Protein Basics

| May 10, 2007 | 0 Comments

by Jacqueline Jacques, ND

Dr. Jacques is a naturopathic doctor with more than a decade of expertise in medical nutrition and serves as Chief Science Officer for Catalina Lifesciences LLC, in Irvine, California.


Protein is like a mantra for most patients who have undergone weight loss surgery. Eat protein first. Eat more protein. Use protein powder. Take protein with every meal. Yet as much as it is emphasized and discussed, there are many questions that are asked by clinicians and patients alike. Questions about protein digestion, quantity, and quality, as well as ways to optimize protein intake, are very common and are important to understand in relation to postoperative nutrition.

Proteins are one of the essential building blocks of the human body. They provide amino acids, which are a nutritional requirement of the body to produce its own proteins and a variety of nitrogen-based molecules. From the proteins we eat, the body synthesizes hormones, enzymes, immune system components, structural molecules, and many more elements indispensable to human life. In addition, protein helps to maintain both fluid and acid-base balance in the body. Needless to say, if there is inadequate protein in the body, health necessarily suffers.

As stated, dietary proteins provide essential amino acids. Generally, amino acids are classified as essential (absolutely needed for normal physiology), conditionally essential (we typically make enough, but under some conditions they are needed from an outside source), and non-essential (those that are manufactured adequately by the body without an outside source)
(Table 1).

Digestion, Absorption, Metabolism

Dietary protein can be provided by a variety of sources, including meat, dairy, and vegetables. This will be discussed in greater detail when we discuss food and eating. Protein digestion begins in the stomach. Hydrochloric acid in the stomach acts to denature a protein and causes the formation of pepsin from pepsinogen. Pepsin then begins the enzymatic digestion of the ingested proteins. The end result of gastric protein digestion is a chyme of mostly polypeptides and some free amino acids.

The next phase of protein digestion normally occurs in the duodenum by the action of pancreatic enzymes, including trypsin, chymotrypsin, elastases, peptidases, and others. The goal of this phase of digestion is to produce tripeptides, dipeptides, and free amino acids. The final digestive phase for protein involves enzymes, which free the final amino acids from the small peptide chains at the brush border of the intestinal lumen.

Amino acids are absorbed throughout the small intestine, with some locational preference based on the individual amino acid itself. The proximal small intestine is generally preferred over the distal small intestine. The colon is not thought to have much, if any, capacity for amino acid absorption.

Protein Requirements

In the calculation of human protein needs, both quality and quantity need to be taken into account. Because it is actually the amino acids of the proteins that are required by the body, proteins with higher amounts and proportions of the essential amino acids are thought of as providing better nutrition for human health. In general, proteins from animal sources have better amino acid profiles than those from plant sources.

Adult protein needs are based on daily losses of amino acids and
nitrogen. The Institute of Medicine (IOM) sets the Recommend Dietary Allowance (RDA)/Adequate Intake (AI) protein requirements at 46g/day for adult females and 56g/day in adult males.[1] These numbers are based on a calculation of 0.8 grams of protein per kilograms of body weight for a man weighing 70Kg. The requirement increases to 71g/day in pregnant or lactating women. If an individual deviates significantly from the standard weights used to calculate protein requirements, an individual protein requirement should be calculated. The IOM guidelines are also designed for patients who have a healthy body weight. If a patient is significantly over or under a healthy body weight, protein requirements should be calculated using ideal, rather than current body weight.

One question that is often raised is whether 0.8 grams of protein per kilograms is too low of a number to use when calculating protein requirement in bariatric surgery patients. There have been no scientific assessments of protein requirements after bariatric surgery. The closest estimations of protein requirements can be taken from knowledge of low and very low calorie diets. Most caloric restriction programs advise a minimum of 50 grams[2] of protein per day or 0.8 to 1.5 grams[3]of protein per kilogram of lean body mass to maintain nitrogen balance. It is very common for bariatric programs to recommend 50 to 70 grams of protein per day—essentially the range of the recommended daily allowance (RDA). Further study in this area would surely benefit our knowledge of nutrition in weight loss surgery patients.

Protein Deficiency

Protein malnutrition has been reported with both Roux-en-Y and duodenal switch. Little is known about overall incidence, as only around eight percent of surgeons track labs such as total protein, albumin, or prealbumin.[4] Limited studies suggest that patients with the most rapid or greatest amounts of weight loss are at greatest risk of protein malnutrition.[5] With surgical resection of the stomach, hydrochloric acid,[6] pepsinogen, and normal churning are all significantly reduced or eliminated. Furthermore, pancreatic enzymes that would also aid in protein digestion are redirected to a lower part of the small intestine. It is thus likely that maldigestion rather than malabsorption is a significant component of the . Some studies have also implicated low intake.[7]

Prealbumin is generally considered to be the most sensitive laboratory indicator of protein status.[8] However, it will respond rapidly to treatment, so it should not be used as a sole endpoint for assessing response to treatment. While albumin has been a traditional marker, it is relatively insensitive to dietary changes, and is also greatly affected by other factors like dehydration and kidney function.[9] Comparatively, prealbumin is relatively unaffected by these factors as well as by liver function. It is affected by alcohol abuse, low zinc status,10 and the use of some medications, such as systemic steroids. Other tests that can be used for assessment include transferrin and retinol binding protein.

Clinical manifestation of low protein status can be varied and, ultimately, can affect every organ system in the body. Muscle loss is common and may include not only peripheral muscle loss but also loss of cardiac and respiratory function. Patients may also have impaired immune function, poor wound healing, lethargy, loss of cognitive function, hair loss, and more.

Protein Quality/Bioavailability

A variety of methods exist for understanding protein quality. The two most commonly utilized methods are amino acid scoring (also called chemical scoring) and biological value (BV). Amino acids scoring compares a protein to the amino acid composition of egg protein (which is given a score of 100). Egg white is used as the reference because it has an amino acids pattern that makes it highly compatible with human nutritional needs. Scores will be below 100 if any single or multiple amino acids fall below optimal needs.

Wheat, for example, has an amino acids score of 51, primarily because it is very low in lysine. This type of scoring can be particularly important in strict vegetarians who need to assure adequate proportional intake of whole grains, legumes, and vegetables to avoid amino acid deficiencies.

Biological value is a measurement of the amount of nitrogen retained from the ingestion of a protein source at a given quantity. Higher quality proteins (those with higher amino acids scores) generally have higher BVs, and can be ingested at lower quantities to meet daily protein needs. In theory, it takes twice the amount of a protein with a BV of 50 to meet the same nutritional criteria as a protein with a BV of 100. In reality, BV is more accurate when protein intake is low; in other words, as one ingests higher and higher amounts of even lower BV foods, the scores tend to even out a bit.
A similar marker to BV is net protein utilization (NPU). Rather than being based on the amount of nitrogen retained, it is based on the amount of nitrogen excreted after a given protein load. The calculations will result in a number between zero and 1—zero representing no protein utilization, and 1 representing total protein utilization. Eggs and milk both have a 1 NPU score.

The latest scoring method to join the ranks is the Protein Digestibility Corrected Amino Acid Score (PDCAAS). This is the method currently accepted by the Food and Drug Administration and the World Health Organization for the rating of protein quality. The PDCAAS takes the amino acid score for a protein and incorporates a correction factor for digestibility in humans (older models typically used rodents). Currently, this is considered to be the International Standard, although adoption has been slow in the US.

When we look at all the data collectively, despite some discrepancies, it is obvious that some general rules apply to protein quality. Meat, dairy, and egg proteins (all animal proteins) clearly rate well above virtually all single sources of vegetable proteins. While some are a bit better than others (egg, for example, is overall a bit better than chicken), they are all very good. Soy is the highest rated of the vegetable proteins, but still falls short of its animal protein counterparts. That said, vegetarians can readily meet their protein needs by selecting from a variety of grains, nuts, seeds, and legumes such that they get a diverse range of amino acids.

Dietary Proteins

As stated, dietary protein can be obtained from a wide variety of food sources. Because the amino acid content of foods differs, the quality of dietary proteins differs. The densest and most complete (containing the most complete sets of amino acids) sources of protein are meat and other animal products, such as eggs and dairy foods. These are followed by legumes, nuts, seeds, grains, and vegetables. Most fruits contain only negligible amounts of protein.

To effectively know the protein content of the diet, one has to know the amount of food eaten, the amount of protein in the food, and something about the quality of the consumed protein. Table 2 shows some common foods, a typical serving size, the grams of protein provided, BV, and PDCAAS. It is also helpful to know that protein provides four calories per gram. One should note, however, that this is for pure protein. Protein containing foods will also contain other dietary elements, such as carbohydrate, fat, and fiber, so calories per serving of a particular food will vary. Protein content for foods also varies by both serving size and macronutrient content. A good rule of thumb when looking at diet is the 5-10-15-25 rule:

Foods provide approximately:
• 5 grams of protein for each egg or handful of nuts/seeds
• 10 grams of protein for a cup of milk or yogurt
• 15 grams of protein for a measured cup of beans, fi cup of cottage cheese, or 3 ounces of tofu
• 25 grams of protein for a 3- to 4-ounce serving of meat, chicken, or fish.

Since many protein foods are cooked, it is good to know that heat alters protein structure to varying degrees. Moderate cooking tends to benefit protein bioavailability as it starts the breakdown between peptide bonds, making the early parts of digestion easier. Overcooking of proteins, however, can render them less useful by resulting in heat-induced cross-linkages that are more resistant to digestion. For some vegetable proteins, especially those from beans and grains, cooking can vastly increase bioavailability as it breaks down cellulose and destroys anti-nutrients like tannins and phytic acid.[11] This may increase bioavailability for some bean proteins by upwards of 90 percent.[11] Nutrition experts will note that some amino acids themselves are harmed through cooking, freezing, and processing. This varies greatly by the food, exposure time, and/or type of processing, so much so that it is highly impractical to take all the variables into account. The easiest strategy is to assure that the diet contains varied protein sources, with a greater emphasis on appropriately cooked (especially not overcooked) whole-food sources.

Supplemental Proteins

A wide variety of supplemental proteins is available to consumers. These proteins range from simple, unadulterated protein powders, such as whey, egg, or soy protein, to liquid hydrolyzed collagen to bars, puddings, and cookies. For the purpose of simplicity, we will focus on protein-only products. It is easiest to think of these products like we think of food. Protein supplements can be rated and evaluated in the same manner as foods, and common sources have also been listed in Table 2. The most common questions regarding protein supplements are:

• Should they be used/are they
• What forms are best?
• Is there a concern about lactose (in dairy proteins)?

Given our current level of knowledge about protein requirements after surgery, it is very hard to state with certainty that protein supplements are a necessity after weight loss surgery. In the early weeks and months after surgery, as patients are transitioning to solid foods and then adjusting to the limitations of their procedure, protein supplements can certainly make it easier for patients to meet their protein requirements. Over the long-term, however, it is not clear whether there would be a general benefit. Some patients with special needs may benefit from long-term use of protein supplements. This could include patients with difficulty eating, vegetarians, and patients with increased malabsorption (such as those who have undergone duodenal switch procedures). There may also be good reason to consider short-term use in pregnancy, lactation, and before or after subsequent surgical procedures such as plastic surgery, as these are all times of conditionally higher protein requirements.

The question of what form of supplemental protein is best has reached new heights since the high-protein diet craze released a flood of designer proteins into the consumer marketplace. When we look purely at protein scoring across all methods, egg protein generally rates the highest in all categories, followed very closely by all forms of dairy protein (whey, casein, and milk protein solids). Soy protein rates highest in the vegetable protein category, trailed by pea, with rice as a very distant third. Hydrolyzed collagen, which is the form most commonly found in “pre-digested” liquid supplements, is very poorly rated on its own as it contains none of the essential amino acid tryptophan. Most commercial preparations, however, now fortify with tryptophan so this is generally not an issue. There is no scoring available for fortified products, but with the appropriate amount of tryptophan, they should be expected to rate very high.

Marketers of commercial protein preparations love to promote great technology in processing and filtration as part of what makes one product vastly superior to another. This article does not comment on all of the finesses of processing, but there are a few general points that can make a difference in taste and quality. When we look at the common proteins (whey, milk, and soy), we may notice that they are sold as two common preparations: Concentrates and isolates. There is a difference. For milk protein solids and whey protein, the filtration process that creates an isolate removes a large majority of the lactose, minerals, and fat content that are naturally-occurring in the raw material. This means that isolates tend to have very little milk sugar, no fat, and provide more protein in a smaller serving size. Not always, but often, they will also have better mixability and a cleaner taste. For soy and other vegetable proteins, isolates tend to markedly improve taste and dispersion. They also remove much of the carbohydrate content. This can be very important as the carbohydrate in legumes, such as soy beans and peas, tends to cause gas in a large number of people. As with dairy proteins, vegetable protein isolates also offer more protein per serving.

Some proteins, usually whey or collagen, are sold in a hydrolyzed form. Hydrolysis breaks large protein molecules down into peptide fragments—thus they are partially pre-digested. Hydrolyzed protein is easier to absorb and has an advantage for patients who have a hard time digesting and absorbing other forms. They are often the preferred form for raising protein status in patients who are protein deficient.

Lactose intolerance can be a concern for some bariatric patients, and many seek protein sources that are low lactose or lactose-free. Of all the dairy proteins, casein has the lowest lactose content. As stated, milk or whey isolates also tend to be very low, but should not be considered to be lactose-free unless specifically labeled as such. Ion-exchange whey proteins are generally the lowest in lactose of the whey proteins, but they are also very costly.

Eating Tips

After surgery, eating is usually staged as healing progresses. Typical stages include the following (durations vary by program and patient):
• Phase I: Clear liquids (2–5 days)
• Phase 2: Full liquids (5 days to several weeks)
• Phase 3: Pureed foods (2 weeks to 1 month)
• Phase 4: Soft foods (1 month to 6–8 weeks)
• Phase 5: Normal (8 weeks on)

The early postoperative phase is the greatest challenge for meeting daily protein requirements both because sources are not as easy to obtain and because patients are healing and adjusting to their procedure. In the early postoperative phase, it is common for patients to report much intolerance to foods, but they should be encouraged to try again at a later time as something that is not well received at two months might be fine at six months.

Since the clear liquid phase is generally quite short, counting protein grams is really not the goal at this stage. This is really an acute healing phase with a focus on maintaining hydration. Some clear liquids that do provide protein include beef or chicken stock and sugar-free gelatins (about a gram/serving for each). Liquid collagen products are also available and may provide a rich source of protein—sometimes 20 grams in a tablespoon. Some programs will also allow basic protein drinks (such as a simple plain or flavored whey protein dissolved in water) in this stage.

Full liquid sources of protein can include skim milk, sugar-free puddings, sugar free yogurt (without fruit pieces), creamed soups, or broth with plain whey protein added, and protein shakes are all good options in this stage. Most patients cannot consume more than 3 to 4 ounces at a time in this phase, but if protein is consumed at most eating occasions, patients should be able to start meeting their daily protein needs in this phase.
Once patients move to pureed foods, meeting their protein needs is only as difficult as what they can tolerate and what they will eat. The diet at this point can include soft scrambled eggs, ricotta, or small curd cottage cheese, soft fish or meat (like tuna or soft chicken), which is blended until smooth, pureed or well-mashed soft tofu, fat-free pureed black or pinto beans, pureed vegetables, and some pureed fruit. Some patients in this phase may do fine with foods cut into very small cubes and chewed well.

After this phase, the diet begins to transition to “normal,” meaning that patients will progressively be able to tolerate foods in more normal textures and, eventually, in larger quantities. In all phases, patients are generally advised to eat protein first and at each eating occasion. Assuming 3- to 4-ounce serving sizes, most patients should be able to accommodate most of their protein needs with food when they are comfortable with a normal food diet. Some, however, may be more easily able to meet their protein needs by continuing to supplement some protein through powders, liquids, or other products.

Since estimating intake of protein over weeks or months can be a challenge, those doing dietary counseling with weight loss surgery patients should consider periodic evaluation of a diet diary. Usually a three-day sample is a good representation. In patients reporting difficulty with protein foods, basic reminders include not overcooking proteins (which makes them tough and harder to digest), chewing well, using small bites, and limiting proteins that are high in fat (which may be poorly tolerated because of the fat content).


Maintaining good protein status is important for good health after weight loss surgery. Bariatric surgery may result in low protein status based on both low intake and altered digestion and absorption. Patients should ultimately be able to meet much of their protein needs through selection of high-quality dietary proteins, but many good choices exist for supplemental protein if it is needed or desired.

1. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Food and Nutrition Board. Institute of Medicine of the National Academies. The National Academies Press. Washington, DC, 2005;589–768. Accessed 10/03/06 at
2. Saris W. Very-Low-Calorie Diets and Sustained Weight Loss. Obesity Res 2001;9:S295–S301.
3. Blackburn GL. Protein requirements with very low calorie diets. Postgrad Med J 1984;60 Supp 3:59–65.
4. Updegraff TA, Neufeld NJ. Protein, iron, and folate status of patients prior to and following surgery for morbid obesity. J Am Diet Assoc 1981;78(2):135–40.
5. Segal A, Kinoshita Kussunoki D, Larino MA. Postsurgical refusal to eat: Anorexia nervosa, bulimia nervosa, or a new eating disorder? A case series. Obes Surg 2004;14(3):353–60.
6. Behrns KE, Smith CD, Sarr MG. Prospective evaluation of gastric acid secretion and cobalamin absorption following gastric bypass for clinically severe obesity. Dig Dis Sci 1994;39(2):315–20.
7. Moize V, Geliebter A, Gluck ME, et al. Obese patients have inadequate protein intake related to protein intolerance up to 1 year following Roux-en-Y gastric bypass. Obes Surg 2003;13(1):23–8.
8. Prealbumin in Nutritional Care Consensus Group. Measurement of visceral protein status in assessing protein and energy malnutrition: Standard of care. Nutrition 1995;11:169–71.
9. Beck FK, Rosenthal TC. Prealbumin: A Marker for Nutritional Evaluation. Am Fam Physician 2002;65(8):1575–8.
10. Bates J, McClain CJ. The effect of severe zinc deficiency on serum levels of albumin, transferrin, and prealbumin in man. Am J Clin Nutr 1981;34(9):1655–60.
11. Rehman Z, Salariya M. The effects of hydrothermal processing on antinutrients, protein, and starch digestibility of food legumes. Intern J Food Sci Tech 2005;40(7):695–700.

Category: Nutritional Considerations in the Bariatric Patient, Past Articles

Leave a Reply