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Nutr Hosp. 2011;26(4): ISSN CODEN NUHOEQ S.V.R. 318 Revisión Impact of different protein sources in the glycemic and insulinemic responses F. C. Esteves de Oliveira 1, A. C. Pinheiro Volp

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Nutr Hosp. 2011;26(4): ISSN CODEN NUHOEQ S.V.R. 318 Revisión Impact of different protein sources in the glycemic and insulinemic responses F. C. Esteves de Oliveira 1, A. C. Pinheiro Volp 2 and R. C. Alfenas 3 1 Candidate for doctoral degree in Food Science and Technology. Federal University of Viçosa. Minas Gerais. Brazil. 2 Assistant Professor. Nutrition School. Federal University of Ouro Preto. Minas Gerais. Brazil. 3 Assistant Professor. Nutrition and Health Department. Federal University of Viçosa. Minas Gerais. Brazil. Abstract Objective: The maintenance of normal blood glucose concentrations is a crucial factor to the achievement of a good health status throughout life. However, the occurrence of abnormalities in this parameter has become increasingly common, which can result in several non-transmissible diseases, such as type 2 diabetes and cardiovascular diseases. Therefore, the purpose of this study was to discuss the role of protein sources in the glycemic and insulinemic responses. Methods: In this review paper, we critically analyzed recently published studies that discussed the role of different protein sources in the glycemic and insulinemic responses in healthy individuals and in those who have cardiovascular diseases or type 2 diabetes. Results: The results of some of these studies suggest that the daily ingestion of at least one high protein meal containing low to moderate amounts of carbohydrate increases insulin secretion and glucose uptake, improving insulin sensitivity. Furthermore, the results indicate that these effects are particularly associated with the consumption of animal protein (p.e. hydrolyzed whey protein), which has a high content of branched-chain amino acids such as leucine, valine and others such as arginine, which leads to improvements in insulin secretion and uptake glucose, since it increases insulin sensitivity. However, there is still no consensus in the literature about the quantity and quality of protein capable of reducing or maintaining blood-glucose concentrations at the desirable range, without causing adverse effects. The difference in the results of the studies may be associated to methodological problems presented by these studies. Conclusions: Well designed studies should be conducted to identify the quantity and quality of protein that can lead to the improvement on blood glucose concentrations, without negative effects to health. These studies should also identify the mechanisms and the magnitude by which protein may affect glycemic response. (Nutr Hosp. 2011;26: ) DOI: /nh Key words: Protein quality. Glycemic response. Type 2 diabetes. Cardiovascular diseases. Correspondence: Fernanda Cristina Esteves de Oliveira. Universidade Federal de Viçosa. Departamento de Nutrição e Saúde. Avda. P. H. Rolfs, s/n Viçosa. Minas Gerais. Brazil. Recibido: 22-III ª Revisión: 4-X ª Revisión: 19-X Aceptado: 16-XI IMPACTO DE DIFERENTES FUENTES PROTEICAS EN LA RESPUESTA GLUCÉMICA E INSULINEMICA Resumen Objetivo: El mantenimiento de la glucemia en concentraciones normales representa un factor crucial para conseguir un buen estado de salud a lo largo de la vida. Sin embargo, en la actualidad, cada vez son más frecuentes las alteraciones de dicho parámetro, siendo un factor de riesgo potencial para el desarrollo de diabetes del tipo 2 y enfermedades cardiovasculares. En este sentido, el objetivo del presente trabajo fue discutir el papel de diferentes fuentes proteicas en la repuesta glucémica y insulinémica a la alimentación. Metodología: Se ha realizado una revisión crítica de la literatura reciente teniendo en cuenta aquellos estudios que se han centrado en el análisis del papel de las diferentes fuentes proteicas en la respuesta glucémica y insulinémica tanto en individuos sanos como en aquellos que ya han desarrollado alguna alteración cardiovascular o bien presentan diabetes mellitus tipo 2. Resultados: Los resultados de los estudios científicos revisados sugieren que la ingesta de una o más comidas con alto contenido en proteínas y un contenido bajo o moderado de hidratos de carbono, mejora la secreción de insulina y la captación de glucosa, mejorando así la sensibilidad insulínica. Además, la fuente proteica (origen animal o vegetal) y la composición aminoacídica de las proteínas ingeridas juegan a su vez un papel importante ya que pueden dar lugar a efectos diferenciados en la glucemia y insulinemia. Así, normalmente aquellos estudios que han utilizado alimentos cuya composición protei - ca se basa principalmente en aminoácidos de cadena ramificada como la leucina, valina y otros como la arginina, han demostrado mejoras en la secreción de insulina y captación de glucosa, favoreciendo una mayor sensibilidad a la hormona. Además, este efecto se encuentra particularmente asociado al consumo de proteínas de origen animal (como las proteínas hidrolizadas del suero de leche), las cuales presentan un alto contenido de aminoácidos esenciales de cadena ramificada. Conclusiones: Sin embargo, todavía no hay un consenso respecto a la cantidad y calidad proteica capaz de reducir o bien mantener en equilibrio homeostático la concentración de glucosa con el fin de conseguir un buen estado de salud sin llegar a producir efectos adversos. (Nutr Hosp. 2011;26: ) DOI: /nh Palabras clave: Calidad proteica. Glucemia. Diabetes mellitus del tipo 2. Enfermedades cardiovasculares. 669 Abbreviations CVD: Cardiovascular diseases. GIP: Glucose-dependent insulinotropic polypeptide. GLP-1: Glucagon-like Polypeptide-1. kg: Kilogram. g: Gram. BMI: Body mass index. kg/m 2 : Kilogram/square meter. %: Percentage. mv: milivolts. BCAA: Branched-chain amino acids. LEU: Leucine. CHO: Carbohydrate. PTN: Protein. DM2: Type 2 Diabetes. PEPCK: Phosphoenolpiruvate carboxykinase. PGC-1α: Receptor and co-activator 1 α of activated reproducer of peroxisome. β: Beta. g/kg/h: Gram/kilogram/hour. g/l: Gram/liter. mmol: milimol. LIP: Lipid. HDL-c: High Density Lipoprotein-cholesterol. LDL-c: Low Density Lipoprotein-cholesterol. Introduction The maintenance of blood glucose concentration at the normal range is important to health throughout life. However, the occurrence of abnormalities in this parameter has become very frequent, leading to the metabolic syndrome and the development of several chronic diseases, such as type 2 diabetes (DM2) and cardiovascular diseases (CVD). 1,2,3 It has been claimed that the level of physical activity, body composition and some characteristics presented by food (For example: fruit ripeness, food physical form, processing method and preparation time, macronutrient contents, etc.) may significantly affect the postprandial glycemic response. 2,4,5 Scientific evidences suggest that the ingestion of high-protein meal, presenting low to moderate quantities of carbohydrate increases insulin secretion. 6 This effect may be attributed to the synergistic effect associated to increased protein and reduced carbohydrate ingestion, which result in the improvement of insulin sensitivity and in glucose uptake. Besides being potent insulin secretagogues (valine, leucine, isoleucine), some amino acids stimulate the incretin system: Glucose-dependent insulinotropic polypeptide (GIP) and Glucagon-like Polypeptide-1 (GLP-1), thus reducing the speed of gastric leakage, stimulating insulin secretion and inhibiting glucagon secretion. 7,8,9,10,11 Furthermore, some authors suggest that the source (animal or vegetable) and the amino acid composition of proteins may also cause different effects on blood glucose concentration. 12 Therefore, the present study aimed at discussing the role of different protein sources on glycemic and insulinemic responses, after critical analysis of studies on this subject already published. Proteins and Glycemic Response The intake of normal quantities of protein (0.8 g protein/kg/day, according to recommendations of the Institute of Medicine 13 ) stimulates insulin secretion, and may reduce significantly blood glucose concentration, depending on the amino acid profile presented by the protein ingested. This insulinotropic effect is also frequently observed after the consumption of high quantities of proteins. 12,14 However, there is no consensus in the literature about how much protein would cause such effect, without affecting health. A study involving normal weight individuals verified that the insulinemic response curve was higher as the protein intake and glycemic response curve were lower in response to the consumption of breakfast containing protein/glucose (g) (50/0, 0/50, 10/50, 30/50, 50/50). However, the gradual increase in protein intake did not result in a significant increase in insulinemic response. 15 In type 2 diabetic people, the increase in insulin secretion stimulation may be beneficial to hyperglycemia reduction, preventing the occurrence of lipolysis and excessive release of fatty acids, thus avoiding the occurrence of problems related to CVD. 16 In a previous study involving non-treated type 2 diabetics, which used the same protocol adopted by the authors of the above mentioned study, 15 the area under the insulinemic response curve increased linearly and the glycemic response decreased according to the quantity of protein ingested. This demonstrates that insulin secretion response is much more sensitive to protein intake by diabetic people. 17 However, it is important to emphasize that, in a long term, the excessive intake of proteins may lead to the occurrence of renal overload, development of CVD and osteoporosis. 18,19,20 Effects on the regulation of blood glucose concentrations The insulinotropic effect presented by proteins may lead to significant increase (over 200%) in insulinemic response and glucose uptake. This effect has been observed specially for the consumption of animal protein (such as whey hydrolyzed protein), which presents high amount of essential branched-chain amino acids. 16,21 Gannon et al. 6 verified that the intake of a high-protein (30% of protein) and low-carbohydrate (40% of carbohydrate) diet, during five weeks, reduced the postprandial blood glucose concentration in type 2 diabetic people and improved the glycemic control, if compared to the control diet (15% of protein, 55% of carbohydrate, and 30% of lipid). It is worth stressing that in this stud, 6 the consumption of high-protein diet 670 Nutr Hosp. 2011;26(4): F. C. Esteves de Oliveira et al. during five weeks did not affect the release of creatinin and urinary microalbumin, which are indicators of renal function. In spite of the importance of such results, it cannot affirm that such parameters would remain unchanged if such diet were applied for a long period. Furthermore, the participants of such study did not present homogeneous characteristics of age (39-79 years-old) or body mass index (BMI of kg/m 2 ), which may have influenced the results. It has been observed that individuals at the age of 60 years and/or BMI higher than kg/m 2 could have reduced insulin secretion and tolerance to the blood glucose level. Anther factor to be considered is that the effect observed in the blood glucose level was due to the increased protein intake or reduced carbohydrate in the diet, or a synergetic effect of both factors. Weigle et al. 14 verified a greater stimulus for insulin secretion after two weeks of eucaloric high-protein diet consumption (30% protein, 20% lipid and 50% carbohydrate) compared to the control diet (15% protein, 35% lipid and 50% carbohydrate) in overweight and obese individuals. This effect was attributed to the higher ability of proteins to stimulate insulin secretion, in comparison to lipids. In this work, the quantities of carbohydrates remained constant in the tested diets in order to isolate the insulinotropic effect of protein. But, since this evaluation was not carried out in this study, 14 the glycemic response could not be inferred from these individuals. It was evaluated the dose-response effect of 0 to 30 g (0, 5, 10 or 30 g) of soy protein concentrate or maize oil on glycemic response and insulin sensitivity, after the intake of 50 g of glucose, in non-diabetic individuals (normal insulinemic and hyperinsulinemic). The consumption of the meals tested did not affect the average blood glucose level in the groups of normal insulinemic and hyperinsulinemic individuals. However, it was observed a higher reduction (p 0.05) in the glycemic response when the dose of 30 g of protein was ingested. 22 This is an interesting result, but it must be emphasized that the meals tested in this study were liquid. Liquid food does not need chewing, and requires less time to pass through the intestine. Besides, its nutrients are more bioaccessible and bioavailable, 23 which could lead to a bigger postprandial glycemic elevation. Thus, it cannot be determined if these results could be inferred for solid food. Some authors 24,25 suggest that different proteins may stimulate the release of insulin differently. The insulinotropic effect of amino acids may occur because they allow the entrance of calcium, by a voltage-dependent mechanism related to the depolarization of the cell membrane. This depolarization occurs when there is a reduction in cell potassium exit, followed by the opening of sodium channels, leading to an intracellular increase of sodium and a reduction of calcium efflux. Consequently, the difference in the membrane potential reaches 0 mv. Thus, the channels of dependent voltage calcium open up themselves, promoting an increase in the cytoplasmatic calcium concentration, which carries on to a maximum insulin secretion. Other possible mechanisms of amino acid actions also occur to the blood glucose level, in other words, the stimulus to insulin secretion does not occur by fixation of glucose to a membrane receptor, but by a receptor that would be an enzyme of its own metabolism. For example, the insulinotropic effects of leucine seems be related to the glutamate dehydrogenase and dehydrogenase of branched keto acids. 24,25 Some authors 26 consider that leucine stimulates insulin secretion, acting both as a metabolic substrate and allosteric activator of the enzyme glutamate dehydrogenase, leading to increased glutaminolysis. Glutamate dehydrogenase is the key enzyme controlling aminoacids and ammonia metabolism in pancreatic β cells, liver, and brain. It is believed that leucine or its transaminated product, a-ketoisocaproate, regulates K ATP channel activity and results in increased free cytosolic Ca 2+,which triggers insulin secretory granule exocytosis via mechanisms involving the activation of some protein kinases and protein acylation. Another possible mechanism by which leucine can stimulate insulin secretion is the regulation of gene transcription and protein synthesis in pancreatic islet β cells through the activation of the protein serine-threonine (mammalian target of rapamycin-mtor). The activation of this protein significantly stimulates the phosphorylation of p70 S6K and increases protein synthesis in pancreatic β cells in a rapamycin-sensitive and insulin-independent manner at physiological concentrations ranging from 0.4 mm to 4 mm. 26 It is known that high concentrations of branchedchain amino acids (BCAA) contribute to the production of glucose in the liver (gluconeogenesis), through the alanine-glucose cycle. The degradation of these amino acids in the skeletal muscles is connected to the production of alanine and glutamine, and the maintenance of glycemic homeostasis. This cycle involves a continuous release of BCAA from the liver and splenic circulation to the skeletal muscle. The amino acid gain by the muscular tissue leads to intracellular concentration increase and stimulates BCAA transamination for alanine production. This amino acid is released from the muscle to the blood and is captured by the liver to participate in the gluconeogenesis, which contributes to the homeostasis of the blood glucose level. The amino acids arising from the alanine-glucose cycle, which serve as the main carbon sources for endogenous production of glucose, represent about 40% of this production during the extended exercise and approximately 70% after nocturnal fasting. 27 Furthermore, it has been verified the occurrence of a positive correlation between the postprandial insulinemia and the increase of amino acid levels in the plasma. This effect is more intense in response to the presence of some specific amino acids, such as: leucine, valine, lysine and isoleucine. This fact can be explained by their structure, making digestion easier and resulting in Protein in glycemic and insulinemic responses Nutr Hosp. 2011;26(4): Membrane depolarization Cell β INSULIN Food Intake Amino Acids (BCAA in special) Liver Increase of gluconeogenesis Blood Increase of plasmatic aa Activation of incretin system GLYCIDIC HOMEOSTASIS Abbreviations: branched-chain amino acids (BCAA). Muscle Increase of alanine Fig. 1. Schematic representation of glycidic homeostasis modulated by amino acid ingestion. faster release of peptides or amino acids bioactive in the bloodstream, where incretins are activated. 9,10,21 Therefore, the postprandial metabolic pathway of amino acids could be a crucial step for the insulinotropic properties of dietary proteins. A schematic view of such effects could be observed in the figure 1. The effects of the hydrolyzed casein and leucine (LEU) on insulinemia and blood glucose concentration were evaluated after the intake of three beverages with different contents: 0.7 g/kg of carbohydrate (CHO) or 0.7 g/kg of CHO g/kg of protein (PTN) or 0.7 g/kg of CHO g/kg PTN g/kg LEU. The beverages were offered to type 2 diabetic people and healthy controls in three different events, with an interval of at least seven days between the tests. It was verified a significant increase in the plasma insulin response after the consumption of beverages CHO + PTN and CHO + PTN + LEU, corresponding to 141 and 204% in the diabetic individuals. In the control individuals, this increase was 66 and 221% respectively, if compared with the beverage that contains CHO (p 0.05) only. There was a significant reduction of 12 and 15% for the glycemic response in the group with DM2 and 92, and 97%, in the controls after the consumption of beverages CHO + PTN and CHO + PTN + LEU, respectively (p 0.05). Although the insulinemic response is not different among the experimental groups, the blood glucose concentration was substantially higher in diabetic patients in all classifications, if compared to the controls (p 0.01). 25 The results of this study indicate that the ingestion of proteins with or without leucine increases the insulin secretion in diabetic and non-diabetic individuals, and lead to a reduced glycemic response of these individuals. Another study evaluated the effects of increased leucine intake in the diet, during 14 weeks, on the improvement of the glycemic control of rats. It was verified that leucine supplementation significantly prevented the hyperglycemia observed after the consumption of a high-fat diet, which is related to the improvement in insulin sensitivity and an increase in glucose tolerance, reduced glucagon concentrations and gene expressions of glucose-6-phosphatase, phosphoenolpiruvate carboxykinase (PEPCK) and peroxisome proliferator-activated receptor-γ co-activator 1α (PGC-1α) involved in hepatic gluconeogenesis. 28 Nilsson et al. 9 evaluated the effect of different protein sources (cod fish, milk, whey, cheese) on insulin secretion in healthy individuals. The quantities of lactose were equivalent to those in dairy meals. Dairy products, mainly whey, presented more insulinotropic properties than other proteins probably because the amino acid composition of such food presents a great quantity of branched-chain amino acids, which are potent insulin secretagogue. 9,10,21 In a study 29 involving type
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