Nutrition and Bioavailability

What You Need to Know About WLS, Food, Nutrition and Bioavailability

January 8, 2016

There is a strong connection between WLS, food, nutrition and bioavailability.  Having weight loss surgery greatly impacts the types of food post-op patients should eat, the nutrition needed for good health along with the bioavailability from those foods.

Bariatric surgery is the most effective solution for long-term weight loss in individuals affected by morbid obesity.(1) On average, bariatric surgery patients experience a 65-80% loss of excess body weight (with some patients reaching 100% loss of excess body weight) at one year after surgery.(2) However, along with substantial weight loss, malabsorption of fat, protein, vitamins and minerals also occur.(1,2)

What is the Digestion Impact on VSG and RNY WLS?

With the Vertical Sleeve Gastrectomy (VSG), the stomach is divided and stapled vertically, removing about 85%, leaving a tube or banana-shaped pouch.(3) The stomach pouch is significantly smaller and can only hold two to four ounces of food compared to a typical normal stomach, which can hold about four cups (32 ounces) of food.(3) The Sleeve works primarily by restricting the amount of food, and therefore, total calories, which can be eaten and absorbed by the body.(1,3) The Gastric Sleeve has the smallest impact on digestion as food still passes through the stomach and all of the intestines.(3)

With a Gastric Bypass (RNY), the stomach is reduced to the size of a small egg.(4) For the bypass portion, the upper segment of the small intestine is divided, and the bottom end of the divided small intestine is brought up and connected to the newly formed small stomach pouch.(4) The bypassed upper segment of the small intestine is then reattached to the small intestine further down so that digestive enzymes will help break down food.(4) By ‘bypassing’ or ‘rerouting’ the flow of food, there is less absorption of total calories, fat, and protein. This can achieve up to a 25% greater weight loss than the Gastric Sleeve, but comes with additional risk.(4)

What are Dumping Syndrome and Ghrelin?

Dumping Syndrome typically only occurs in Gastric Bypass patients when food empties too quickly from the stomach into the small intestine.(5) Following bypass surgery, high fat and high sugar foods are unable to be broken down in the stomach and are sent straight to the intestines.(5) The intestine cannot break down high fat or high sugar foods, so it tries to dilute the food by ‘dumping’ water into your intestines causing nausea, cramping, diarrhea, sweating, faintness, and heart palpitations.(5)

Bariatric surgery also affects gut hormones that control hunger and satiety by removing the upper curvature of the stomach, called the fundus.(1) The fundus is where the hunger hormone Ghrelin is produced.(1) Without Ghrelin, most patients do not have feelings of hunger after surgery.(1)

How Does the Loop Duodenal Switch or SADI-S Work?

With the Loop Duodenal Switch or SADI-S (Single Anastomosis Duodeno-Ileal Bypass with Sleeve Gastrectomy), the stomach is reduced to a small tube or banana-shaped pouch, identical to the Sleeve Gastrectomy.(6) The small intestine is then divided in half, and the midpoint of the divided small intestine is brought up or ‘looped’ and connected to the sleeved small stomach pouch.(6) The Loop Duodenal Switch bypasses a significant portion of the small intestine, greater than that of the Gastric Bypass surgery. This means less absorption of calories, particularly fat and protein, and achieves a greater weight loss than the Sleeve Gastrectomy and Gastric Bypass (long-term, patients can lose 75-100% of their excess body weight). (6) However, decreased absorption puts patients at a much higher risk for nutritional deficiencies.

Additionally, with the removal of the majority of the stomach, acid production is also reduced and some foods may be more difficult to digest after surgery (such as tougher meats and fibrous vegetables). With decreased acid production, many patients also experience taste changes and no longer have an appetite for certain foods, most commonly eggs, milk, beef, and chicken. Furthermore, about 10% of patients develop a lactose intolerance or sensitivity to dairy products after bariatric surgery.(5)

The altered anatomy of the digestive tract after bariatric surgery results in decreased absorption of carbohydrates, fat, protein, vitamins and minerals which your body needs to function properly.

Nutrition and Bioavailability: What Happens With Carbohydrates and Fats?

Carbohydrates are well absorbed after surgery as starches are digested and absorbed all throughout the length of the intestinal tract.(7) Since carbohydrates are easily absorbed after surgery, it is important to watch the number of carbohydrates in the diet as too much carbohydrate consumption can hinder weight loss.(5,6) Additionally, refined starches high in sugar can cause dumping syndrome.(5) While fruits and vegetables are considered starches these should not be avoided as they are low in calories and provide essential nutrients.(6) A diet including fruits and vegetables and limiting excessive carbohydrate intake from starchy foods (breads, pastas, crackers, refined cereals) and sweetened foods (cookies, cakes, candy, or other sweets) is recommended.(5)

Fat is primarily broken down and absorbed in the upper portion of the intestine, where fat mixes with bile salts from the liver.(7) When the upper portion of the intestines is removed, fat does not mix with bile salts and is not as easily broken down and absorbed. Fat malabsorption is present with Gastric Bypass and Loop Duodenal Switch surgeries (up to 75% of fat is malabsorbed  with the loop duodenal switch).(6) However, with decreased absorption of fat comes decreased absorption of vitamins including vitamins A, D, E, and K.(6) Additionally, eating too much fat can result in dumping syndrome in Gastric Bypass patients and causes increased occurrences of diarrhea in Loop Duodenal Switch patients.(5) Therefore, with bariatric surgeries than involve bypassing the intestine, a lower fat diet is recommended.(5,6)

Why is Protein so Important?

Protein intake is key after surgery as it provides energy, provides a sense of fullness for a longer period of time, and spares lean muscle mass during periods of rapid weight loss. Protein is broken down by stomach acids produced by the stomach and is absorbed in the intestine.(7) With the removal of the majority of the stomach, protein is not as easily broken down and absorbed after all types of bariatric surgery (up to 25% of protein is malabsorbed).(6)   Therefore, it is recommended that bariatric surgery patients consume 60-80 grams of protein per day (up to 90-100 grams per day for Loop Duodenal Switch patients).(6)

Initially after surgery, it can be difficult to meet these protein goals through food, and often times protein shakes are used to meet these goals. Whey protein isolate (best absorbed), whey protein concentrate, whey protein, soy protein isolate, and soy protein are recommended sources of protein in protein shakes and protein bars.(6) Collagen and gelatin should be avoided as they are low-quality sources of protein and are not well absorbed by the body.(6)

Complete sources of protein are best absorbed by the body and come from animal-based proteins such as meats, fish, eggs, and dairy.(6) Soy products are also considered complete sources of protein. Incomplete sources of protein are not as well absorbed by the body and include plant foods such as grains, legumes, nuts, seeds, and vegetables.(6) Incomplete proteins provide valuable nutrients and variety in the diet, and simply must be combined with other incomplete proteins to form a complete protein source.(6) Examples include rice with beans, cheese or peanut butter with crackers, and salad with nuts.

Why All the Focus on Vitamin Supplements?

After bariatric surgery, patients have a significantly decreased food intake and can no longer obtain adequate amounts of vitamin and minerals through food alone.

Additionally, removal of the majority of the stomach results in decreased production of stomach acid needed to break down and absorb nutrients.(1,6) Vitamin B12 and calcium both require stomach acid to be broken down and absorbed.(1,6) With decreased production of stomach acid, patients need to take a sublingual form (already broken down form) of Vitamin B-12 in order to absorb the vitamin.(1,6) Since calcium is best absorbed in an acidic environment, calcium citrate is the best supplemental form of calcium after surgery.(6)

Finally, the majority of vitamin and mineral absorption occurs in the upper portion of the intestine, which is bypassed in the Gastric Bypass and Loop Duodenal Switch surgeries.(6) Vitamins and minerals can be absorbed in other parts of the intestinal tract, but to a lesser degree.(6) Therefore, these surgeries have a higher risk of developing deficiency, particularly the Duodenal Switch which bypasses a larger portion of the intestine.(6)

To meet specific vitamin and mineral requirements to prevent deficiency, it is recommended that Gastric Bypass and Duodenal Switch patients take 200% DV of Multivitamin (100% DV of Multivitamin for Gastric Sleeve as patients have a complete intestinal tract), 3,000 IU vitamin D, B-50 Complex, 250-500 mcg vitamin B-12, 1,500 mg calcium, and iron.(6)

How Can You Live a Healthy WLS Life Long-Term?

For individuals afflicted by morbid obesity, bariatric surgery is the most effective treatment for long-term weight loss.(6) For all bariatric surgery patients, consuming adequate amounts of protein, limiting the intake of fat and starches, and taking a daily vitamin and mineral supplements is a lifelong commitment to ensure optimal health. Frequent regular follow-ups with the surgeon and registered dietitian are keys to success after bariatric surgery so that any problems or concerns that arise can be addressed by an experienced team.

Nutrition and Bioavailability References

  1. Sawaya RA MD, Jaffe J MD, Friedenberg L, et al. Vitamin, mineral, and drug absorption following bariatric surgery. Current Drug Metabolism. 2012 November; 13 (9): 1345-1355.
  2. Buchwald H, Estok R, Fahrbach K, et al. Weight and type 2 diabetes after Bariatric Surgery: systematic review and meta-analysis. American Journal of Medicine. 2009;122(3):248–256.
  3. U.S. National Library of Medicine- National Institutes of Health. Vertical sleeve gastrectomy. 2011. Accessed November 2015 from
  4. U.S. National Library of Medicine- National Institutes of Health. Gastric Bypass surgery. 2011. Accessed November 2015 from
  5. ASBS Public/Professional Education Committee - American Society for Metabolic and Bariatric Surgery. Bariatric surgery: postoperative concerns. 2008. Accessed November 2015 from
  6. Sánchez-Pernaute, Rubio MÁ, Pérez Aguirre E, et. al. Single-anastomosis duodenoileal bypass with sleeve gastrectomy: metabolic improvement and weight loss in first 100 patients. Surgery for Obesity Related and Diseases. 2013 Sep-Oct; 9(5):731-5.
  7. Sareen S. Gropper and Jack L Smith. Advanced Nutrition and Human Metabolism. 6th ed. Wadsworth: Cengage Learning; 2013.

Dena Dobbs is a Registered Bariatric Dietitian at Bariatric and Metabolic Center of Colorado

Nutrition and Bioavailability


Dena Dobbs is a Registered Bariatric Dietitian working alongside Dr. Joshua Long at the Bariatric and Metabolic Center of Colorado in Parker, Colorado. She received both her Bachelor’s and Master’s Degree in Human Nutrition and Food Science with a concentration in Dietetics from Colorado State University. She completed her dietetic internship at Penrose-St. Francis Health Services in Colorado Springs. Dena has a passion for building one-on-one relationships with bariatric patients to help them modify their eating habits to achieve healthier lifestyle behaviors.