Protein: a component of every living cell provides structure and framework in the body plays a role in fluid balance and acid--base balance used to transport substances through the blood provides 4 cal/g of energy
Amino Acids: building blocks of protein composed of carbon, hydrogen, oxygen, and nitrogen atoms 20 common amino acids 9 are considered essential because the body cannot make them--indispensable; they must be consumed through food 11 are considered nonessential because they can be made by the body if nitrogen and other precursors are available dispensable
All amino acids have; A carbon atom core Four binding sites An amino group An acid group A hydrogen atom The side (R group) that gives each AA its distinctiveness
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
Amino Acids (cont d) are joined in different amounts, proportions, and sequences to form the thousands of different proteins in the body are used to make proteins, nonessential amino acids, and other nitrogen-containing compounds are also components of enzymes, hormones, neurotransmitters, and antibodies
Amino Acids (cont d) The small intestine is the principal site of protein digestion; amino acids and some dipeptides are absorbed through the portal bloodstream Some amino acids can be converted to glucose Amino acids consumed in excess of need are burned for energy or converted to fat and stored
Protein Digestion Begins in the stomach Hydrochloric acid converts pepsinogen to the active enzyme pepsin, which begins the process of breaking down proteins into small polypeptides and amino acids
Protein Digestion Majority of protein digestion occurs in the small intestine, where pancreatic proteases reduce polypeptides into shorter chains, tripeptides, dipeptides, and amino acids Enzymes located on the surface of the cells that line the small intestine complete the digestion: aminopeptidase splits amino acids from the amino ends of short peptides, and dipeptidase reduces dipeptides to amino acids
Hepatic Functions in Protein Metabolism Clearing house for amino acids Regulates release of amino acids into bloodstream Enzyme synthesis Removes nitrogen from amino acids Non-essential amino acid formation
Nitrogen Balance Healthy adults are in nitrogen balance, which means that protein synthesis (anabolism) is occurring at the same rate as protein breakdown (catabolism) Nitrogen balance is determined by comparing the amount of nitrogen consumed (intake) with the amount of nitrogen excreted (output) in urine, feces, hair, nails, and skin
Nitrogen Balance Positive nitrogen balance: when protein synthesis exceeds protein breakdown Negative nitrogen balance: an undesirable state that occurs when protein breakdown exceeds protein synthesis
Protein Turnover Sources of amino acids Endogenous Exogenous Rate of turnover differs Liver, pancreas, small intestine Nerve tissue
Proteins: Quantity and Quality Except for the Fruit Group and apex, all Food Guide Pyramid groups provide protein in varying amounts The RDA for protein for adults is 0.8 g/kg of body weight Most experts recommend that protein contribute 10% to 20% of total calories in the diet Most Americans consume more protein than they need
Proteins: Complete The quality of proteins varies: complete or incomplete Complete proteins, those with high biologic value, provide adequate amounts and proportions of all essential amino acids needed for protein synthesis necessary to support tissue growth and repair Animal proteins and soy protein are complete proteins
Proteins: Incomplete Incomplete proteins lack adequate amounts of one or more essential amino acids Except for soy protein, all plants are sources of incomplete proteins Gelatin is also an incomplete protein
Complementary proteins: 2 proteins that when combined, provide adequate amounts and proportions of all essential amino acids needed to support protein synthesis
Examples of two 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
Vegetarian Diets Pure vegetarians or vegans eat no animal products eat only plants they form the smallest group of vegetarians. Most American vegetarians are lacto-vegetarians whose diets include milk products, or lacto-ovo vegetarians, whose diets include milk products and eggs
Vegetarian Diets (cont d) Most vegetarian diets meet or exceed the RDA for protein and are nutritionally adequate across the life cycle Pure Vegan diets may need supplementation: Vitamins B12, D Zinc Iron Calcium
Muscle building = resistance/exercise + protein/calories Although bodybuilding requires more than the RDA for protein, the limiting factor in adding muscle mass is energy intake, not protein Muscle building requires resistance exercise, adequate calories, and moderate protein
High Protein Diets Many dieters believe a high protein diet is superior to a high carbohydrate diet in promoting weight loss American Journal of Pediatrics 2010 study
High Protein Diets Clients who choose to eat a high-protein diet in an attempt to lose weight should be reminded to choose low-fat sources of protein to keep calories under control: low-fat or skim milk and milk products lean cuts of meat and skinless poultry that are baked, broiled, or roasted egg whites, whole eggs dried peas and beans
Protein Supplements Some people take amino acid capsules or powders, based on the belief that the more protein the better However, the average American diet provides approximately 50% more protein than needed. So why take supplements? More is not better
Protein Supplements (cont d) People most vulnerable to adverse effects of protein supplements include: Pregnant and lactating women Infants, children, and adolescents The elderly People who need to limit their protein intake (e.g., people with liver or renal disease)