Relevance of Dual Protein in Blood Glucose Maintenance and Satiety

Introduction

Type 2 diabetes is ranging from predominantly insulin resistance with relative insulin deficiency to predominantly an insulin secretory defect with insulin resistance. The transition from the early metabolic abnormalities that precede diabetes, impaired fasting glucose (IFG) and impaired glucose tolerance (IGT), to diabetes is occurring in most individuals (perhaps up to 70%).1,2 Medical nutrition therapy can have substantial effects on diabetes delay/prevention. We should focus on non-carbohydrate sources, especially protein.

Proteins form majority of the structure of muscles and other vital tissues in the body. They are also the building block of hormones, enzymes, and haemoglobin. They can also be utilized as a source of energy, though a secondary one.3 With proper combination of sources, vegetable proteins possess similar benefits as protein derived from animal sources (eggs, milk, meat, fish and poultry).3,4 The effectiveness of a protein is decided by finding its quality and rate of digestibility. The protein quality refers to the bioavailability of amino acids that it supplies. This is important when determining the nutritional benefits it can provide. While, digestibility refers to how a source of protein is best utilized by the body.3,4

Protein Digestibility Corrected Amino Acid Score (PDCAAS) has been adopted by FAO/WHO as the preferred method for determining the protein value in human nutrition. The method is based on comparison of the concentration of the first limiting essential amino acid in the test protein with the concentration of that amino acid in a reference (scoring) pattern.5

Types of Protein

All proteins are not the same. Classification based on the source of protein molecule is given below.

  Plant proteins Animal Proteins
Common source groundnuts, beans, pulse, cereal eggs, milk, meat and fish
Source used Soybeans Milk
Protein Soy protein isolate Whey, Casein
PDCAAS 1 1
  1. Plant proteins

Soy is a most widely used vegetable protein source.3

Soy protein isolate (Supro®) is derived from soybeans (leguminous)7 that is highly purified soy protein. Isolated soy protein is at least 90% protein by weight. It is the only vegetable food that has all essential amino acids. The quality of soy protein is higher than other plant proteins and comparable to animal protein. According to PDCAAS scale, soy protein is rated with a score of 1.0, the highest possible rating. It also has good digestibility.3,7,8  The protein which is predominantly present in soybean are storage proteins, namely 7S globulin (conglycinin) and 11S globulin (glycinin), which forms 80% of total proteins. Other storage proteins in lesser amount include 2S, 9S, and 15S. Soy protein is also associated with fatty acids, isoflavones, saponins, and phospholipids.9

Amino acid profile:

Soy protein is considered a complete protein as it contains most of the essential amino acids that are present in animal proteins. However, soy proteins contain low levels of methionine/glycine and lysine/arginine ratios as compared to casein.9

  1. Milk protein

Two important milk proteins are whey and casein.

Whey protein is a set of globular proteins made up of α- lactoalbumin and β-lactoalbumin. It has composition of well-balanced amino acids along with high-content essential amino acids. It is also composed of bovine serum albumin, immunoglobulins and lactoferrin (enzyme).10,11,12 Whey protein digests very quickly giving a fast, high and transient release of amino acids.3 It also stimulates rapid protein synthesis but a larger part of it is used as fuel as it is oxidized.3

Casein is a phosphoprotein. Casein at its phosphorylated site binds to calcium forming calcium phosphate–micelle complexes that increases the digestibility of casein.10,12 It is absorbed slowly in the body as casein micelle has an attractive property to form a clot or gel in the stomach which makes it power pack in the supply of nutrients. Casein micelle clot gives a steady release of amino acids in the blood so that protein formation continues for a prolonged period of time.3 Thus, casein results in a greater protein accumulation over a longer period of time.3

Amino acid profile:

On the basis of human amino acid requirements, digestibility, and bioavailability both whey protein and casein are designated as high quality proteins as they incorporate high proportion of essential amino acids. However the difference in physiological effects of both milk proteins is attributed to the difference in their amino acid composition. Whey protein contains relatively higher proportion of the branched chain amino acids (BCAA) like leucine, isoleucine, and valine as compared to casein. The branched chain amino acids and particularly leucine is shown to trigger increase in protein synthesis in type 2 diabetes mellitus. Among other essential amino acids casein contains a high proportion of histidine, methionine, phenylalanine, and valine than whey protein. In addition, casein also has high content of several non-essential amino acids like arginine, glutamic acid, proline, serine, and tyrosine.15

Role of Protein in Blood Glucose Maintenance

The impact of proteins on the metabolism of glucose depends on the composition of amino acid, kinetics of protein digestion and utilization of amino acids in the gastrointestinal tract.6 Protein stimulates the secretion of insulin which regulates the metabolism of glucose by decreasing the plasma glucose concentration.6,12,16,17

Insulin also potentiates the uptake of amino acids and in the synthesis of muscle protein. Amino acids, especially branched chain amino acids (leucine) and arginine, promotes the secretion of insulin.16

The process through which insulin secretion is stimulated by proteins are:16

  • Direct effect on pancreatic β-cells by absorbed amino acids
  • Indirect effect by release of hormone Incretins that enhances the secretion of insulin

Role of milk proteins in blood glucose maintenance:

Both the milk proteins, whey and casein stimulate increased insulin secretion that not only have potential to alter glucose uptake by tissue but also control the rise in postprandial blood glucose. Whey protein reduces glucose concentration in blood due to its structure of branched chain amino acids consisting of leucine, isoleucine, valine, lysine, threonine that are potent amino acids in stimulating insulin.15

Whey is more potent than casein in increasing the glucose uptake by tissues and decreasing the concentration of plasma glucose as whey is digested more quickly causing increase in insulin release than casein that are digested slowly.6

Role of Soy protein in blood glucose maintenance:

Soy protein is useful in nondiabetic and diabetic individuals in the control of blood glucose. According to Chang et al., soy diet helps individuals with impaired glucose tolerance and reduced insulin sensitivity.18

Genistein, which is the most abundant isoflavone in soy19has antidiabetic property with its direct role in pancreatic β-cell proliferation and insulinotropic effect19 with glucose-stimulated insulin secretion. Genistein also has a potential therapeutic role for managing the complications of diabetes.  It helps to relieve diabetic peripheral painful neuropathy, revert proinflammatory cytokine and restoration of the nerve growth factor (content of the diabetic sciatic nerve).20

Protein and Satiety

Satiety or feeling of fullness with protein is greater as compared to carbohydrates and fat.10,21

Milk proteins especially whey proteins are more satiating as compared to other sources of protein. According to Veldhorst et al., whey protein had a greater effect in suppressing hunger than casein or soy.  Whey proteins induce satiety signals that are responsible for short- and long-term food intake regulation. Glycomacropeptide, a peptide that is found in whey protein, induces satiety by causing a release of hormone cholecystokinin which is involved in the control of intake of food.10,11

Soy protein may improve satiety by optimizing gut peptide response and increasing gut transit time. Also soy protein has shown to decrease the levels of peptide ghrelin, which is associated with hunger.22

Thus, diet that is high in protein causes less desire to eat causing increased post-meal and long-term satiety.23 This is achieved by a protein diet through following ways:11

Protein Appetite Theory

Satiety is mediated by total calorie intake and protein intake. Evidence supporting a link between dietary protein consumption and appetite was formalized as the aminostatic theory of feeding.  An inverse association between serum amino acid concentrations and reports of hunger was noted.24

According to protein leveraging hypothesis, when the proportion of protein needs are not met, food intake will increase until an appropriate amount of protein is ingested. Conversely, protein rich diets are purportedly ingested in low quantity as they provide the requisite amount of protein with relatively less total energy.24

Summary

This review has illustrated the importance of dual proteins in the regulation of amino acid, glucose, and food intake. A high-protein diet lowers blood glucose post-prandial in persons with type 2 diabetes and improves overall glucose control. The impact of proteins on the metabolism of glucose depends on the composition of amino acid, kinetics of protein digestion and utilization of amino acids in the GI tract. Protein is also beneficial in increasing and maintaining weight loss due to effects on appetite regulation and energy expenditure. Whey decreased hunger more than casein or soy coinciding with higher leucine, lysine, tryptophan, isoleucine, and threonine responses.

References

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