[OS 201] E02-T10-Biochemistry of Macronutrients_compressed
I. Introduction
A. Overview of Macronutrients
Macronutrients are substances required in large amounts in the diet that are metabolized by the body for various critical functions including:
Energy provision: They provide fuel for all body activities, from cellular functions to physical exercise.
Maintenance of homeostasis: They help maintain a stable internal environment in the body, crucial for survival.
Support of bodily functions and structures: They are essential for the structure and function of cells, organs, and systems in the body.
Three Major Types:
Carbohydrates: Primary source of energy, especially for the brain and during physical activity.
Proteins: Vital for the growth and repair of tissues, production of enzymes and hormones, and serving as an energy source when carbohydrates are scarce.
Lipids: Important for energy storage, cell membrane integrity, and signaling processes in the body.
B. Digestion of Carbohydrates
Mouth: Enzymatic action begins with salivary amylase, which breaks down polysaccharide starches into oligosaccharides, facilitating nutrient absorption.
Stomach: The acidic environment (HCl) deactivates salivary amylase, halting further digestion of carbohydrates.
Small intestine:
Cholecystokinin (CCK) induces the release of pancreatic juices, including pancreatic amylase, which continues the breakdown of carbohydrates to trisaccharides and disaccharides.
Brush border enzymes convert these disaccharides into monosaccharides for absorption.
C. Digestion of Proteins
Mouth: Mechanically breaks down food and begins protein denaturation through mixing.
Stomach:
Gastrin, a hormone, triggers the secretion of HCl, which unfolds proteins and activates pepsinogen to pepsin, cleaving peptide bonds and resulting in oligopeptides.
Small intestine:
CCK stimulates the secretion of digestive enzymes, including trypsinogen, which is activated to trypsin. It further cleaves peptides into amino acids, which are absorbed via specific transporters.
D. Digestion of Lipids
Mouth: Lingual lipase initiates the breakdown of triglycerides (TAG) into diglycerides and free fatty acids.
Stomach: Gastric lipase continues the breakdown of TAG, albeit less significantly due to the acidity of gastric contents.
Small intestine:
Pancreatic lipase, aided by bile acids, emulsifies fats for digestion, breaking them down into monoacylglycerides and free fatty acids for absorption into enterocytes.
E. Summary of Macronutrient Metabolism
Functions: Macronutrients provide energy, maintain bodily structure, and are critical in metabolism.
Key metabolic pathways:
Glycolysis: Converts glucose to pyruvate, resulting in ATP production.
Gluconeogenesis: Converts non-carbohydrate sources (like amino acids) into glucose during fasting states.
Lipolysis: Breakdown of TAG into fatty acids for energy utilization, particularly during prolonged fasting or exercise.
F. Digestion in Enterocytes
Carbohydrate absorption: Disaccharides are converted into monosaccharides by brush-border enzymes before entering the bloodstream.
Protein absorption: Amino acids and small peptides are absorbed via specific transport mechanisms.
Lipid absorption: Fatty acids and monoglycerides passively diffuse through the plasma membrane of enterocytes, where they are reassembled into TAG.
G. Macronutrients & The Provision of Energy
Energy Production: Macronutrients are oxidized to produce ATP.
Non-digestible components: Some are non-digestible or converted to fecal energy, impacting overall caloric intake and metabolism.
Net energy usage: The energy produced from oxidation supports metabolic functions, physical activity, and maintenance of bodily functions.
II. Carbohydrates as Macronutrient
A. Carbohydrates as Energy Source
Carbohydrates serve as the primary energy source in the diet, being metabolized to carbon dioxide (CO2) and water (H2O) during aerobic respiration, with excess carbohydrates stored as glycogen in the liver and muscles for later use.
B. Carbohydrates as Dietary Fibers
Dietary fibers are indigestible plant cell wall components, including cellulose and hemicellulose. They play a crucial role in digestive health by promoting regular bowel movements, preventing constipation, and supporting the growth of beneficial gut microbiota.
C. Glycemic Index (GI)
The Glycemic Index measures blood glucose levels following carbohydrate consumption, with a scale from 0 to 100, where glucose is set as the reference point at 100. Foods with a high GI cause rapid spikes in blood sugar, while low-GI foods lead to gradual increases.
D. Glycemic Load (GL)
Glycemic Load considers both glycemic index and the carbohydrate quantity in a serving, providing a more comprehensive view of a food's impact on blood sugar. This measure helps individuals manage blood sugar levels, especially in those with insulin sensitivity or diabetes.
E. Carbohydrate Intolerance
Conditions such as lactose intolerance result from the body’s inability to digest certain carbohydrates, leading to symptoms such as bloating, gas, and diarrhea due to malabsorption.
III. Proteins as Macronutrient
A. Roles of Proteins in Nutrition
Proteins are essential for numerous physiological functions, including:
Growth and repair of body tissues,
Hormone production (e.g., insulin, growth hormone),
Immune system function (e.g., antibodies),
Serving as a secondary energy source during prolonged fasting or intense physical activity.
B. Measures of Protein Quality
The quality of dietary proteins can be assessed using measures such as:
Amino Acid Score (AAS): Reflects the proportion of essential amino acids.
Protein Digestibility-Corrected Amino Acid Score (PDCAAS): Accounts for amino acid content and digestibility.
Digestible Indispensable Amino Acid Score (DIAAS): Assesses protein quality based on the amino acid digestibility at the end of the small intestine.
C. Ways of Improving Protein Quality
Improving protein quality can be achieved through strategies like:
Fortification: Adding essential nutrients to low-protein foods.
Enrichment: Enhancing protein content in foods through processing and formulation.
Supplementation: Using protein supplements to meet dietary requirements.
Complementation: Combining plant proteins with different amino acid profiles to achieve complete protein intake.
D. Amino Acids and Health
The availability and metabolism of amino acids can be affected by various conditions, such as illness-induced nitrogen loss, which may require higher protein intake to meet physiological needs and support recovery.
IV. Lipids as Macronutrient
A. Roles of Dietary Lipids in Human Body
Dietary lipids are essential for numerous bodily functions, including:
Major energy storage, as they provide more than double the energy per gram compared to carbohydrates or proteins.
Structural components of cellular membranes, maintaining fluidity and integrity.
Precursors to bioactive molecules such as hormones and signaling compounds (cytokines and eicosanoids).
B. Lipids in Food
Types of dietary lipids include:
Saturated fatty acids: Found in animal fats and some tropical oils.
Unsaturated fatty acids: Found in vegetable oils, nuts, and seeds, categorized into monounsaturated and polyunsaturated fats.
Trans fatty acids: Industrially produced fats linked to negative health outcomes.
C. Lipids in Vascular Disorders
There are established links between industrial trans fats and an increased risk of cardiovascular diseases, including atherosclerosis and heart attacks, highlighting the importance of dietary fat quality over quantity.
V. Supplementary
A. Reference Macronutrient Intake
The Philippine Dietary Reference Intake (PDRI) establishes nutrient intake levels to guide dietary decisions for various populations, recommending appropriate macronutrient distribution ranges for maintaining health and preventing chronic diseases.
VI. References
A comprehensive list of cited resources on nutritional guidelines and detailed mechanisms of macronutrient metabolism, ensuring the quality and reliability of the information presented.