protein into peptides. The pleted usually by the first 100 cm of small intestine.
stomach absorbs very few peptides or amino acids.
When the peptides and amino acids enter the duode- num, the duodenal and jejunal epithelial cells release Lipids are the largest energy source in the body and 93% cholecystokinin Presence of food in the an- of dietary lipids are absorbed in the proximal two-thirds trum, antral distention, and vagal stimulation increase of jejunum.Dietary lipids include free fatty acids, tri- the production of gastrin. Gastrin stimulates CCK al- glycerides, phospholipids, and cholesterol. When lipids soCCK stimulates release of trypsinogen from pan- enter the duodenum, the I and S cells, located within the creatic acinar cells. Trypsinogen is released into the du- duodenal mucosa, secrete CCK and secretin, respec- odenum and cleaved into its active form, trypsin, by tively. CCK and secretin stimulate gallbladder contrac- enterokinase, an enzyme released from the duodenal tion and pancreatic secretion. The pancreas secretes the mucosal cells. Trypsin activates chymotrypsin and pro- lipolytic enzymes lipase, cholesterol esterase, and phos- carboxypeptidases A and B. Chymotrypsin and the pro- pholipase A2. These enzymes digest distinct lipid com- carboxypeptidases are produced from the pancreatic aci- ponents. Lipase hydrolyzes triglyceride molecules into a nar cells as well. All of these pancreatic enzymes monoglyceride and two fatty acids. Phospholipase A2 hydrolyze proteins into amino acids or oligopeptides.
hydrolyzes phospholipids into a fatty acid, a phosphoric Trypsin and chymotrypsin are endopeptidases that acid, and a nitrogenous base. Cholesterol esterase cleaves cleave internal peptide bonds while procarboxypepti- cholesterol esters into free cholesterol. After fat digestion dases cleave the carboxy terminals of the peptide chains.
by these distinct enzymes, bile-saltϪformed micelles In the duodenum, brush-border peptidases digest oli- emulsify the lipid byproducts. The strongly polar bile gopeptides that are absorbed through the sodium- acids align their hydrophilic tails externally and their dependent amino acid cotransporters located along the nonionized, hydrophobic heads internally to form a mi- luminal border of the enterocyte. Although 50% of pro- celle. This emulsification process provides a nonpolar tein absorption occurs in the duodenum, the entire environment to harbor and transport lipid byproducts small bowel is capable of protein absorption. By midje- into the aqueous blood. Ultimately, enterocytes absorbthe micelles. Once absorbed into the enterocyte, the junum, the overwhelming majority of protein absorp- triglycerides and cholesterol are resynthesized. These compounds combine with other nonpolar lipids, phos-pholipids, and proteins to form lipoproteins called chy- lomicrons. Chylomicrons are transported from the en- Monosaccharides, including glucose, fructose, and ga- terocyte into the lymphatic system and eventually into lactose, are the most basic forms of carbohydrates. Di- the thoracic duct that enters the bloodstream through etary carbohydrates exist as disaccharides, such as su- the left subclavian vein. Fat absorption, including fat crose, maltose, or lactose or as polysaccharides such as soluble vitamins, occurs throughout the entire small starch. Initially, salivary and pancreatic amylase hydro- bowel although excess bile salts are absorbed in the ter- lyze polysaccharides into oligosaccharides, such as disac- charides and trisaccharides. These oligosaccharides areadditionally broken down into monosaccharides by oli- gosaccharidases in the brush border of the intestinal mu- Surgical therapy for morbid obesity occurs through ei- For example, the intestinal enzyme sucrase di- ther restricting food intake or decreasing absorption gests sucrose into the monosaccharides glucose and from the intestinal tract. Several techniques reduce the fructose. Maltase divides maltose into two glucose mol- size of the stomach to induce early satiety and limit oral ecules and lactase digests lactose into the monosacchar- intake. These techniques include gastric banding or sta- ides glucose and galactose. Glucose and galactose are pling. Malabsorptive operations decrease the amount of transported into the cell by active transport using a bowel available for absorption and divert bile and pan- Naϩ-Kϩ ATPase pump. Fructose is transported pas- creatic enzymes from the proximal to the distal bowel.
sively through carrier-facilitated diffusion. Carbohy- These two alterations limit food digestion and produce Physiologic Basis of Roux-en-Y Gastric Bypass malabsorption. There are several surgical options avail- proteins function as enzymes and provide structural sup- able to construct this restrictive or malabsorptive state.
port to the body, protein deficiency has serious conse-quences, including poor wound healing, decreased im- mune function, muscle weakness, weight loss, mental apathy, anemia, pressure sores, depigmented hair or In addition to a decreased dietary intake of proteins skin, and diarrhea. The most severe form of protein de- secondary to the gastric pouch, the RYGB induces pro- ficiency is referred to as kwashiorkor. Currently, little tein malabsorption. Under physiologic conditions, the data exists about protein levels after RYGB. Protein de- majority of protein is absorbed in the duodenum. After ficiency was documented in 4.7% of patients after RYGB, protein never enters the duodenum. Protein ab- sorption is limited to the distal jejunum and ileum.
centage is low, the real percentage of protein deficiency is Other components of the RYGB contribute to pro- difficult to document because of the lack of data.
tein malabsorption. Pepsinogen, as discussed alreadycleaves protein into peptides. After RYGB, Sundbom and colleaguesfound a substantial decrease in pepsino- Similar to protein, RYGB alters carbohydrate digestion gen levels from the gastric remnant. Decreased levels of and absorption. Under physiologic conditions, most pepsinogen are secondary to the vagotomy that occurs carbohydrate absorption occurs within the first 100 cm with gastric partitioning. Vagotomy negates cephalic of small intestine after interacting with pancreatic stimulation and decreases acid secretion and ultimately amylase. Because ingested carbohydrates bypass the du- decreases pepsinogen levels. Without pepsinogen, many odenum, pancreatic amylase stimulation and release is protein molecules are left intact and are not absorbed.
reduced. After passing from the gastric pouch, carbohy- Even if the gastric remnant releases a basal level of pep- drates travel through the Roux limb as intact polysaccha- sinogen into the duodenum, pepsinogen does not inter- rides. After passing the jejunojejunostomy, the polysac- act with the ingested protein until it traverses the jeju- charides interact with a small amount of basal amylase nojejunostomy. This delays protein cleavage and small that is secreted from the pancreas through the biliopan- creatic limb. The limited pancreatic amylase hydrolyzes Along with the restrictive component and decreased the polysaccharides into oligosaccharides and allows pepsinogen levels, decreased production of pancreatic minimal carbohydrate absorption in the remaining enzymes contributes to protein malabsorption. In 1971, small intestine. The combination of decreased absorp- Ito and Mdemonstrated a 40% reduction in pan- tive surface area, decreased pancreatic amylase secretion, creatic enzyme secretion after a gastric bypass. They con- and delayed interaction of amylase with the polysaccha- cluded that this finding resulted from decreased gastrin rides limits carbohydrate absorption.
levels.Discussed here already, food in the antrum, an- Carbohydrates are a major source of energy for body tral distention, and vagal stimulation induce gastrin se- metabolism and the sole energy source for the brain and cretion. Gastrin then stimulates the release of CCK.
red blood cells. The liver and skeletal muscle store car- Without food in the excluded antrum, antral distention bohydrates as glycogen. In the absence of adequate car- and vagal stimulation of G cells do not occur. Interest- bohydrate substrate, fat and protein are broken down ingly, Ito and Mason17 showed increased pancreatic se- through gluconeogenesis into carbohydrates to provide a cretion after restoration of gastric continuity; diminish- nutritional substrate for the brain and red blood cells. A ing the role of vagotomy on pancreatic secretion. Under deficiency in carbohydrates can lead to a deficiency in physiologic conditions, as peptides pass through the du- odenum, the duodenum releases CCK and causes the The anatomic alterations created during the RYGB pancreas to secrete proteolytic enzymes. Theoretically, probably contribute to dumping syndrome. The early because the peptides never traverse the duodenum, this phase of dumping occurs immediately after eating and is physiologic process is omitted as well. Several studies do characterized by abdominal cramping, nausea, vomit- not show any changes in CCK levels after RY ing, diarrhea, dizziness, palpitations, and diaphoresis.
Unlike fat or carbohydrate, protein is not stored. All Increased gastric emptying after RYGB leads to rapid protein in the body has some biologic function. Because filling of the small bowel with hyperosmolar chyme.
Physiologic Basis of Roux-en-Y Gastric Bypass This causes an osmotic shift of extracellular fluid into decrease in serum calcium. Many patients experience the bowel to restore isotonicity. Also, this shift causes progressive bone loss over time. Vitamin D deficiency is intestinal distention, abdominal cramping, nausea, more likely to occur after a biliopancreatic diversion vomiting, and diarrhea. Bowel distention causes the re- than RYGB because a greater degree of fat malabsorp- lease of vasoactive intestinal peptide and serotonin.
tion. Deficiencies of vitamin D and calcium after both These two hormones are associated with diaphoresis, types of malabsorptive procedures can increase bone dizziness, and palpitations. Fluid shifts from the intra- turnover and decrease bone mass.It is imperative vascular space can accentuate the vasomotor symptoms.
to monitor hemoglobin, calcium, and thiamine levels Early phase symptoms are accentuated by a surge in In addition to vitamin D, any of the fat-soluble The late phase occurs 1 to 4 hours after eating and is vitamins can become deficient after RYGB. This in- characterized by sweating, dizziness, and fatigue. A de- cludes vitamins A, E, and K. There are reports of ocular layed surge in insulin secretion with rebound hypogly- deficiency after biliopancreatic diversions as a result of cemia causes this phase. The primary treatment for vitamin A deficiencyThere is no such data after dumping syndrome includes a decrease in simple carbo- RYGB. Vitamin K deficiency can lead to a decrease hydrate intake. This can be a problem in patients with a in vitamin K-dependent clotting factors. Coagulation carbohydrate deficiency. Symptom improvement can studies should be monitored closely for patients taking occur by modifying the diet to small, frequent meals that are high in fiber, protein, and complex carbohydrates Cobalamin (vitamin B12) is absorbed in the terminal ileum. In order for absorption to occur, cobalamin must curs to some degree in approximately 70% to 75.9% of be linked to the glycoprotein, intrinsic factor. Intrinsic patients after bariatric surgeryAlthough this per- factor is produced by parietal cells in the stomach and centage seems high, the overwhelming majority of pa- released after hormonal stimulation from acetylcholine, tients only experience transient dumping as they modify histamine, and gastrin. Pepsin and hydrochloric acid are needed to separate B12 from the protein bolus in thestomach. Without hydrochloric acid in the proximal pouch, RYGB patients cannot cleave B12 from the pro- Under physiologic conditions, fat passes into the duode- tein moiety. The excluded stomach and bypassed duo- num and stimulates CCK. In turn, CCK stimulates the denum prohibit binding of free B12 to intrinsic factor.
gallbladder and pancreas to release bile and lipolytic en- Several investigators showed that bound B12 is not ab- zymes. After RYGB, the secretion of bile and lipolytic sorbed as well as administered crystalline B12 orally after enzymes is reduced because lipids never pass through the a RYGB or partial gastrectomySmith and col- duodenum. Lipids, including triglycerides, phospholip- leaguesfound that B12 was not digested and essentially ids, and cholesterol, travel through the Roux conduit as malabsorbed. They recommended supplementing with intact structures until they reach the jejunojejunostomy.
either monthly parenteral B12 or daily oral crystalline Delayed breakdown of dietary fats and the delayed for- preparations. Meanwhile, a normal Shilling test can oc- mation of micelles limit the amount of fat available for cur after RYGB. This finding suggests that the bypassed absorption. Undigested fat passes into the colon produc- stomach continues to secrete some degree of intrinsic ing fat malabsorption and steatorrhea.
factorand that B12 deficiency results from malab-sorption; not a decrease in intrinsic factor.
Restrictive procedures, such as gastric banding and RYGB reconstruction can cause deficiencies in certain gastroplasty, can cause vitamin and mineral deficiencies vitamins and minerals. Iron, calcium, and thiamine (vi- as well.There are several reports of vitamin A, C, D, tamin B1) are absorbed primarily in the duodenum.
E, K, B1, B2, and B6 deficiencies. Many of these find- After RYGB, these vitamins never enter the duodenum ings were present in patients taking regular multivitamin and their absorption is decreased considerably. Because vitamin D is a fat-soluble vitamin, it is poorly absorbed tients admitted for either malnutrition or excessively after the RYGB. Ultimately, this leads to an additional rapid weight loss after a horizontal gastroplasty. They Physiologic Basis of Roux-en-Y Gastric Bypass found substantially decreased levels of serum thiamine sume adequate amounts of protein and calories can feel and folate compared with preoperative levels. Thiamine weak and develop nutritional deficiencies. Caloric in- and folate levels were low in 50% and 65% of these take should be between 1,000 and 1,500 cal/kg and patients, respectively. These values can have far-reaching patients with an initial body mass index of Ͻ 50 kg/m2 indications as more patients undergo the LapBand should consume 60 to 70 g of protein per day. Patients with a body mass index 50 to 70 kg/m2 can need up to100 g of protein per day.
Prevention and control of deficienciesRYGB decreases the quantity of food patients consume.
Daily oral supplementation with a chewable multivita- Ursodiol, which increases the solubility of bile salts, re- min, as a result, is recommended. Although there is little duces the risk of developing gallstones to approximately published data about treatment regimens for micronu- 2% if taken at 300 mg four times a day. Without urso- trient deficiencies after RYGB, low serum levels of iron, diol, gallstones can occur in 30% of patients with con- B12, and folate require either a multivitamin or specific siderable weight loss from either medical or surgical supplementation of the deficient micronutrient. A min- methods.Ursodiol is given as long as the patient imum daily dose of 350 ug of oral B12 is sufficient to continues to lose considerable weight (Ͼ 3% of body maintain normal serum levels after a RYGBand 500 ug weight per month) or for the first 6 months after an of oral B12 is sufficient to correct the majority of defi- operation. Cost and the rather large size of the pill limit ciencies. Intramuscular injections can be used as well.
This regimen entails an injection of 1,000 ug followed Physical activity is recommended postoperatively. A every 2 to 4 weeks with 100 to 500 ug to maintain strenuous activity program on a very low calorie diet normal serum levels. Nasal sprays and gels can also pro- causes an excessive loss of lean body mass and an unde- vide effective prophylaxis. Meanwhile, low folate levels sirable effect. A knowledgeable physical therapist or ex- are corrected with multivitamin supplementation, ercise physiologist should guide any activity greater than which typically contains 400 ug of folate.
walking in the early postoperative period.
A complete blood count and serum levels of iron, Meanwhile, adequate fluid intake prevents constipa- total iron-binding capacity, and vitamin B12 should be tion, hypotension, urinary tract infections, kidney obtained in patients preoperatively and again postoper- stones, and hyperuricemia. Patients should sip small atively at 6-month intervals during the first 2 years.
quantities of liquid on a regular basis to avoid these These levels should be checked annually thereafterAl- problems. They should avoid excessive sweating to limit though most surgeons do not measure serum calcium volume depletion in the early postoperative period. In addition, most patients should discontinue diuretics relatively harmless. In addition to a multivitamin, 1,200 postoperatively as some degree of volume depletion Multivitamins and oral iron supplements do not con- The demand for bariatric surgery increases as the epi- sistently correct low serum iron levYet, only 50% demic of morbid obesity continues to worsen. In 1990, of low Hgb levels are associated with iron deficiency.
surgeons performed approximately 4,900 bariatric proce- Many patients with severe iron deficiency anemia (Hgb dures compared with 100,000 procedures in 2000. Clearly Ͻ 10 g) respond rarely to oral iron supplementation bariatric surgery achieves a sustained weight loss with long- alone. Patients do respond to a variety of therapeutic term folloand resolves associated comorbidities measures including IM iron injections, IV iron dextran, including diabetes and hypertension.The principles SC injections of erythropoietin and blood transfusions.
that induce weight loss can result in deficiencies of protein, Total abdominal hysterectomy might be required in fat, carbohydrate, vitamins, and minerals. Understanding some patients with menorrhagia. Prophylactic oral sup- the physiologic basis of weight loss is crucial to produce plements containing 650 mg of elemental iron in pre- weight loss without causing malnutrition. Because the number of RYGB continues to increase, more research is Adequate protein and calorie consumption is neces- needed to define the physiologic mechanisms associated sary to maintain muscle mass. Patients unable to con- Physiologic Basis of Roux-en-Y Gastric Bypass 17. Ito C, Mason EE. Gastric bypass and pancreatic secretion. Sur-
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