top of page

Protein and Gastric Bypass Surgery

One topic that the International Protein Board (iPB) is often asked about is protein and gastric bypass surgery. Basically, people and health practitioners want to know “how important is protein after gastric bypass surgery” and “are some proteins better than others”? These are important questions because food intake goes down after gastric bypass surgery, however the contribution of protein to those calories should go up. What's more, because the structure of the stomach and the path of food is modified, there are some important considerations. In this article the iPB, in collaboration with will provide important information and considerations on this key Protein Matter.

First, diet protein is needed for efficient repair and healing of the stomach and small intestine. And second, diet protein helps limit the loss of body protein during weight loss. The second point is critical as protein is the structural and functional basis of the body. Therefore, its best to hold to as much protein as possible during the significant weight loss following surgery. Knowing this, dietary protein must be evaluated based on its quality and potency in meeting body protein needs. Moreover, the sources and type of protein chooses is also influenced by how well they overlap with an individual’s lifestyle, food philosophies and/or restrictions. Lastly, since the capacity of the stomach is dramatically reduced as result of gastric bypass, the impact of digestion rate and efficiency should be evaluated as well, especially in the form of high-protein drinks and other foods. In this article we will address all these considerations.

Gastric Bypass Surgery

Before we get into the protein part of this discussion, lets dig into the basics of what is happening when someone has gastric bypass surgery. In the more common Roux-en-Y Gastric Bypass (RYGB) surgery, the upper stomach is segmented off from the bulk of the stomach and small 1-2oz pouch is created.(1) In addition, the middle segment of the small intestine is then connected to the pouch allowing access of the food from the upper stomach directly to the small intestine. This rearrangement leads to a bypass of most of the stomach, a.k.a. the gastric chamber, hence “gastric bypass”.

While the newly created stomach pouch drains into the middle segment of the small intestine, the remaining stomach still functions and drains into the upper small intestine into middle segment of the small intestine through a surgically created opening just below where the stomach pouch drains. One interesting point about gastric bypass is that food no longer depots in the stomach, but instead moves into the small intestine more rapidly than before surgery, and not dealing with a lot of stomach acid.(2-5) 

Protein Options

Choosing better protein sources after gastric bypass shouldn’t be difficult. However, there are few things to consider including the digestibility and quality of different proteins as well as your previous experiences. For some proteins, bypassing the acidic stomach and some of the digestive actions in the first part of the small intestine can influence how a protein is digested. Also, because food volume intake is reduced at meals and daily, protein quality and potency will be hugely important! To be more specific, the amino acid (protein’s building blocks) composition that is more important, specifically the essential amino acids. These amino acids must be plentiful in the diet for efficient conversion of food protein to body protein.


The highest quality proteins are milk-based proteins, so your thinking should start there. Milk protein is a combination of casein and whey, with more of the former than latter. Whey and casein boasts exceptional essential amino acid compositions and are the top of the list when it comes to leucine content, the most important of the essential amino acids. Whey is known to be the fastest digesting and absorbing protein as it moves through the stomach and into the small intestine quickly. Whey also has the highest leucine content and is often stated as the best protein post gastric bypass surgery.


Oppositely, casein tends to get jammed up in the stomach as it gels in the acid bath therein, much like making cheese. This slows its release into the small intestine and makes it a digestive turtle compared to the more rabbit-like digesting whey. Now here’s where it gets interesting. After gastric bypass surgery, casein moves through the stomach (pouch) and into the small intestine quickly since the stomach is generally bypassed.(4) This means that debates as to whether casein, or milk protein which is about 80% casein, may not be a good choice after gastric bypass, need to be redirected if the push back is based on digestion rate pre-surgery versus post-surgery. In support, a recent research study presented at Obesity Week 2018 suggested that a milk protein-based shake was able to help individuals get more protein daily in the first 12 weeks after gastric bypass surgery versus the standard of care.(6) 


The application of different protein sources after gastric bypass is vastly under-researched.(7,8) However, if one applies logic, proteins less reliant on stomach acid for greater digestibility as well as demonstrate higher general digestibility and higher biological value would make the more sense as protein targets. This means that as someone introduces more food proteins in the weeks following gastric bypass, egg, meat and poultry protein is more desirable while connective tissue protein (e.g. collagen, gelatin) and many plant-based may be targeted less when trying to achieve protein goals.


Wrapping this one up, the iPB recognizes that after post-bariatric surgery, protein is critical and should be evaluated more on amino acid composition and digestibility in the small intestine and not based on speed at which it might move through the stomach during pre-surgery times. Milk proteins, including casein is a desirable protein and has been shown to be beneficial post gastric bypass.


1.      Sjöström L. Review of the key results from the Swedish Obese Subjects (SOS) trial—a prospective controlled intervention study of bariatric surgery. J Intern Med 2013; 273:219–34.

2.      Falkén Y, Hellström PM, Holst JJ, Näslund E. Changes in glucose homeostasis after Roux-en-Y gastric bypass surgery for obesity at day three, two months, and one year after surgery: role of gut peptides. J Clin Endocrinol Metab 2011; 96:2227–35.

3.      Jacobsen SH, Bojsen-Møller KN, Dirksen C, Jørgensen NB, Clausen TR, Wulff BS, Kristiansen VB, Worm D, Hansen DL, Holst JJ, et al. Effects of gastric bypass surgery on glucose absorption and metabolism during a mixed meal in glucose-tolerant individuals. Diabetologia 2013; 56:2250–4.

4.      Bojsen-Møller KN et al. Accelerated protein digestion and amino acid absorption after Roux-en-Y gastric bypass. Am J Clin Nutr. 2015 Sep;102(3):600-7.

5.      Medeiros DM, Wildman REC 2019, Digestive System; In: Advanced Human Nutrition (4th ed); Jones & Bartlett Learning, Burlington, MA, USA

6.      Smith-Ryan AE, Hirsch KR, Blue MNM, Trexler ET, Wildman REC. Provision of a ready to drink protein shake post bariatric surgery: a pragmatic clinical trial. Obesity Week 2018. Nashville, TN.

7.      Khoo CM, Muehlbauer MJ, Stevens RD, Pamuklar Z, Chen J, Newgard CB, Torquati A. Postprandial metabolite profiles reveal differential nutrient handling after bariatric surgery compared with matched caloric restriction. Ann Surg 2014; 259:687–93.

8.      Lindqvist A, Spégel P, Ekelund M, Mulder H, Groop L, Hedenbro J, Wierup N. Effects of ingestion routes on hormonal and metabolic profiles in gastric-bypassed humans. J Clin Endocrinol Metab 2013;98: E856–61.



bottom of page