A Component of Every Living Cell

• Protein Chapter 3• Protein• A component of every living cell• Accounts for 20% of adult weight• Immune to the controversy over optimal intake • Protein (cont’d)• Amino acids – Basic building blocks of protein– Composed of carbon, hydrogen, oxygen, and nitrogen atoms – 20 common amino acids o 9 are considered essential because the body cannot make them—indispensable; they must be consumed through food o 11 are considered nonessential because they can be made by the body— dispensable • Essential Amino Acids • Histidine• Isoleucine• Leucine • Lysine• Methionine • Phenylalanine• Threonine • Tryptophan• Valine • Nonessential Amino Acids • Alanine • Arginine • Asparagine • Aspartic acid• Cystine (cysteine)• Glutamic acid• Glutamine• Glycine • Proline • Serine• Tyrosine • Protein (cont’d)• Protein structure– Most contain several dozen to several hundred amino acids– Shape determines function • Protein (cont’d)• Functions of protein– Major structural and functional component of every living cell– Body structure and framework– Enzymes– Other body secretions and fluids– Acid–base balance– Transport molecules• Protein (cont’d) • Functions of protein (cont’d) – Other compounds– Some amino acids have specific functions within the body– Fueling the body• Protein (cont’d)• How the body handles protein– Digestion o Begins in the stomach o Hydrochloric acid converts pepsinogen to the active enzyme pepsin o Small intestine is the principal site of protein digestion o Enzymes located on the surface of the cells that line the small intestine complete the digestion o Protein (cont’d)• How the body handles protein (cont’d)– Absorption o Amino acids, and sometimes a few dipeptides or larger peptides, are absorbed through the mucosa of the small intestine – Metabolism o Liver acts as a clearinghouse  Retains amino acids to make liver cells, nonessential amino acids, and plasma proteins such as heparin, prothrombin, and albumin Regulates the release of amino acids into the bloodstream o Protein (cont’d)• Metabolism (cont’d)– Liver acts as a clearinghouse (cont’d) o Removes the nitrogen from amino acids o Converts protein to fatty acids which form triglycerides for storage in adipose tissue o Forms urea from the nitrogenous wastes of protein o Protein (cont’d)• Protein synthesis– Complicated but efficient process that quickly assembles amino acids into proteins the body needs– Part of what makes every individual unique is the minute differences in body proteins– Important concepts o Protein turnover Important Concepts • Protein turnover• Continuous process • Body proteins vary in their rate of turnover• Metabolic pool• Contains supply of each amino acid • Consists of recycled amino acids from body proteins that have broken down and also amino acids from food• In a constant state of flux• Important Concepts (cont’d)• Nitrogen balance– Reflects the state of balance between protein breakdown and protein synthesis– Determined by comparing the amount of nitrogen consumed (intake) with the amount of nitrogen excreted (output)– Healthy adults are in neutral nitrogen balance– Important Concepts (cont’d)• Nitrogen balance (cont’d) – Positive nitrogen balance: when protein synthesis exceeds protein breakdown– Negative nitrogen balance: an undesirable state that occurs when protein breakdown exceeds protein synthesis– Protein (cont’d)• Protein catabolism for energy– Physiologic and economic waste– Overtime, loss of lean body tissue occurs– Loss of 30% of body protein causes: o Impaired breathing o Altered immune function o Altered organ function o Ultimately death• Sources of Protein • Protein quality– Differs based on content of essential amino acids– Quality can become a crucial concern– Sources of Protein (cont’d)• Complete and incomplete proteins– Complete proteins o High biologic value o Provide adequate amounts and proportions of all essential amino acids needed for protein synthesis necessary to support tissue growth and repair o Animal proteins and soy protein are complete proteins• Sources of Protein (cont’d)• Complete and incomplete proteins (cont’d)– Incomplete proteins o Lack adequate amounts of one or more essential amino acids o Except for soy protein, all plants are sources of incomplete proteins o Gelatin is also an incomplete protein– Complementary proteins o 2 proteins that when combined provide adequate amounts and proportions of all essential amino acids needed to support protein synthesis o Examples of Complementary Plant Proteins • Black beans and rice• Bean tacos• Pea soup with toast• Lentil and rice curry• Falafel sandwich (ground chickpeas on pita)• Peanut butter sandwich• Pasta e fagioli (pasta with white beans)• Examples of a Plant Protein Complemented by a Small Amount of an Animal Protein to Form a Complete Protein • Bread pudding• Rice pudding• Corn pudding• Cereal and milk• Macaroni and cheese• Cheese fondue• French toast• Cheese sandwich• Vegetable quiche• Dietary Reference Intakes • RDAs– For healthy adults is 0.8 g/kg– Acceptable macronutrient distribution range for protein for adults is 10% to 35% of total calories• When the RDA does not apply – Intended for healthy people only• Dietary Reference Intakes (cont’d)• Protein deficiency– Protein-energy malnutrition (PEM) o Kwashiorkor  Results mainly from acute critical illnesses  Aggressive nutritional support is used to restore metabolic balance as quickly as possible o Marasmus  Occurs secondary to chronic diseases Nutritional therapy is started slowly and advanced gradually – Dietary Reference Intakes (cont’d)• Protein excess– No proven risks from eating an excess of protein– Conflicting data as to whether high-protein diets increase the risk of: o Osteoporosis o Renal stones• Protein in Health Promotion • Not addressed in the Dietary Guidelines for Americans• Vegetarian diets– Pure vegetarians or vegans o Eat no animal products o Eat only plants o They form the smallest group of vegetarians – Protein in Health Promotion (cont’d)• Vegetarian diets (cont’d)– Most American vegetarians are: o Lacto-vegetarians whose diets include milk products o Lacto-ovo vegetarians, whose diets include milk products and eggs• Nutrients of concern– Most vegetarian diets meet or exceed the RDA for protein and are nutritionally adequate across the life cycle– Iron, zinc, calcium, vitamin D, and alpha-linolenic acid are nutrients of concern – Protein in Health Promotion (cont’d)• Is vegetarianism for everyone?– A personal choice, subject to personal interpretation– Proper planning means paying close attention to the nutrients of concern and using a vegetarian food guide for planning

A Component of Every Living Cell

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