Protein and the Bariatric Patient

| July 18, 2008

by Laura Frank, PhD, MPH, RD, CD

Dr. Frank is with the St. Francis Center for Weight Management, Franciscan Health System, Federal Way, Washington.

Part 1 of a 3-part series.

The importance of protein in nutrition cannot be overstated. The word protein is derived from the Greek word proteos, meaning primary, or “taking first place.”1 Protein is made up of building blocks of amino acids that provide nitrogen to the body. Nitrogen-based compounds are constantly being degraded and resynthesized, which is a process called protein turnover. When protein turnover results in a net synthesis of body protein, the body is in a state of positive nitrogen balance.
Positive nitrogen balance is associated with increased protein tissue synthesis (e.g., building of lean body mass) and growth. In contrast, net degradation of protein is termed negative nitrogen balance and occurs in conditions such as malnutrition.

Protein is an important macronutrient for bariatric patients because it provides energy (4 calories per gram) and promotes wound healing and muscle protein synthesis. Furthermore, calories provided by protein may be more thermogenic and may help the bariatric surgery patient achieve weight loss goals more efficiently than the equal amount of calories provided from either carbohydrates or fats.2 However, due to restriction of dietary intake and/or malabsorption of dietary protein, bariatric surgery can put patients at risk for protein malnutrition.3

It is important for clinicians to have a thorough understanding of protein and its metabolism, including digestion and absorption pre- and post-surgery. It is also important for clinicians to monitor protein status either by serum protein biomarkers or body composition measures. Close monitoring of bariatric patients can decrease the patient’s risk of protein malnutrition.

Dietitians can determine patients’ protein needs and help patients identify high-quality food and supplemental sources of protein in order to meet these needs. However, best practice for nutrition guidelines pre- and post-surgery has not been established for any type of bariatric surgery. Continued multidisciplinary efforts to establish best practice for protein intakes are warranted in order to promote optimal health for our patients.

Overview of Protein
The building blocks of protein are referred to as amino acids.1 Amino acids are molecules that contain a nitrogen group attached to two hydrogens (-NH2), referred to as an amine group. There are essentially 20 amino acids.4 Nine of these 20 amino acids need to be obtained by the diet as the body cannot synthesize them; these are called the indispensable amino acids (IAAs). Approximately six amino acids are termed the conditionally indispensible (essential) amino acids (CIAAs), which become indispensable when the body is put under additional stress as in wound healing4 or after bariatric surgery. The Institutes of Medicine (IOM) have established a recommended dietary allowance (RDA) for the IAAs that should be used as a reference value when assessing dietary intake and protein supplements, shown in Table 1.5

Roles of Protein in the Body
Protein plays functional roles in the body, providing constituent amino acids for the synthesis of enzymes, hormones, and immunoproteins.1 Enzymes are catalysts used in chemical reactions. Hormones are signaling proteins that regulate many of our body’s metabolism and function. Immunoproteins are proteins that help protect our bodies from infection and illness.6,7 Structural proteins include fibrous proteins that make up our tissues, including collagen (found in our skin, bone, cartilage, and blood vessels), elastin (found in our skin, bone, cartilage, and blood vessels), keratin (found in our hair and nails), and the muscle contractile proteins (actin and myosin) are also considered structural proteins.

Transport proteins are vehicles of transport for several substances, including biomarkers of nitrogen balance, such as albumin, prealbumin (transthyretin), hemoglobin, myoglobin, transferrin, and ceruloplasmin.1,7 Each of these transport proteins is responsible for carrying nutrients and/or oxygen to various places in the body, and are often used as indicators of nutritional status. Lipoproteins are lipid-containing proteins, which transport cholesterol, triglycerides, and phospholipids to and from the liver.1,6

Digestion and Absorption
Once swallowed, protein foods enter the stomach from the esophagus. Protein digestion begins in the stomach with the action of hydrochloric acid (HCL). The release of HCL is stimulated by several mechanisms, including the release of gut peptides and hormones (gastrin, gastrin-releasing peptide [GRP], the neurotransmitter, acetylcholine [Ach], and the amine [nitrogen compound] histamine).1 HCL denatures or modifies the protein by causing the protein to unfold or uncoil. HCL also activates the conversion of the proteolytic enzyme pepsinogen (inactive pre-enzyme form or zymogen) to pepsin (active form). Pepsin hydrolyzes peptide bonds at a pH of less than 3.5 and creates large polypeptides, oligopeptides, and/or free amino acids (especially leucine, methionine, phenylalanine, tyrosine, tryptophan, glutamate, and aspartate).8 The end products of stomach protein digestion mixed with an acid-chyme are emptied through the pyloric sphincter into the duodenum for continued digestion.1

In the duodenum, the acid-chyme product stimulates gut peptides, such as secretin and cholecystokinin (CCK), released from the mucosal endocrine cells.1 Both secretin and CCK stimulate the pancreas to release pancreatic juices and alkaline digestive zymogens, which need to be converted by catalysts into active proteolytic enzymes (Table 2). Enteropeptidase is secreted from the intestinal brush border in response to secretin and CCK. Once trypsin is formed, it too can activate trypsinogen and chymotrypsinogen to yield active proteolytic enzymes.

The cavity of the small intestines, called the lumen, is a fertile ground for the initial holdings of free amino acids as well as oligopeptides. Proteolytic peptidases, located in the brush border of the intestine, hydrolyze and allow further breakdown of oligopeptides into tripeptides and dipeptides.

Although protein absorption occurs throughout the small intestine, the jejunum and ileum are particularly important in this role.3 Digestibility and absorption rates of several foods have been established, specifically for milk, peas, casein, whey, and free amino acids from enteral protein products. However, it is less clear about the specific absorption rates of meat, chicken, fish, and legumes.9 Absorption rates from the gut can vary from 1.4g/h for raw egg white to 8 to 10g/h for whey protein isolate; however, there have been no studies that have evaluated the upper limit of amino acid intake,10 especially in the bariatric community.

Liver Processing of Protein and Elimination
The urea cycle, which is found in the liver, is important for the removal of ammonia (NH4+) from the body. Ammonia is toxic because of its effects on the pH and can lead to brain malfunction and coma.1 The liver’s capacity to deaminate proteins and produce urea for excretion of excess nitrogen through the kidneys is one of the rate-limiting factors of protein digestion and is termed the maximum rate of urea excretion (MRUE).9,12 Rudman et al12 studied 10 healthy males and the MRUE associated with various levels of protein intakes (measured in grams (g) protein nitrogen (N)/kg of body weight (BW)/day). This study showed that with a protein dose of 0.53g protein N/kg/day, a MRUE was reached, averaging 55mg urea N/h/kg, persisting for 16 hours. Higher doses of protein (~1.33g protein N/kg/day) did not further accelerate urea excretion, but prolonged the duration of MRUE to 28 hours. These findings indicate that MRUE corresponds to a period of maximal rate of urea synthesis (MRUS). According to Rudman et al, the MRUS in healthy individuals has been reported as an average of 65mg N/h/kg (range of 55–76). Based on the range of MRUS, the recommended daily protein intake for structural use would be 56g for a 70kg individual. However, the level of dietary protein that can be deaminated and processed through the urea by the liver in a 24-hour period is dependent upon body weight and individual variation in efficiency.

The elimination of protein continues through the colon. However, in general, less than one percent (approximately 10g) of ingested protein is excreted daily in the feces.1 Usually, colonic bacterial proteases have little effect on protein digestion and amino acid absorption. However, the colon over time has been shown to adapt to protein absorption among post-bariatric patients.13 More research is needed within the bariatric population to determine how much the colon contributes to overall amino acid kinetics.

Sources of Protein
Protein is found in both animal and plant sources. Good quality protein is an expression of a protein’s ability to provide the nitrogen and amino acids requirement for growth, maintenance, and repair,5 and therefore contains the IAAs. Animal sources of protein, including meat, poultry, fish, eggs, and dairy, are good quality proteins and provide all of the IAAs. Plant sources of protein other than soy, (legumes, vegetables, and grains) tend to have one or more indispensable amino acids lacking, thereby decreasing protein quality.

There are biologic methods used to assess protein quality, including the net protein utilization (NPU), defined as the nitrogen retained in the body/nitrogen consumed; biologic value (BV), defined as the nitrogen retained in the body/nitrogen absorbed from the gut; and the protein efficiency ratio (PER), defined as weight gain/nitrogen consumed.14 Other researchers have utilized the efficiency of nitrogen utilization (ENU) to measure protein quality. Rand et al15 reported that the ENU for nitrogen retention is approximately 50 percent in healthy adults, with no apparent differences in sex, diet, or climate. Significantly lower ENU was found for older subjects compared to younger subjects (p=0.003). In 1991, the protein digestibility corrected amino acid score (PDCAA) was established as a superior method for the evaluation of protein quality.14 This score indicates the overall quality of a protein because it represents the relative adequacy of its most limiting amino acid.5 The PDCAA indicates the body’s ability to use that product for protein synthesis. The PDCAA is equal to 100 for milk, casein, whey, egg white, and soy.4,5

Protein Requirements in Humans
The protein requirement for humans is the sum of the requirement for each of the IAAs plus the non-specific nitrogen requirement.5 The 2005 dietary reference intakes (DRIs)5 suggest that the average protein requirement for adults is 0.66g/kg/day. Nitrogen balance data (corrected for nitrogen intake and dermal and miscellaneous nitrogen losses) are valuable in assessing protein requirements. A meta-analysis from 235 subjects were gathered from 19 studies examining the estimation of basal or maintenance requirements of nitrogen intakes in healthy adults.15 The authors reported that the median estimated average requirement (EAR) of nitrogen from these data was 105mg N/kg/day and the 97.5th percentile (defined as the recommended dietary allowance [RDA]) was estimated as 132mg N/kg/day or 0.65 and 0.83g good quality protein/kg/day. Therefore, the RDA for protein is set at 0.8g/kg/day or approximately 50g per day for normal adults.5 The Institute of Medicine (IOM) has also published RDAs for each of the IAAs (Table 1).5

The protein requirement for an individual is significantly increased when an individual is not meeting his or her overall energy/caloric needs compared to an individual who is in positive nitrogen balance.16 Pellett and Young17 showed that nitrogen balance was severely compromised when dietary energy intakes were less than 35kcal/kg. Researchers have concluded that adding 100g of carbohydrate per day decreases nitrogen loss by 40 percent in modified protein fasts.7, 18
Because muscle protein accretion or anabolism is indicative of positive nitrogen balance, measurements of protein synthesis are important. Indispensable amino acids, including branched chain amino acids (BCAA), are primarily responsible for acute amino acid-stimulated muscle anabolism.19 Studies on resting human muscle indicate that administration of BCAAs, particularly leucine, has an anabolic effect on protein metabolism either by increasing the rate of protein synthesis, decreasing the rate of protein degradation, or both.20, 21 Furthermore, IAAs, including the BCAAs, have been shown to activate key enzymes in protein synthesis after physical exercise.22, 23 Therefore, postprandial leucine balance can be used as an index of protein deposition9, 24 and can help in the clinical assessment of nitrogen balance. However, practically speaking, assuming adequate total energy intakes, a diet providing all of the IAAs, including the BCAAs, should result in positive nitrogen balance and protein accretion.

Upper Limits of Protein Ingestion

Less is known about the maximum amount of nitrogen that a body can digest and utilize. To date, there have been no studies that have established a tolerable upper limit (TUL) of amino acid uptake.10 According to the IOM, there is insufficient data in humans to establish a TUL for either total protein or any of the amino acids.5 Lack of scientific data about TULs, however, does not diminish the fact that consuming large amounts of amino acids, especially disproportionate amounts of specific amino acids, may cause adverse physiological effects.10 For example, large quantities of methionine, cysteine, and histidine have been found to cause tissue damage with chronic administration.25

The maximum safe protein intakes for humans have been estimated at approximately 285g/d for an 80kg male.9 However, Bilsborough and Mann9 suggest the maximum protein intake should be based on bodily needs, weight control evidence, and avoidance of protein toxicity. These authors recommend that maximum protein intakes should be approximately 25 percent of total energy intakes at approximately 2 to 2.5g/kg/d, corresponding to 176g protein per day for an 80kg individual on a 2,800-calorie diet.

There is conflicting data regarding the metabolic disadvantages of high-protein diets. Bernstein et al26 concluded that long-term consumption of high-protein diets by persons with normal kidney function may cause renal injury; however, other researchers have shown discordant results.27-29 Matrin et al29 concluded that high-protein diets promote normal adaptive responses that are well within the function of a normal healthy kidney. More studies are needed to determine if high-protein diets are deleterious to the kidneys. Also, promoting a high-protein diet from high-saturated fat food sources (animal sources of protein) versus monounsaturated or polyunsaturated food sources (plant sources of protein) would not be prudent dietary practice due to the increased risk of heart disease.

Clinicians should have a broad understanding of protein nutrition, including food sources, digestion and absorption, and the various roles that protein plays in the body. Protein’s role of providing energy and nitrogen to the body underscores its importance in human nutrition. Continued research regarding protein intake in the pre- and post-bariatric patient is warranted in order to establish best practice nutritional guidelines.

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Category: Nutritional Considerations in the Bariatric Patient, Past Articles

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