Protein

Proteins are macromolecules that exert several physiological functioning. They are the building blocks of the cellular structures, organs, and also maintain the muscular structure of the body. There are twenty natural amino acids that assemble in different ways depending upon the genetic code and stereochemical properties; thus it forms polypeptide chains or proteins. Therefore, protein is essential for normal functioning of the life1.

Recent dietary guidelines reported that the RDA of protein is 0.8 g/kg body weight for adults. Each essential amino acid has their specific RDA and that can be fulfilled by consuming a different variety of protein-containing foods. Dietary Guidelines Advisory Committee recommended that remaining calorie consumption should be compensated by the greater variety of plant protein products2.

 

Classification of Protein

Body's protein requirement is fulfilled by dietary protein intake. Dietary protein sources can be classified into two groups - animal-derived proteins and plant-based proteins. Some basic differences of animal-derived proteins and plant-based proteins are as follows3:

Animal-derived proteins

Plant-based proteins

Contain greater amounts of essential amino acids

All the essential amino acids may not be sufficiently present.

Easily digestible

Digestion process is comparatively  slow

Less splanchnic extraction

Greater splanchnic extraction

Urea synthesis is less

Urea synthesis is high

 

Health benefits of protein

Dietary protein intake has several health benefits and preventive action against metabolic disorders, which include:

  • Assist to maintain body weight and decrease weight gain,
  • Improve the blood lipid profile
  • Reduce blood pressure
  • Decrease the risk of cardiovascular disease  
  • Improve glycemic response and provide low–glycemic index
  • Delaying gastric emptying
  • Stimulate insulin secretion  and prevent diabetes
  • Increase HDL (Good) cholesterol level
  • Reduce fat mass and
  • Maintain lean body mass during energy-deficient weight-loss diets3

These health benefits of protein also depend upon the source of protein.

 

Health benefits of plant-based protein

Control hypercholesteremia

Soy-based protein can control hypercholesteremia by decreasing total cholesterol level. In addition, some study result showed that isoflavones present in soy protein can improve LDL:

HDL cholesterol ratio, decrease triglycerides level and increase HDL cholesterol concentrations. Serum cholesterol-lowering effect of soy protein in association with isoflavones is significant.  

Soy protein containing isoflavones that increase the expression of sterol regulatory element binding protein2, which maintain the intracellular cholesterol concentration by clearing excess blood cholesterol. Therefore, soy protein lowers cholesterol level by up-regulating sterol in hepatic cells and decrease triglycerides level and plasma LDL level3.      

 

Hypertension

Soy protein in association with isoflavones has arterial-vasodilation effects and provide a greater decrease in diastolic and systolic blood pressure. However, blood pressure status is influenced by smoking and alcohol consumption habit, body weight and physical activity3. In addition, fruit and vegetable are rich sources of a fiber, magnesium, and potassium intake, therefore intake of this whole food along with plant-based protein definitely assist to maintain blood pressure2.  

 

Blood glucose level maintenance  

Soy protein-based diet able to reduce fasting blood glucose level in diabetic patients. Wheat gluten also lower insulinotropic effect and assist maintain Glucose homeostasis3.

 

Body weight management

There is a negative association between the intakes of plant-sourced protein and BMI or waist circumference. Plant-sourced protein can decrease body fat.  Research studies showed that soy protein supplementation is more potent than milk protein in reducing BMI, body fat, and body mass in postmenopausal women with hyperglycemia.  However, it has been also reported that soy protein exhibit a higher gain in lean body mass3.

 

Cardiac benefits  

It is observed that there is a close relationship between cardiac risk with obesity, hypertension, high blood glucose level and increase serum blood cholesterol level. Plant-based protein can control all these metabolic syndromes and reduce the risk of cardiac diseases. Cardiac benefits are more prominent with soy protein than animal protein3.

 

Evidential facts about Plant-Sourced Protein and Animal-Sourced Protein2, 3

  • The cholesterol-lowering effect depends upon amino acid content. Increased plasma level of arginine in the postprandial course provides hypocholesterolemic effect; whereas leucine acts as a cholesterol precursor and also stimulates insulinotropic effect. Both of these can decrease cholesterol synthesis in the liver.  Soy protein has low leucine-to-arginine ratio compared with that of animal proteins and potentially prevent hypercholesterolemia and atherosclerosis. Animal- or meat-based diets are rich in dietary cholesterols and saturated fats and can directly elevate plasma cholesterol and triglycerides concentrations. Moreover, the benefits of Soy protein in lowering the cholesterol level is an association with isoflavones, as both protein and isoflavones have a synergistic effect. But without isoflavones, Soy protein does not provide this benefit to hypercholesterolemic patients. The protein related decrease event of cholesterolemia and triglyceridemia is significantly observed in dyslipidemic patients, but protein does not affect plasma cholesterol and triglyceride concentrations in normal healthy person.
  • However, the cholesterol-lowering effect is not observed in every plant-based protein like a pea, barley, and fava bean proteins and some may increase serum cholesterol level. For example, intake of wheat gluten can increase triglycerides response, as gluten-based diet stimulates lipid production in the liver, but whey protein obtained from milk protein can reduce lipid absorption–lowering and plasma triglycerides level.
  • Dietary protein intake can reduce blood pressure, but the significant effect can only observe with plant-sourced proteins because plant-sourced proteins have a lower ratio of methionine and alanine and greater ratio of threonine and histidine. This amino acid pattern is only specific in plant-sourced proteins, but not present in animal proteins. Increased intake of methionine and alanine causes hypertension, whereas threonine and histidine intake lower down the blood pressure.
  • In general, the insulin response after protein intake may result in the prevention and management of obesity and type 2 diabetes. Isoflavones or soy protein can improve glycemic control and insulin sensitivity among other plant-based protein. Wheat gluten and soy protein increase postprandial insulin response. Whereas, whey protein increases the insulinotropic effect and increases insulin secretion by enhancing the discharge of glucagon-like peptide 1 and glucose-dependent insulinotropic polypeptide.  
  • Dairy-sourced proteins such as whey and casein contain a higher concentration of essential amino acids like leucine, isoleucine, and valine (BCAAs) and stimulate muscle protein synthesis and also alter body composition. Supplementation of animal protein can increase BMI, waist circumference, and body weight in adults and produce harmful effects on body composition in children aged between 6 to 7 years. However, meal-based protein intake or protein supplementation do not cause alteration of body composition.

 

Overall Advantages of Plant-based protein in comparison to Different Animal protein2

CVD risk associated difference

Evaluation of risk of developing CVD and associated mortality report showed that a higher vegetable low-carbohydrate score being associated with lower CVD mortality and a higher animal low-carbohydrate score increasing CVD mortality. Legumes, nuts, beans are usual source of plant-based protein and red meat, poultry, fish, dairy, eggs are considered as animal-based protein. The following findings are obtained from different research data in analyzing the effect of animal protein and plant-based protein on cardiovascular events and CVD risk and related mortality.  

 

IHD risk associated difference

A study result reported that increased protein intake (both animal and plant-based protein) inversely associated with low risk of ischemic heart disease development. However, detail investigative result showed that vegetable protein only reduces the risk of IHD; but high intake of animal protein, which also increases the total protein intake increase the IHD risk.  

 

CHD risk associated difference

Both animal proteins and plant proteins replacement of carbohydrate-containing food, significantly decrease the CHD mortality. But plant-based protein with low score carbohydrate-containing food is effective to lower the risk of CHD than animal proteins with low score carbohydrate-containing food.

 

CVD mortality associated difference

Reduction of consumption of red meat and processed meat able to reduce the incidence CVD mortality. In addition, it has also been observed from research data that higher consumptions of red meat, processed meat, and high-fat dairy products lead to increase CHD risk. Higher intake of meat products also increases CVD and cancer mortality. But increasing the intake of poultry, fish, and nuts can decrease the CHD risk. The detail evaluation showed that replacement of red meat with plant-based proteins (nuts) can provide higher protection (30%) from CHD than fish (24%), poultry (19%) and low-fat dairy products (13%) respectively.

Replacing meat-containing diet with nuts, poultry, whole grain legumes, low-fat dairy product and fish can reduce and prevent CVD related mortality in both the gender. The reduction of risk is 19% in the nuts intake, 14 % in poultry and whole grain diet, 10 % in legumes and low-fat dairy intake and 7% in fish intake respectively for replacing meat intake. Processed meat consumption also increase the incidence of heart failure.

 

Facts about protein diet2, 3, 4

All the above-discussed evidence showed that plant-based proteins are effective to provide cardiovascular health benefits by reducing hypertension, cholesterolemia. However, plant-based proteins do not show a significant effect in balancing of blood glucose level and body composition. In addition, plant-based proteins are insufficient to supply all the essential amino acids. Some experts may recommend for required essential amino acid substitution along with only plant-based protein. A complete meal with a variety of protein sources including fish, poultry, and dairy products are always best option to maintain the adequate protein supply to the body. The experts recommended whole dietary sources of protein provide more significant health benefits than protein supplementation. This difference is obtained because of total dietary fibers and polyunsaturated fats content is high in whole dietary sources than protein supplementation. The naturally obtained protein sources are better consumable than a single dietary supplement and thus whole protein sources lead to greater physiologic adaptations. Research report mentions avoiding high quantity red meat intake, as it increases the cardiovascular diseases. The different components such as heme iron, cholesterol, and advanced glycation and lipoxidation end-products in red meat cause different metabolic disorders, including diabetes, cancer etc.

Protein is an important dietary component for muscle building, maintaining the muscle strength and prevent sarcopenia4. Source of protein can greatly affect these factors, like beef protein intake can higher the muscle protein synthesis than Soy protein. The muscle anabolic response is reduced with wheat protein but is higher with the consumption of egg or whey protein. Therefore, the selection of a right protein source is very important to decide to depend upon body's own requirement, which can vary from individual to individual.

 

References

  • Ardala Breda, Napoleão Fonseca Valadares, Osmar Norberto de Souza, and Richard Charles Garratt. Chapter A06Protein Structure, Modelling and Applications. Created: May 1, 2006; Last Update: September 14, 2007. Bioinformatics in Tropical Disease Research: A Practical and Case-Study Approach [Internet].  National Center for Biotechnology Information (US); 2008.
  • Chesney K Richter, Ann C Skulas-Ray, Catherine M Champagne, Penny M Kris-Etherton. Plant Protein and Animal Proteins: Do They Differentially Affect Cardiovascular Disease Risk? American Society for Nutrition. Adv Nutr 2015;6:712–28; doi:10.3945/an.115.009654
  • Tristan Chalvon-Demersay, Dalila Azzout-Marniche,  Judith Arfsten, L´eonie Egli, Claire Gaudichon, Leonidas G Karagounis,  Daniel Tom´. A Systematic Review of the Effects of Plant Compared with Animal Protein Sources on Features of Metabolic Syndrome.  American Society for Nutrition. Manuscript received August 15, 2016. Initial review completed September 19, 2016. Revision accepted December 20, 2016. 281. First published online January 25, 2017; doi:10.3945/jn.116.239574.
  • Stephan van Vliet, Nicholas A Burd, Luc JC van Loon. The Skeletal Muscle Anabolic Response to Plant- versus Animal-Based Protein Consumption. American Society for Nutrition. Manuscript received September 24, 2014. Initial review completed November 20, 2014. Revision accepted June 30, 2015. 1981. First published online July 29, 2015; doi:10.3945/jn.114.204305.
  • Rachel R. Deer1 and Elena Volpi. Protein Intake and Muscle Function in Older Adults. Curr Opin Clin Nutr Metab Care. 2015 May; 18(3): 248–253. doi:10.1097/MCO.0000000000000162