Unit 4 Proteins and Amino Acids PDF

Introduction to Human Nutrition UNIT 04: Nutrition and Metabolism of Proteins and Amino Acids Dr. Mahpara Safdar Key Points Protein and Amino - Protein is the most abundant nitrogen-containing compound in the diet and body. Acids Overview - Formed...

Introduction to Human Nutrition UNIT 04: Nutrition and Metabolism of Proteins and Amino Acids Dr. Mahpara Safdar Key Points Protein and Amino - Protein is the most abundant nitrogen-containing compound in the diet and body. Acids Overview - Formed by polymerization of L-α-amino acids through peptide bonds. Amino Acids – Structure - Amino acids have a similar central structure with different side chains. and Function - Side chains determine their multiple metabolic and physiological roles. Essential - Cannot be synthesized fast enough by the body to meet growth/maintenance needs. (Indispensable) Amino Acids - Intake should maintain nitrogen balance without major changes to protein turnover. - Special groups (infants, children, pregnant/lactating women) have additional needs. Conditionally Essential Amino Acids - Synthesized in limited amounts, often reliant on another amino acid. - Increased metabolic demand may exceed the body’s synthesis capacity. - Determined by protein synthesis, maintenance, and turnover rates. Protein and Amino Acid - Influenced by genetics, life stage, physical activity, dietary intake, disease, Requirements hormones, immune products. Methods for - Nitrogen excretion and balance. Determining Protein - Factorial estimations. Needs - Tracer techniques. - Previous methods underestimated digestibility by only comparing nitrogen intake to Protein Quality and fecal output. Digestibility - Tracer techniques show high true digestibility for most dietary proteins. - Quality assessed by Protein Digestibility-Corrected Amino Acid Score (PDCAAS). - Animal proteins generally have higher concentrations of indispensable amino acids. Animal vs. Plant Protein - Limiting amino acids in plant foods include lysine, methionine, cystine, tryptophan, and threonine. OUTLINE 4.1 Introduction 4.2 A Historical Perspective 4.3 Structure and Chemistry of Amino Acids 4.4 Classification of Amino Acids 4.5 Biology of Protein and Amino Acid Requirements 4.6 Estimation of Protein and Amino Acid Requirements 4.7 Meeting Protein and Amino Acid Needs 4.8 Factors other than Diet Affecting Protein and Amino Acid Requirements  4.1 Introduction to Proteins  4.1 Introduction to Proteins Protein is the most abundant nitrogen-containing compound in the diet and the human body. Classes of Complex Biomolecules: Proteins, DNA, RNA, polysaccharides, and lipids. Protein Formation: L-α-amino acids polymerize via peptide bond formation. Creates structural framework of proteins. Multimeric Proteins: Proteins can have two or more polypeptide chains. Each chain is called a subunit. PROTEIN STRUCTURE AND FUNCTION Proteins can be single or multiple polypeptide chains. Proteins are essential for cell function and organ activity. Building Blocks of Proteins: Amino acids linked in sequences directed by DNA. Directed by the Genome: Protein sequences are based on DNA (the genome).  4.1 Introduction to Proteins. PROTEIN NUTRITION A ND META B OLISM Amino acids are the currency of protein WHAT IS NUTRIGENOM IC S? nutrition and metabolism. Defi nition: Nutrigenomics is the study of how nutrition and the genome interact to infl uence health. Proteins are essential for cellular processes. Nutritional intake can infl uence gene expression, which The genome’s base sequence directs amino aff ects protein synthesis. acid assembly. Nutrigenomics is a new, rapidly evolving fi eld in nutrition Proteins impact tissue, organ function, and science. overall health. I M PA C T O F I N A D E Q UAT E P R O T E I N A N D THE HUMAN GENOME AND PROTEINS A M INO A C ID INTA KE The genome contains only about 30,000 genes. Insuffi cient intake of proteins and specifi c amino acids Despite this, hundreds of thousands of proteins leads to dysfunction in tissues and organs. exist. Poor protein nutrition aff ects growth, immune function, These proteins contribute to human uniqueness. and overall well-being. Gene expression leads to diverse proteins that Chronic protein defi ciency can lead to diseases such as determine characteristics. Kwashiorkor and other metabolic issues.  4.1 Introduction to Proteins  Protein’s Role in Tissue and Organ Function Proteins in Tissues and Organs: Proteins maintain the structural integrity of tissues (e.g., muscles, skin). Regulatory Functions: Proteins regulate processes like enzyme reactions, hormone function, and transport mechanisms. Example: Hemoglobin, a protein, is essential for oxygen transport in blood.  Amino Acids – The Building Blocks of Protein Types of Amino Acids: 1. Essential vs. non-essential amino acids. 2. Essential amino acids must come from the diet. Role in Protein Synthesis: Amino acids are assembled in a sequence to create specifi c proteins. Examples: Leucine, lysine, valine are essential amino acids necessary for protein synthesis. Essential Vs Non Essential aminoacids 4.2 A Historical Perspective Proteins contain about 16% nitrogen by weight, and nitrogen conversion to protein requires a factor of 6.25  Discovery of Nitrogen Daniel Rutherford discovered nitrogen in 1792 and called it "phlogisticated air" His Doctorate in Medicine thesis at Edinburgh marked the fi rst recognition of nitrogen  Discovery of Amino Acids First Amino Acid Discovered: Cystine: Cystine was the fi rst amino acid discovered, extracted by Wallaston in 1810 from a urinary calculus Discovery of Threonine: Threonine, the last essential amino acid to be discovered, was identifi ed in 1935 by W.C. Rose at the University of Illinois An essential nutrient for mammals, including humans  Origin of the Term 'Protein' Jöns Jakob Berzelius coined the term "protein" Gerhardus Mulder accepted and promoted the term in 1838  4.2 A Historical Perspective  The Nutritional Importance of Nitrogen In 1816, Magendie recognized nitrogen's importance in nutrition through experiments on dogs Dogs fed only sugar and olive oil died within weeks, proving that a nitrogen source is essential in the diet  Justus von Liebig and Nitrogen Metabolism Justus von Liebig investigated the chemical basis of protein metabolism Discovered urea as an end-product of protein breakdown Founded schools of biochemical studies in Gießen and Munich  Carl Voit's Work on Nitrogen Balance Carl Voit, a student of Liebig, laid the foundation of modern studies on body nitrogen balance His work infl uenced many leading scientists  4.2 A Historical Perspective  Pioneers of Protein Metabolism Max Rubner: Studied specifi c dynamic action of proteins and their eff ect on energy metabolism Wilbur Atwater and Graham Lusk (USA): Researched food composition, protein requirements, and energy metabolism Theories of protein metabolism were developed and refi ned through their work  Rudolf Schoenheimer’s Seminal Work Rudolf Schoenheimer developed contemporary views of protein metabolism at Columbia University Introduced the use of stable isotopes to study protein turnover and amino acid metabolism Stable isotopes such as 13 C, 18 O, and 15 N are safe for human metabolic studies and naturally present in the environment Schoenheimer’s research established the principle of continuous tissue and organ protein loss and renewal This principle explains the dietary need for protein, the supply of amino acids, and utilizable nitrogen  4.3 Structure and Chemistry of Amino Acids   Overview of Amino Acids 4.3 Structure Amino acids are the building blocks of peptides and proteins. They have a central structure (with the exception of proline) consisting of: and Chemistry 1. Amino group (-NH2) of Amino Acids 2. 3. Carboxyl group (-COOH) Hydrogen atom (-H) 4. Side chain (R group) that distinguishes each amino acid. The linkage between amino acids is called a peptide bond. Formed through a condensation reaction between the carboxyl group of one amino acid and the amino group of another, releasing a molecule of water. Peptide bonds create linear structures of peptides and proteins.  Importance of Side Chains Side chains (R groups) determine the physical and chemical properties of each amino acid. They contribute to the overall structure and function of proteins. Some side chains are critical for the metabolic and physiological roles of free amino acids versus protein-bound amino acids. 4.3 Structure and Chemistry of Amino Acids  Chemical Properties and Functions of Amino Acids Specifi c chemical properties enable various functions: Methionine: Donates a methyl group in one-carbon metabolism. Glutamine: The amide group serves as a nitrogen source for pyrimidine synthesis. Cysteine: The sulfhydryl group forms disulfi de bonds for cross-linking. Amino acids can link carbohydrate and protein metabolism. Alanine and Glutamate/Glutamine: Act as intermediates connecting these metabolic pathways. Branched-chain amino acids: Serve as a “universal” fuel throughout the body when required.  Derivatives of Amino Acids Some amino acids serve as precursors for important compounds: Creatine: Formed from glycine, arginine, and methionine; involved in intracellular energy transduction. Dopamine: Synthesized from tyrosine; acts as a neurotransmitter. Ornithine: Derived from glutamate; functions in the urea cycle and as a precursor for polyamines (spermine and spermidine) used in DNA packaging.   Post-Translational Modifi cations Specifi c amino acid residues in a polypeptide chain undergo post- 4.3 Structure translational modifi cations during protein synthesis. and Chemistry These modifi cations can aff ect protein function, stability, localization, and interactions with other molecules. of Amino Acids  Role of Amino Acids in Protein Synthesis Amino acids serve as precursors for protein synthesis. They also act as signaling molecules modulating protein synthesis processes.  Steps in mRNA Translation Initiation 1. Binding of met-tRNA to the 40S ribosomal subunit to form the 43S preinitiation complex. 2. Binding of the 43S complex to mRNA, localizing to the AUG start codon. 3. Release of initiation factors from the 40S ribosomal complex, leading to the formation of the 80S ribosomal complex through the joining of the 60S ribosomal subunit. The 80S complex then proceeds to the elongation stage of translation. 4.3 Structure and Chemistry of Amino Acids Mediators of the 43S Preinitiation Complex: Formation is mediated by a heterotrimeric complex of eIF–4F proteins. The mTOR (mammalian target of rapamycin) pathway plays a crucial role in regulating mRNA translation. mTOR Signaling Pathway: mTOR regulates the formation of the eIF–4F complex through a series of phosphorylation–dephosphorylation processes of downstream targets. Traditionally involved in mediating hormone actions, recent studies show its role in amino acid signaling. Role of Branched-Chain Amino Acids (BCAAs) (in mTOR Signaling) Leucine plays a unique role in regulating mRNA translation via the mTOR signaling pathway. Increased availability of leucine activates mTOR and its downstream targets. Inhibition of the mTOR pathway by rapamycin partially inhibits leucine’s stimulatory effect on protein synthesis, suggesting mTOR-independent pathways also exist. Non-Protein Functions of Amino Acids Regulatory Roles of Individual Amino Acids in health and disease: Glycine: Anti-infl ammatory, immunomodulatory, cytoprotective agent via glycine receptors. Cysteine: Regulates glutathione synthesis; protects against oxidative damage. 4.3 Structure Arginine-Nitric Oxide Pathway: An active area of investigation with signifi cant physiological implications. Importance of Amino Acids in Physiology Amino acids are essential for maintaining: and Chemistry of Immune and protective functions Amino Acids Digestive function Cognitive and neuromuscular function Nutritionally dispensable amino acids exert these functions signifi cantly. Synthesis Pathways De novo synthesis pathways and the supply of exogenous amino acids or their precursors are critical in modulating physiological and pathophysiological conditions. Ensuring adequate amino acid supply can support overall health and combat diseases. 4.4 Classification of Amino Acids  4.4 Classification of Amino Acids Amino acids have traditionally been classifi ed into two categories: 1. Indispensable (Essential) 2. Dispensable (Nonessential) This classifi cation aids in understanding amino acid nutrition.  What are Indispensable Amino Acids? Defi ned as amino acids that cannot be synthesized by the organism at a speed suffi cient for normal growth using materials ordinarily available to the cells. Important phrases in the defi nition: 1. Ordinarily Available 2. At a Speed 3. Normal Growth 4.4 Classification of Amino Acids  Importance of "Ordinarily Available" Some essential amino acids can be synthesized from analogous α-keto acids through transamination (e.g., branched-chain amino acids, phenylalanine, methionine). These keto acids are not typically available in the diet, thus not considered “ordinarily available” to the cells. Example: In conditions like renal failure, these keto acids can help maintain nitrogen metabolism.  Signifi cance of "At a Speed" The rate of amino acid synthesis can be limited by the availability of nonessential nitrogen. Certain amino acids, such as arginine, cysteine, proline, and glycine, are often described as conditionally indispensable based on physiological conditions. Example: In states of stress or illness, the body's demand for these amino acids may exceed its capacity to synthesize them. 4.4 Classification of Amino Acids  Understanding "Normal Growth" The defi nition emphasizes the context of growth for classifying amino acids. For example: Arginine is indispensable for growing rats but becomes dispensable for adult rats. If the ability to synthesize arginine is impaired (e.g., due to intestinal removal), it becomes essential again. Amino acids can be classifi ed based on their chemical and metabolic properties rather than solely on their role in growth. Each amino acid exhibits specifi c structural features that aff ect its synthesis and metabolic fate. Essential amino acids have structures that cannot be synthesized due to the absence of necessary mammalian enzymes. In obligatory carnivores like cats, the loss of critical enzymes increases dependency on dietary sources of certain amino acids. Example: Lack of arginine in a single meal can be fatal for cats.   The Concept of De Novo Synthesis 4.4 De novo synthesis refers to the creation of amino acids from non-amino acid precursors. Classification of Some amino acids can be synthesized from similar structural precursors, e.g.: Methionine can be synthesized via transamination of its keto acid analogue Amino Acids and through remethylation of homocysteine. Threonine and lysine cannot be formed through transamination or from other carbon precursors, making them truly indispensable. A dispensable amino acid is one that can be synthesized de novo from non-amino acid nitrogen sources (e.g., ammonium ion) and carbon sources (e.g., glucose). True metabolically indispensable amino acids include: Glutamic acid (synthesized from glucose and ammonium ions) Glycine (synthesized from carbon dioxide and ammonium ions)  Metabolic Complexity in Vivo In vivo conditions may diff er qualitatively and quantitatively from isolated studies or test-tube experiments. The complexity of amino acid metabolism in living organisms surpasses what is observed in simplifi ed biochemical pathways. Real-life metabolic processes involve intricate interactions that are not fully captured in laboratory settings. 4.4 Classification of Amino Acids  Ammonia Incorporation  Understanding Nonspecifi c Nitrogen Reactions NSN is essential for supporting body protein and nitrogen metabolism. Glutamate Ammonia Ligase Reaction: Traditionally, it was considered suffi cient to use a simple nitrogen-containing Glutamate + NH₄⁺ + ATP → Glutamine + ADP + mixture (e.g., urea, diammonium citrate). Pi + H⁺ This perspective is evolving to refl ect the complexities of human nitrogen needs. Glutamate Dehydrogenase Reaction:  The Nitrogen Cycle α-Ketoglutarate + NH₄⁺ + NADPH ⇌ L- Certain organisms can fi x atmospheric nitrogen into ammonia. Glutamate + NADP⁺ + H ₂O Plants utilize ammonia or soluble nitrates (reduced to ammonia by nitrifying Note: High Km for NH₄⁺ in the dehydrogenase bacteria). reaction limits its contribution to ammonia Vertebrates, including humans, must obtain dietary nitrogen from amino acids or organic compounds (urea, purines, pyrimidines). assimilation in mammals. Glutamate and glutamine play key roles in introducing ammonia from the nitrogen cycle into amino acids. These amino acids help maintain the nitrogen economy of the individual. The nitrogen Cycle Glutamate Production in Plants and Bacteria Glutamate Synthase Reaction: 4.4 Classification of α-Ketoglutarate + Glutamine + NADPH + H⁺ → 2 Glutamate + NADP⁺ Combined reactions yield: Amino Acids α-Ketoglutarate + NH₄⁺ + NADPH + ATP → Glutamate + NADP⁺ + ADP + Pi In animals, ammonia incorporation primarily occurs through glutamate rather than glutamine. Pathways of Ammonia Incorporation Serine Formation: Ammonia Incorporation into Glycine Can be formed from glucose via 3-phosphoglycerate. Glycine Synthase Reaction: CO ₂ + NH ₄⁺ + H⁺ + NAD⁺ + N ₅ ,N ₁₀ -Methylenetetrahydrofolate ⇌ Its nitrogen is obtained from glutamic acid via transamination with α-ketoglutarate. Glycine + NAD⁺ + Tetrahydrofolate Glutamate emerges as a key amino acid in providing net Glycine can be incorporated into proteins, glutathione, creatine, amino nitrogen to mammals, primarily derived from porphyrins, and converted to serine. plant protein. Glycine cleavage is more signifi cant in glycine catabolism than synthesis. 4.4 Classification of Amino Acids  What is Conditional Indispensability? Conditional indispensability refers to amino acids that may become essential under certain physiological conditions. Their synthesis may be limited by various determinants, requiring dietary sources.  Factors Infl uencing Synthesis Precursor Availability: Synthesis requires other amino acids as precursors. Example: Citrulline is needed for arginine synthesis; serine is needed for glycine synthesis. Tissue-Specifi c Synthesis: Some amino acids are synthesized in limited tissues. Example: Proline and arginine are heavily dependent on intestinal metabolism. 4.4 Classification of Amino Acids Examples Metabolic Demand vs. Biosynthetic Capacity: 1. Proline and Arginine: The synthesis of conditionally essential amino acids may be Nutrition in severely burned individuals shows increased needs beyond limited even with abundant precursors. biosynthetic capabilities. Increased demand in states of stress or immaturity can surpass 2. Cysteine and Glycine: Premature infants often have elevated requirements that exceed their ability synthesis capacity. to synthesize these amino acids.  Implications for Nutrition Understanding conditional indispensability is crucial for dietary planning, especially in vulnerable populations (e.g., infants, burn victims). Clinicians and nutritionists should monitor and possibly supplement conditionally essential amino acids when synthesis may be inadequate. 4.5 Biology of Protein and Amino Acid Requirements 4.5 Biology of Protein and Amino Acid Requirements Dietary α-amino acid nitrogen is crucial for organ protein synthesis. Requirements for protein vary based on body protein mass, age, gender, and physiological state. Measuring Body Protein Mass Direct measures of total body protein in living subjects are currently unavailable. Indirect methods provide estimates of body nitrogen content at various life stages. Body nitrogen rapidly increases from birth through childhood, reaching a peak around age 30, followed by gradual decline. Age-Related Changes in Protein Mass Body Protein Body nitrogen declines more rapidly in men than in women with aging. Mass and Skeletal musculature is a major contributor to age-related nitrogen loss. Strength training can help attenuate or partially reverse this Dietary decline, improving function. Understanding Protein Requirements Needs Adult protein requirements are defined as the intake needed for "maintenance" of body nitrogen. Infants, children, and pregnant women require additional protein for tissue deposition. The dynamic state of body chemistry affects nitrogen content and requirements.  Factors Influencing Protein Turnover of Proteins Turnover and Amino Acid Metabolism 1. Dietary Factors: 2. Hormonal and Immune Factors:  Protein Turnover and Its Importance Proteins undergo continuous synthesis and degradation in a process known as turnover. Levels of dietary intake Hormones and immune The rate of turnover relative to and theand protein balance amino of synthesis systemand degradation products also regulate acid needs protein metabolism impact nitrogen and amino acid needs. Dietary requirements are not only determined by body protein mass but also by turnover rates. Turnover ofForm Proteins and Amino Acid and route of nutrient delivery (parenteral vs. Metabolism enteral) Principal systems involved in maintaining protein and amino acid homeostasis: Protein Synthesis Timing of nutrient intake in Protein Degradation relation to carbohydrate and fat consumption Amino Acid Interconversions (transformation and oxidation) Amino Acid Synthesis (for dispensable and conditionally indispensable amino acids) Turnover of Proteins and Amino Acid Metabolism The major systems in amino acid uptake, utilization, and catabolism, with an indication of the processes involved and some factors that can affect them. TNF, tumor necro- sis factor, IL, interleukin.  Nitrogen Balance Turnover of Proteins and Amino Acid Metabolism Two main cycles determine body protein balance:  Dynamics of Nitrogen Cycles Protein Synthesis and Breakdown:fl ow of nitrogen and amino acids in protein synthesis/breakdown is Intake vs. Synthesis vs. Excretion: Breakdown: approximately three times greater than in the intake/excretion cycle. Effective balance is essential for maintaining overall protein Balance of In adults, both Balance of protein nitrogen intake synthesis and cycles typically homeostasis. and nitrogen operate in degradation. excretion. balance.  Impact of Dietary and Nutritional Factors Adequate dietary intake is crucial for maintaining nitrogen balance. Imbalances can lead to negative nitrogen balance, impacting muscle mass and overall health. Timing and composition of meals infl uence the effi ciency of protein metabolism. The two endogenous nitrogen cycles that determine the status of body protein (nitrogen) balance.  Prematur e Protein synthesis: 11–14 Newborn g/kg/day s: Turnover of Proteins and Amino Acid Metabolism Term Protein synthesis: 7 Babies: g/kg/day  Hormonal Regulation Young Hormones Protein such as insulin, glucagon, synthesis: 4–5 and cortisol play signifi cant roles in regulating protein synthesis and Adults: g/kg/day degradation. Immune system products can also infl uence protein metabolism, especially during stress or infection.  Protein Synthesis Rates by Age: Premature Newborns: Protein synthesis: 11–14 g/kg/day Term Babies: Protein synthesis: 7 g/kg/day Young Adults: Protein synthesis: 4–5 g/kg/day Observation: Synthesis rates decline with growth and development.   Nutritional Implications 1. Higher Synthesis in Young: Net protein deposition occurs during growth (~30% in 6- month-olds) High protein turnover for tissue remodeling and abnormal protein removal. Turnover of 2. Reutilization of Amino Acids: Proteins and Amino Rates of synthesis and breakdown far exceed dietary intake (1-1.5 g/kg/day for adults). Acid Metabolism Extensive reutilization suggests that humans are not obligate carnivores. 3. Energy-Protein Relationship: Protein synthesis and degradation require energy. Approx. 15–20 kJ (4–5 kcal) is expended to synthesize each gram of protein.  Adequate dietary protein and energy intake. Balancing needs for amino acids, nitrogen, Turnover of Proteins and Amino Acid and daily energy expenditure. Additional energy for new tissue during Metabolism growth. Amino acids are crucial for synthesizing important nitrogen-containing compounds. These metabolites play vital roles Interrelationships in cell, Between Protein and organ, and system functions. TheyEnergy Metabolism require replacement by nitrogen and indispensable amino acids from protein Protein and amino acid metabolism accounts for ~20% of total intake. basal energy metabolism. Basal Metabolic Rate (BMR): Achieving Optimal A signifi cant proportion of total daily energy expenditure. Body Protein Interdependence between protein and energy intakes aff ects nitrogen balance. Nutrition  Body Nitrogen Balance Dynamics Changes in body nitrogen balance depend on: Level of energy intake (above/below requirements). Degree of change in nitrogen balance in response to nitrogen intake. Relationship between nitrogen balance and energy intake with diets of different protein levels. Between energy intake A (low) and B (higher) the two lines are parallel.  Amino Acids as Precursors of Physiologically 1. Arginine and Nitric 2. Arginine and Creatinine 3. Glutathione Oxide (Nitric Oxide Important Nitrogen Compounds (NO) Synthesis): Represents less than (Creatinine Synthesis): Accounts for 10% of Synthesis Composition: Formed 1% of whole body whole body arginine from glutamate, arginine flux. Quantitative Utilization of Amino Acids Accounts for less than flux. Represents 70% of glycine, and cysteine. High cysteine 1% of daily arginine daily arginine intake. utilization exceeds the Current estimates on the utilization of amino acids for precursor roles are limited. intake. Importance: Creatinine typical daily intake. Significance: NO plays is a marker for kidney Reutilization:  Examples: a critical role in function. Continuous synthesis vascular function and relies on endogenous signaling. cysteine. Low dietary intake of methionine and cysteine negatively affects glutathione levels.   Clinical Implications Amino Acids as Low intake of key amino acids can lead to compromised Precursors of glutathione status. Physiologically Particularly signifi cant in: Important 1. Trauma patients Nitrogen 2. Patients with Acquired Immunodefi ciency Syndrome (AIDS) Compounds Importance of Nutritional Therapy: Focus on amino acid intake to improve antioxidant defenses.  Urea Cycle Enzymes and Urea Production The urea cycle is essential for removing amino Conditions Affecting Urea nitrogen from the body. Production Urea production helps adjust nitrogen loss relative High protein or to nitrogen intake. amino acid supply. Involves fi ve key enzymes distributed in the mitochondrion and cytosol.  The Urea Cycle and Metabalon High rates of protein breakdown The fi ve enzymes of urea biosynthesis work as a tightly (e.g., in severe trauma or connected metabolic pathway known as a metabalon. infections). Function: Converts potentially toxic ammonia. Urea production Removes excess amino acids through oxidation. and excretion adjust with Transfers nitrogen to arginine and ultimately to changes in dietary nitrogen intake. urea. Urea and Nitrogen Homeostasis Urea enters the intestinal lumen; some nitrogen is salvaged through hydrolysis. Hydrolysis produces ammonia, available for the synthesis of dispensable or conditionally indispensable amino acids. The extent of this nitrogen fl ow's contribution to whole-body nitrogen homeostasis is uncertain.  Reutilization of Urea Nitrogen Begins with hydrolysis of intact urea. Infusion studies show the minimal appearance of [15N]-urea in plasma. Linear relationship exists between protein intake and urea production/hydrolysis.  Metabolic Pathways of Ammonia Assimilation Possible pathways include: Citrulline synthesis L-glutamate dehydrogenase pathway in mitochondria Glycine synthase Net formation of amino nitrogen from these pathways is minimal compared to overall amino acid metabolism. The urea cycle enzymes and their distribution in the liver. CPS, carbamoyl phosphate synthetase; OTC, ornithine transcarbamylase; Asy, argininosuccinic synthetase; AS, argininosuccinate; Arg, arginase. Metabolic  Maintenance of Body Protein Stores Basis for Most nitrogen and amino acid requirements stem from Protein and maintaining body protein stores. Amino Acid Factors contributing to maintenance needs: Requirements Ineffi cient recycling of amino acids from tissue degradation. Catabolism related to free amino acid concentrations in tissues.  Importance of Amino Acid Turnover Turnover of functionally important amino acid products is necessary for health. Though not a major quantitative requirement, it is qualitatively crucial for maintaining health.  Metabolic Basis for Acid Requirements Protein and Amino Intestine: Phenylalanine & Absorptive and Tryptophan: protective Maintain functions. adrenergic and Immune serotonergic andneurotransmitter Repair systems. System: Methionine: Defense against Methyl group donor for diseases. creatine synthesis. Skeletal Musculature Branched-Chain Amino System: Acids: Nitrogen Structural and precursors for functional support. cerebral glutamate synthesis. Central Nervous Dispensable and conditionally System: indispensable amino acids serve as necessary Essential precursors for cognitive for health function. System Specifi Critical Amino Health Roles Acid for c s The involvement of amino acids in physiological systems and metabolic function  4.6 Estimation of protein and amino acid requirements 4.6 Estimation of protein and amino acid requirements  Protein Requirements Estimating total protein needs often begins with measuring dietary nitrogen needed for zero nitrogen balance in adults. Additional requirements arise for: 1. Growing infants and children 2. Pregnant or lactating women 3. Individuals recovering from trauma or infection  According to the UN Expert Consultation (1985): Nitrogen Balance “Protein requirement is the lowest dietary protein intake to balance body and Definition of losses while maintaining energy balance at modest physical activity levels. Protein Includes needs for tissue deposition and milk secretion in children and Requirement pregnant/lactating women”.  Nitrogen Balance Nitrogen Balance=Nitrogen Intake−Nitrogen Excretion Nitrogen excretion occurs via urine, feces, skin, and minor routes. Limitations of Nitrogen Balance Technique Technical and interpretative limitations exist: Inherent sources of error in nitrogen balance measurements. Experimental requirements for reliable data: Energy intake must match energy needs. Stabilization period on the experimental diet. Suffi cient duration to establish full dietary response. Accurate urine collection timing and completeness. Absence of mild infections or other stressors.   Methods for Estimating Requirements When direct nitrogen balance data is lacking: Interpolation between age groups based on body weight. Factorial approach to determine obligatory nitrogen losses: Nitrogen Urine and fecal nitrogen losses measured after adaptation Balance and to a protein-free diet. Definition of Summation of all obligatory losses (sweat, integument). Protein Requirement For children, include estimates of nitrogen deposition.  Recommendations for Infants Recommendations for meeting protein requirements are often based on: Estimated protein intakes from fully breast-fed infants. Special considerations for very young infants regarding protein needs.  Protein Requirements for Various Age and Physiological Groups  Protein Needs Across Adults and Aged Groups Protein requirements for young adult men and women are based on short- and long-term nitrogen balance studies. Healthy elderly individuals have similar protein requirements to younger adults.  Adjusting Protein Recommendations To make practical recommendations: Adjust the average requirement by a factor accounting for individual variations. This factor is typically 2 × CV (coeffi cient of variation). Adjusted value aims to cover the needs of 97.5% of the population. This adjusted requirement is considered a safe practical protein intake for healthy adults.  Protein Requirements for Various Age and Physiological Groups Population Variability Most individuals require less than the adjusted intake to maintain adequate protein nutritional status. Variation exists even among apparently similar individuals, necessitating adjustments.  UN Recommendations on Protein Intake Current UN recommendations apply to healthy individuals of all ages. Recognizes that sick or less healthy individuals likely have higher protein needs. Recommendations based on UN values should serve as a starting point for evaluating dietary protein needs in the context of disease and stress. Quantifying protein needs for hospitalized patients remains challenging.  Must occur under energy balance with modest physical activity. Indispensabl e Amino Acid The lowest level of intake of an indispensable amino acid that achieves nitrogen balance or Requirement balances irreversible oxidative loss without major changes in normal protein turnover. For infants, children, pregnant, and lactating women, additional amounts are needed for net protein deposition and milk synthesis.  Methods Used:  Functional vs. Operational Defi nitions Nitrogen Growth Breast Milk Factorial Balance: Assessment: Analysis: Predictions: Operational Defi nition: Focuses on nitrogen balance and intake levels needed. Functional Defi nition: Aims forForindices For infants and both like disease resistance or enhanced physical For adults. For infants. infants and children. performance. adults. Challenge: Choosing appropriate indices and quantifying them for future research.  Determination Methods Similar to total protein needs estimation: nitrogen excretion and balance, factorial estimation. Indispensabl Methods Used: e Amino Acid 1. 2. Nitrogen Balance: For adults. Growth Assessment: For infants and children. Requirement 3. Breast Milk Analysis: For infants. 4. Factorial Predictions: For both infants and adults.  Factorial Approach for Adults Total obligatory nitrogen losses are approximately 54 mg/kg/day (~0.36 g protein/kg/day). Average amino acid composition of body proteins helps estimate contributions to nitrogen output. At requirement intake levels, absorbed amino acids balance obligatory oxidative losses with ~70% effi ciency. 1. Direct Amino Uses a labeled tracer of the dietary amino acid being tested (e.g., [13C]-lysine). Acid Oxidation (DAAO) Determines the amino acid requirement by measuring its oxidation rate. Tracer Techniques Used for leucine, valine, lysine, threonine, and phenylalanine. 2. Indicator Uses an indicator tracer to assess oxidation status of a test amino acid. Amino Acid Oxidation Types of Tracer Studies (IAAO) Example: Measures [13C]-phenylalanine oxidation or [13C]-leucine balance at varying lysine intake levels. Advances in stable 3. Indicator isotope enrichment Similar approach to IAAO, but focuses on amino acid balance. Amino Acid measurements have Balance (IAAB) enhanced human 4. Kinetic metabolic research. Assesses protein retention during the postprandial phase of amino acid metabolism. Studies ; EarlyUses Postprandial tracer studies as a tracer. [13C]-leucine Protein Utilization beganThis in promising the 1980s approach has limited use in human amino acid requirement studies (PPU) at the Massachusetts Institute of Technology (MIT) to determine amino acid requirements in adults. 1. Direct Amino Uses a labeled tracer of the dietary amino acid being tested (e.g., [13C]-lysine). Acid Oxidation (DAAO) Determines the amino acid requirement by measuring its oxidation rate. Tracer Used for leucine, valine, lysine, threonine, and phenylalanine. Types of Tracer Studies Techniques 2. Indicator Amino Acid Uses an indicator tracer to assess oxidation status of a test amino acid. Oxidation (IAAO) Example: Measures [13C]-phenylalanine oxidation or [13C]-leucine balance at varying Advances lysine in stable intake levels. isotope enrichment 3. Indicator measurements haveto IAAO, but focuses on amino acid balance. Similar approach Amino Acid enhanced human Balance (IAAB) metabolic research. 4. Kinetic Early Assesses tracer studies protein retention during the postprandial phase of amino acid metabolism. Studies ; began in the 1980s Postprandial Uses [13C]-leucine as a tracer. Protein at the Utilization This promising approach has limited use in human amino acid requirement studies Massachusetts (PPU) Institute of Technology (MIT) to determine amino acid requirements in adults.  Tracer Techniques Limitations of Tracer Reference Methods Techniques While all methods have limitations, the IAAO and IAAB approaches are currently considered the Each method has inherent reference methods for estimating amino acid requirements in adults. limitations, including: Studies typically involve continuous tracer Potential inaccuracies in administration over a 24-hour period.of tracer studies. measuring tracer kinetics.  UN 1985 Amino Acid Requirements The complexity of The United Nations (UN) in 1985 proposed amino interpreting results due to acid requirement values for various age groups. metabolic variations These values are widely used but have been questioned among individuals. due to: Resource-intensive nature Limited and dated data. of tracer studies. Potential inaccuracies in nitrogen balance studies. Tracer Techniques  IDECG 1994 Amino Acid Requirements The International Dietary Energy Consultancy Group (IDECG) reassessed amino acid needs, particularly for infants. They used a factorial method which led to much lower amino acid requirements for infants compared to UN values. Key Point: IDECG values approximate average requirements for amino acids in infants.  Diff erences Between UN and IDECG Values The diff erence in UN and IDECG values highlights: Diff erences in interpretation of data. Diff erent criteria for judging adequacy of intake. Diff erent methodologies used for calculating requirements. IDECG values focus on average needs, while UN values are based on a higher intake derived from breast milk consumption, covering most infants.  Tracer Techniques Why Do Recommendations Vary? Interpretation of the same data: Different expert groups interpret available data differently. Use of different datasets: Some recommendations are based on data from specifi c populations or regions. Differences in criteria: Criteria used to judge the adequacy of amino acid intake differ between groups.  Factors Infl uencing Adult Requirements The values for adults are particularly questionable due to: Inappropriate experimental design in earlier studies. Inadequacy of nitrogen balance technique for determining amino acid needs. Result: Adult requirement values from 1985 may not be reliable.  Contemporary Estimates More recent estimates of amino acid requirements for adults are often very different from those proposed in 1985. The UN has convened new expert groups to reassess requirements based on updated data. New recommendations were expected by 2002 or 2003, but at the time of writing, they have not yet been published.  4.7 Meeting protein and amino acid needs  4.7 Meeting protein and amino acid Determinants of Protein Quality: 1. Indispensable Amino Acid Content: needs Proteins that lack sufficient essential amino acids are of lower quality. Examples: Animal proteins (high quality), plant proteins (variable quality). Total protein intake should meet physiological needs. 2. Availability of Amino Acids: Protein The amino acids must be digestible quality istoassessed based on the indispensable amino acid content. and absorbable be of use to the body. digestive Factors such as protein source and Amino acids that must be obtained from the diet (e.g., lysine, threonine, valine). processes play a role in this. These are required for protein synthesis and metabolic functions.  Determinants of Protein Quality: 1. Indispensable Amino Acid Content: Proteins that lack suffi cient essential amino acids are of lower quality. Examples: Animal proteins (high quality), plant proteins (variable quality). 2. Availability of Amino Acids: The amino acids must be digestible and absorbable to be of use to the body. Factors such as protein source and digestive processes play a role in this.   Protein Digestibility 1 Apparent Digestibility: Traditional measure: Difference between nitrogen intake and fecal nitrogen output. Limitations: Bacterial Proteins: Fecal nitrogen is largely bacterial, not undigested dietary protein. Digestibility and Endogenous Nitrogen: Secretions and urea nitrogen also intestinal amino contribute to fecal nitrogen. acid metabolism 2. True Digestibility: Measures nitrogen that is truly absorbed. Uses isotopic labeling (e.g., 15N) to track protein and amino acid breakdown. Adults given 15N-labeled proteins. True digestibility measured by nitrogen fl ow at the terminal ileum. Findings: True digestibility is higher than apparent digestibility. Over 50% of fecal nitrogen comes from endogenous sources, not the diet.   Splanchnic Amino Acid Metabolism Digestibility and Amino Acid Digestion and Utilization: Most dietary proteins are fully digested in the small intestine. intestinal amino A signifi cant portion of amino acids is metabolized in the splanchnic bed acid metabolism (gut and liver) before reaching other tissues. Key Findings: 50% of the body's amino acid utilization occurs in the gut. Threonine and glutamate are heavily utilized by the gut.  Impact of Age on Protein Metabolism Age-related Metabolism: Infants and elderly have higher splanchnic amino acid metabolism. Higher splanchnic metabolism affects the effi ciency of amino acid use for protein synthesis and overall nitrogen balance. Implications: Different age groups may require adjustments in protein intake to compensate for higher gut utilization of amino acids. Amino Highlight the importance of amino acid-specifi c metabolism and its role in Percentage Utilized in the Gut Acid nutrition. Threonin High utilization e Glutamat Nearly 100% utilization e Aspartate Nearly 100% utilization  Digestibility and intestinal amino acid metabolism  Practical Implications for Diet Optimizing Protein Intake: Ensure dietary protein sources provide all essential amino acids. Include high-quality proteins (e.g., from animal sources or well- balanced plant combinations). Impact on Dietary Planning: Special considerations for infants, elderly, and those with digestive issues. Consider true digestibility and amino acid availability when formulating diets. Challenges in Plant-Based Diets: Lower digestibility of plant proteins due to fi ber and anti- nutritional factors. Need for complementary protein sources (e.g., rice + beans) to provide all essential amino acids. Strategies: Pairing foods to create a balanced amino acid profi le. Consideration of protein digestibility-corrected amino acid score (PDCAAS).  Defi nition: “Protein nutritional quality refers to a protein’s ability to meet physiological nitrogen and amino acid requirements”. Factors Affecting Protein Quality: Protein Nutritional 1. Indispensable Amino Acids: Essential for protein synthesis and must Quality and PDCAAS be obtained from the diet. 2. Protein Source: Animal vs. plant proteins diff er in amino acid profi les and availability.  Diff erences in Protein Quality Indispensable Amino Acid Composition: Animal proteins: Generally contain a complete profi le of indispensable amino acids. Plant proteins: May lack one or more indispensable amino acids, making them incomplete. Limiting Amino Acids: The amino acids most likely to be limiting in plant-based proteins are lysine, threonine, tryptophan, and methionine. Lysine is typically the most limiting amino acid in plant proteins. Protein Nutritional Quality and PDCAAS Importance of Complementary Proteins: Protein Quality in Animal Combining different plant proteins (e.g., rice + beans) can provide a vs. Plant Sources complete amino acid profi le.  Assessing Protein Quality: Amino Acid Scoring Animal Proteins: Amino Acid Scoring: High in all essential amino acids. Examples: Eggs, meat, dairy. Compares the amino acid composition of a given protein to a reference amino acid requirement pattern. Reference Pattern: Plant Proteins: Often lack one or more Based on the amino acid requirements of a 2-5-year-old child indispensable amino acids. Examples: Legumes (low in (considered to have the highest needs). methionine), Grains (low in lysine). Importance: Ensures that the diet provides adequate amounts of essential amino acids.  Determine which essential Introduction to PDCAAS 1. Identify the Most Limiting amino acid is in the shortest Amino Acid: supply Steps in PDCAAS PDCAAS (Protein Digestibility-Corrected Amino relative to Calculation Use true Acid Score): human digestibility needs. A method to evaluate protein quality based on both data to 2. Measure amino acid composition and digestibility. correct for Protein Equation: Digestibility: how much PDCAAS = (Concentration of the most limiting, protein is Use absorbed bythe digestibility-corrected amino acid in test protein) ÷ amino acid the body. (Concentration of that amino acid in the FAO/WHO requirement reference pattern). 3. Compare to pattern for a Why Is It Important? Reference 2–5-year-old Pattern: Uses human amino acid requirements for evaluation child as the (not animal models). reference standard. Takes digestibility into account, ensuring that the protein source is not only high in essential amino acids but also absorbable. Helps assess the quality of food protein sources in human diets. Useful in formulating diets that meet human nutritional needs effi ciently.  Helps optimize diets by suggesting Introduction to PDCAAS Food Blending: combinations of foods that provide a complete amino acid profile. Applications of PDCAAS: Can inform decisions in food  Example Dietary Planning: manufacturing, nutrition policy, and individual diet formulation Soy Protein: Example of a high-quality plant-based protein with a PDCAAS of 1.0 (maximum score). Wheat Protein: Example of a lower-quality protein with a PDCAAS of 0.47 due to lower lysine content.  Benefi ts of PDCAAS over Other Methods PDCAAS is based on human amino acid needs, not animal models, making it more accurate for human nutrition. Applications: Food Blending: Helps optimize diets by suggesting combinations of foods that provide a complete amino acid profi le. Dietary Planning: Can inform decisions in food manufacturing, nutrition policy, and individual diet formulation. Introduction to PDCAAS  Limitations of PDCAAS Incomplete Representation of Protein Quality: PDCAAS does not consider certain factors like: Anti-nutritional factors that might affect protein digestion. Processing effects on protein quality. Future Adjustments: PDCAAS can be modifi ed as new knowledge on amino acid requirements and digestibility emerges.  Research Developments: Improved Digestibility Measurements: Advances in understanding how to measure protein digestibility more accurately. New Amino Acid Requirements: Adjustments may be made as we learn more about human amino acid needs across different life stages. Alternative scoring systems like Digestible Indispensable Amino Acid Score (DIAAS) are being developed to address PDCAAS limitations.  Common in developed More prevalent in regions. developing regions. Major sources of Typically provide 60-70% of Make up 60-80% of total food proteins in the the total protein intake. Examples: Meat, dairy, fish, protein intake. Examples: Cereals (e.g., poultry, eggs. diet rice, wheat), legumes (e.g., beans, lentils), nuts, seeds. Geographical and Socioeconomic Infl uences: Developed Regions: Animal Protein Higher Plantproteins reliance on animal-based Proteindue to Sources: Sources: availability and economic access. More likely to consume suffi cient amounts of indispensable amino acids due to the higher quality of animal protein sources. Developing Regions: Diets are more dependent on plant-based proteins like cereals and legumes due to aff ordability and cultural practices. Greater reliance on plant-based proteins can lead to lower intake of essential amino acids, particularly: Lysine Sulfur amino acids (methionine and cysteine) Tryptophan Threonine  Major sources of food proteins in the diet Commonly limiting in Lysine: cereal-based diets (e.g., wheat, rice, maize). Often limited in legume- Methionine & Cysteine: Concer based diets. Challenges in Plant-Based Diets: Amino Acids o Can be limited in some Incomplete Protein Profi les: Many plant-based sources Tryptophan: cereal and grain diets. lack one or more essential amino acids. Also limited in some plant Need for Food Pairing: Combining complementary Threonine: proteins, especially in developing countries. proteins (e.g., rice and beans) can improve amino acid profi les and overcome defi ciencies. Advantages of Animal Protein Sources: Generally provide all indispensable amino acids in suffi cient quantities. Higher protein digestibility compared to plant-based proteins.  4.8 Factors Other changes Than Diet Affecting Protein and Physiological Amino Environmental Acid Requirements Psychological influences factors Not everyone of the same age, body build, and gender has identical nutrient requirements. Factors infl uencing nutrient requirements Factors Genetic influencing Pathological include: background nutrient conditions requirements  1. Genetic Factors Genetic predispositions can affect metabolism and nutrient needs. Variations in genes related to protein metabolism can lead to differences in: o Amino acid utilization o Protein synthesis effi ciency o Examples of genetic disorders impacting protein requirements (e.g., phenylketonuria). 4.8 Factors Other Than Diet  2. Environmental Infl uences Affecting Protein and Factors such as climate, altitude, and lifestyle can impact nutrient needs. Amino Acid Requirements Examples: Higher protein needs in physically active individuals. Nutrient needs may vary based on living in extreme environments (e.g., high altitude).  3. Physiological Factors  4. Pathological Infl uences Diff erent life stages have varying protein requirements: Chronic diseases can alter nutrient needs: Infants & Children: Higher protein needs per unit o Increased catabolism during illness leads to greater body weight. nitrogen and amino acid loss. Adults: Nutrient needs stabilize; dietary intake o Examples: Cancer, kidney disease, infections. adjusts to lifestyle. Infections can lead to: Elderly: Nutritional needs similar to younger adults, o Increased protein and nutrient requirements for but may be infl uenced by chronic conditions. recovery. Increased incidence of disease aff ects dietary o Anabolic responses to support immune function. requirements. 4.8 Factors  Impact of Aging Other Than Diet Aging is often associated with increased Affecting morbidity. Protein and Chronic diseases (e.g., diabetes, Amino Acid cardiovascular diseases) signifi cantly Requirements impact nutrient requirements. Nutrient absorption and metabolism may decline with age, necessitating adjustments in dietary intake. 4.8 Factors Other Than Diet Affecting Protein and Amino Acid Requirements  Metabolic Response to Stress and Illness 1. Catabolic Phase: Increased nitrogen loss due to infection or trauma. Loss of other nutrients (potassium, magnesium, vitamin C). 2. Anabolic Phase: Body’s response during recovery involves: Increased nitrogen retention. Enhanced production of immune cells and tissue repair enzymes. Duration of Responses: Anabolic response duration exceeds the catabolic phase. Nutritional Needs During Recovery: Increased protein and amino acid needs during recovery from illness or trauma. Importance of tailored nutrition for individuals recovering from acute or chronic conditions.  1. Agent 3. Environmental 2. Host factors (dietary) factors factors Chemical form of Physical (unsuitable nutrition (protein and Age housing, inadequate amino acid source) heating) Sex Biologic (poor Energy intake sanitary conditions) Food processing and Genetic makeup Socioeconomic preparation (may (poverty, dietary increase or decrease habits and food dietary needs) Pathologic states choices, physical Effect of other dietary activity) constituents Drugs Agent, host, and environment factors that Infection affect protein and amino acid requirements and the nutritional status of the individual Physical trauma References Gibney, M. J., Lanham-New, S. A., Cassidy, A., & Vorster, H. H. (Eds.). (2009). Introduction to human nutrition (2nd ed.). Wiley-Blackwell. Smolin, L. A., & Grosvenor, M. B. Basic nutrition (2nd ed.). THANK YOU