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 structure

– Most contain several dozen to several hundred amino acids

– Shape determines function

• 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

• 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

• 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

• 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