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The anatomical changes that occur to the gastrointestinal tract after bariatric surgery have a significant impact on the nutritional needs of an individual in the pre-, peri- and post-operative periods.
Nonsurgical pre-operative weight loss is a common practice aimed to promote positive peri- and postoperative outcomes, including decreased surgery time, length of stay, and complications.1 Whether or not these benefits are significant enough to have nonsurgical weight loss a requirement for bariatric surgery, as many insurance plans currently do, remains controversial. While some argue that medically-supervised weight loss programs create and maintain good habits needed after surgery, others note it is an unnecessary barrier for individuals who have already had multiple unsuccessful weight loss attempts.
Pre-surgical weight loss has very few risks and many benefits. Medical weight loss programs typically have comprehensive approaches that cover nutrition, fitness and behavior strategies to give patients tools for long-term success. Significant weight loss in these programs use meal replacements to achieve a calorie deficit with high protein to facilitate patient adherence.1
In contrast to a long-term (3-12 month) weight loss requirement from insurance companies, short-term (1-4 weeks) loss with intention on reducing liver volume is common. An enlarged or fatty liver may increase technical aspects for the surgeon, increase risk of bleeding, and lead to complications that convert laparoscopic procedures to open ones.2-4 The ASMBS guidelines recommend initiating a “liver shrinkage diet” prior to surgery for these individuals.5 In order to reduce glycogen stores in the liver, very-low-calorie-diets (VLCD) or low-calorie diets (LCD) utilizing ketogenic methods and/or meal replacement products are common. While these diets vary in composition and duration, the literature has shown that 2–4-week pre-op diets between 800-1500 calories successfully reduces liver volume prior to surgery. Baseline liver volume, lean body mass, and adherence are also variables that can impact desired results.2,6-7
| Calories | 100-200 |
| Protein (g) | 25-30 |
| Sugar (g) | <10 |
| Carbohydrates (g) | <15 |
Immediately after MBS, individuals progress through multiple diet phases with varying textures to minimize post-operative complications. The length of time spent in each phase varies by patient needs and practitioner preference.
During this diet progression, protein shakes and meal replacements provide a tool to meet protein, fluid, and other nutrient needs that can be difficult to meet on foods/liquids alone. Whey protein with branched-chained amino acids (BCAA) are typically recommended to attenuate lean muscle loss during the rapid weight loss phase.8
| Clear Liquids | Full Liquids | Pureed/Soft | Regular | |
| Time (Post OP) | 24 - 48 hours | 3 days - 2 weeks | 2 - 4 weeks | 4 - 6 weeks |
| Description | Sipping clear, room temperature liquids slowly to reach the recommended 8 cups/day. Recommend sipping 15mL every 15-30 minutes, with a maximum of ½ cup at one serving. Estimated intake of 400-500kcal | Continued liquid diet, with no more than ¾ cup volume at one serving. Introduce full liquids and high-protein shakes to meet nutrition requirements. Initiate bariatric supplementation when tolerated. May introduce pureed or soft foods when tolerated | Incorporate pureed, semisoft, and smooth foods, increasing to 3-5 small meals per day if tolerated. Protein recommended first while separating liquids from meals. One protein shake is still recommended to meet nutrient needs. Soft, mashed textures can be added if tolerated | Adding solid foods, including beans, red meat, fresh vegetables, fruits, bread, rice, and pasta slowly and monitor for tolerance. Chew adequately and slowly to prevent blockage |
| Example of Foods to Eat | Water, broth, strained cream soups, and unsweetened gelatin and juices | Milk, soy milk, plain yogurt, broth, strained cream soups, protein shakes. Pureed foods when tolerated | Scrambled/boiled eggs, blendered meat, fish, chicken, turkey. Protein Shakes. Peeled/cooked vegetables, peeled/soft fruits if tolerated | Most regular foods as tolerated by case-by-case basis |
| Foods to Avoid | Beverages with sugar, caffeine, or carbonation | Beverages with sugar, caffeine, or carbonation | Foods that include nuts, seeds, fibrous skins or peels | Foods that include nuts, seeds, fibrous skins or peels |
After an individual progresses to regular textures, their long-term diet is dependent on their own experiences. Patients typically maintain between 800-1500 calories daily. Small, frequent meals and snacks should not exceed 1 cup volume.9 Balanced, well-rounded eating patterns (MyPlate, DASH) that provide adequate fruits, vegetables, fiber, and phytochemicals is recommended.5 Protein is emphasized as the primary macronutrient of focus. The ASMBS recommends a minimum of 60 grams daily, but many require a high goal.5,8 “Protein First” is a common phrase among the community used to encourage individuals to eat protein before more filling grains, fruits, and vegetables. Meeting the minimum 130 grams of carbohydrates is recommended, with an acceptable distribution of 35-50% carbohydrate from total Calories.8-9 A focus on complex carbohydrates, with no more than 25 grams of simple sugar per meal is suggested to prevent dumping syndrome.
Nutrient absorption happens throughout the entire digestive tract. While more than 90% food is digested and absorbed before the proximal to mid-jejunum,11 various nutrients are (or can be) absorbed in the stomach and large intestine. The digestive system is an interconnected network requiring secretions from ancillary organs like the gallbladder, pancreas, and salivary glands for proper absorption. Various malabsorptive procedures (RNY, BPD/DS, SADI-S) bypass different sections of the small intestine, which acts as the major source of nutrient absorption. Even the variability between these procedures can be influenced by differences in surface area, access to digestive enzymes, and hormone regulation.
The Vertical Sleeve Gastrectomy limits the stomach to a volume of 150-200mL while a pouch created in a Roux-en-Y gastric bypass is about 30mL. While restricting gastric volume was thought to be the main driver of weight loss, hormonal effects have shown their influence as well. VSG removes 80% of the stomach, including the fundus and greater curvature, which is where parietal cells that produce ghrelin and leptin are located. The resection of the antrum may also have a role in increasing gastric emptying, glucose/insulin response, and hormone regulation, but more is needed to understand these effects.13-14 These differences may contribute to the reasons why weight loss, hunger and satiety cues, and metabolic changes after VSG are more similar to that of a RNY rather than Adjustable Gastric Band.
Procedures like the AGB and VSG do not alter the GI tract posterior to the stomach, therefore, pancreatic secretions are able to continue to resume its path along the entirety of the small intestine. However, in malabsorptive procedures like RNY, BPD/DS, and SADI-S, segments of the small intestine are relocated to create two different pathways/channels. The limb in which partially digested food is carried is called the alimentary or Roux limb. While the partially digested food does pass through, the absence of biliary and pancreatic secretions significantly stunts absorption.15 This “bypassed” segment of the GI tract is no longer utilized for absorption but rather as a path to transport biliopancreatic secretions.
The biliopancreatic limb reconnects to the alimentary limb to create the common channel. This shorter segment of the GI tract is where chyme interacts with the digestive secretions and allows for absorption.
| Stomach | 1 foot |
| Duodenum | 1 foot |
| Jejunum | 8 feet |
| Ileum | 12 feet |
| Large Intestine | 5 feet |
It is thought that the length of the common channel may play a role in managing nutrient absorption.15 Logically, the shorter the common channel, the more malabsorption, the more weight loss. However, this also means less exposure to pancreatic digestive enzymes and less surface area for micronutrient absorption. In 2022, Wang and colleagues reviewed studies comparing different limb lengths in RNY and found that to avoid risks such of protein malnutrition and revision surgery, total alimentary limb length should be greater than 400 cm and common channel should be greater than 200 cm.17 However, other procedures like the BPD/DS maintain smaller common channels around 75-150cm long.17 Clinical research is continuing to investigating how various limb lengths balance expected weight loss, protein malnutrition, and micronutrient deficiencies.
Gastric changes not only influence exposure of nutrients to the small intestine, but change hormone regulation. Faster gastric emptying is thought to promote GLP-1 release, while intestinal transit time alters insulin and glucagon homeostasis.13,14 Lastly, the ileal break, which senses food in the jejunum and duodenum, act as a negative feedback loop to delay gastric emptying and decrease hunger.37 While these mechanisms are sure to support weight loss and glucose balance, in extremes it can also increase risk of hypoglycemia and dumping syndrome.13,14
A meal high in simple carbohydrates, can cause distention that results in unpleasant gastrointestinal and symptoms of early dumping syndrome.16,18 Drinking with meals can contribute to this as well.16 Late dumping, or reactive hypoglycemia can occur 1-3 hours later due to the rapid insulin response of that high-sugar meal.16,18 If left unmanaged, these complications can have significant consequences. Fear of dumping symptoms may cause decreased intake of energy and much needed nutrients.18 To ensure adequate intake and avoid deficiencies, those experiencing symptoms should be counseled on various dietary guidelines to prevent and manage dumping syndrome. If dietary changes are ineffective, pharmacological and surgical approaches may be needed.
| Nutrient | Deficiency Prevalence |
| Vitamin D | As high as 90% |
| Thiamin | 15-29% |
| Vitamin B12 | 2-18% |
| Folate | 2-54% |
| Iron | 14-45% |
| Vitamin A | 7-14% |
| Vitamin E | 2.2% |
| Zinc | 24-74% |
| Copper | As high as 70% |
| Selenium | 58% |
There are various nutrition concerns after surgery, but nutrient deficiencies are the most common. In many cases, these can be properly prevented and managed with adequate diet and supplementation.
Individuals with obesity are more likely to experience micronutrient inadequacies and deficiencies compared to normal-weight individuals.20 This is likely due to the altered metabolic activity influenced by excess adipose tissue. Vitamin D illustrates this well, because it adipose tissue is the nutrient’s main storage site. So it is not the vitamin’s inability to be absorbed (seen after MBS), but rather dysregulated release and utilization of the nutrient.
Many practitioners recommend a multivitamin-multimineral supplements be taken the weeks leading up to sugery. This builds the habit of taking daily supplements. Monitoring and repletion should be conducted as needed, as it is increasingly more difficult to correct after surgery
| Nutrient | AGB | VSG | RNY | BPD/DS |
| Iron | 14% | <18% | 20-55% | 13-62% |
| Vitamin B12 | 4-20% | 20% | ||
| Vitamin E | Uncommon | Up to 9% | Up to 65% | Up to 28% |
| Zinc | Up to 34% | Up to 19% | Up to 40% | Up to 70% |
| Copper | No data | Uncommon | 10-20% | Up to 90% |
| Thiamin | Up to 45% of patients depending on procedure | Up to 45% of patients depending on procedure | Up to 45% of patients depending on procedure | Up to 45% of patients depending on procedure |
| Folate | Up to 65% of patients depending on procedure | Up to 65% of patients depending on procedure | Up to 65% of patients depending on procedure | Up to 65% of patients depending on procedure |
| Vitamin A | Up to 70% of patients depending on procedure | Up to 70% of patients depending on procedure | Up to 70% of patients depending on procedure | Up to 70% of patients depending on procedure |
After bariatric surgery, anatomical, physiological, and dietary changes increase one’s risk of micronutrient deficiency. In addition to the decreased intake and malabsorptive anatomy, other factors such as decreased gastric secretions, food intolerances, and nausea and vomiting can also contribute.11 Incidence of deficiency is dependent on the nutrient and type of surgery.
Measuring weight can provide important information, but anthropometric analysis that distinguishes between adipose tissue, skeletal muscle mass, and fluid retention can provide information on protein status. While reductions in skeletal or lean muscle mass can be expected, excess loss can be a result of protein-calorie malnutrition that may require supplementation
| Nutrient | Laboratory Monitoring |
| Thiamin | High risk patients of all procedures |
| Vitamin B12 | All patients of all procedures |
| Folate | All patients of all procedures Focus on women of childbearing age |
| Iron | All patients of all procedures |
| Calcium | All patients of all procedures |
| Vitamin D | All patients of all procedures |
| Vitamin A | All patients who’ve had RNY or BPD/DS |
| Zinc | All patients who’ve had RNY or BPD/DS |
| Copper | All patients who’ve had RNY or BPD/DS< |
More details with Parrot et al., SOARD 2017, 13(5): 727-741
Laboratory assays can provide accurate and reliable results for micronutrient levels. Due to the high pre- and post-op risk of micronutrient deficiency, regular monitoring of lab values is recommended. The ASMBS’s 2016 Update for Nutrition identifies laboratory indices of deficiencies after bariatric surgery as well as repletion recommendations.19 Some providers suggest abstaining from supplements for eight hours prior to the lab draw for best accuracy.16 It is widely thought that supplemental biotin can influence certain laboratory assessments.
Clinical analysis, including physical exams are an integral part of an assessment for nutrient deficiencies. The Nutrition Focused Physical Exam (NFPE) is a standardized methods to evaluate clinical signs of the skin, nails, hair, eyes, and mucus membranes can provide clues for nutrient deficiencies. Almost all vitamins and mineral deficiencies are associated with clinical signs that can be observed with physical exams. Individuals trained in NFPE also evaluate changes in muscle mass that inform protein-calorie malnutrition, including those with excess adipose tissue.
Evaluating dietary intake is quick and easy way to identify certain nutrient deficiencies. However, it is not entirely reliable due to over- and underreporting. Nevertheless, having general idea of foods absent can provide valuable information about the general nutrient intake of an individual.
Other factors that influence an individual’s ability to eat healthy foods or take their recommended dietary supplements should always be considered when evaluating micronutrient deficiencies.
The practice of utilize two over-the-counter adult multivitamins has been used for many years, however, it usually does not meet the ASMBS recommendations. Adding additional vitamin and minerals to fill the gaps increases supplement burden and can jeopardize adherence. Having too many dietary supplements to take is one of the number one reasons individuals did not adhere to their recommended supplement schedule.25 The introduction of bariatric-specific multivitamin supplements provides options for individuals to meet these guidelines with less supplements. Providing various forms such as liquids, shakes, soft chews, hard chews, tablets, and soft chews considers individual variability in preference and tolerance.
