Nutrient Digestion, Absorption, and Metabolism Overview

Dry Matter

Determination through drying feeds in a drying oven to a constant weight.

Importance of Dry Matter

1. The value and quality depend upon how much of their weight is made up of water. 2. Feed analyses are performed on the dried sample.

Ash

Total mineral content. Equipment: Furnace.

Crude Protein (Kjeldahl Analysis)

The crude protein content of a feed sample represents the total nitrogen (N) in the diet which includes not only true protein but also non-protein nitrogen (e.g., urea and ammonia in a feed but nitrate is not included). Total N present in a feed sample is first determined and then the crude protein is calculated as N x 6.25 (since protein is 16% N).

Crude Fat/Ether Extract

An estimate of the total fat content of feeds from the older collection of methods known as proximate methodology. Crude fat is estimated using either extraction. Crude fat contains true fat (triglycerides) as well as alcohols, waxes, terpenes, steroids, pigments, esters, aldehydes, and other lipids.

Crude Fiber

The insoluble carbohydrates remaining after digestion with dilute acid and alkali. It includes cellulose, hemicellulose, lignin, and other indigestible components.

Nitrogen-Free Extract (NFE)

The difference between the total dry matter and the sum of crude protein, crude fat, crude fiber, and ash. It includes easily digestible carbohydrates such as starches and sugars.

Digestion

The process of breaking down large, complex food molecules into smaller, simpler molecules that can be absorbed.

Absorption

The process by which digested nutrients pass from the gastrointestinal tract into the bloodstream or lymphatic system.

Monosaccharides

Simple sugars like glucose, fructose, and galactose. They are the absorbable form of carbohydrates.

Polysaccharides

Complex carbohydrates like starch and cellulose. They must be broken down into monosaccharides for absorption.

Enzymes for Carbohydrate Digestion

Amylase (salivary and pancreatic), disaccharidases (maltase, sucrase, lactase).

Active Transport of Glucose and Galactose

Requires sodium-glucose co-transporter (SGLT1) for uptake into enterocytes.

Facilitated Diffusion of Fructose

Through GLUT5 transporter into enterocytes.

Transport out of Enterocytes

GLUT2 transporter facilitates movement of all monosaccharides into the bloodstream.

Protein Digestion in Stomach

Initiated by pepsin, an enzyme activated by acidic pH. Breaks proteins into polypeptides.

Protein Digestion in Small Intestine

Pancreatic proteases (trypsin, chymotrypsin, carboxypeptidases) break polypeptides into smaller peptides and amino acids. Brush border peptidases further break down peptides.

Amino Acid Absorption

Individual amino acids are absorbed via specific amino acid transporters, often sodium-dependent co-transport.

Peptide Absorption

Small peptides (di- and tripeptides) can be absorbed directly via peptide transporters (e.g., PEPT1), then hydrolyzed to amino acids inside the enterocyte.

Emulsification of Fats

Bile salts emulsify large fat globules into smaller micelles, increasing surface area for enzyme action.

Lipase

Pancreatic lipase breaks down triglycerides into monoglycerides and free fatty acids.

Micelle Formation

Monoglycerides and free fatty acids combine with bile salts to form micelles, which transport them to the brush border.

Absorption into Mucosal Cells

Monoglycerides and free fatty acids diffuse into mucosal cells.

Re-esterification

Inside the mucosal cells, monoglycerides and free fatty acids are re-esterified to form triglycerides.

Chylomicron Formation

Triglycerides are packaged with cholesterol, phospholipids, and apolipoproteins to form chylomicrons.

Lacteals

Small lymphatic vessels in the intestinal villi. Chylomicrons are too large to enter capillaries and instead enter lacteals.

Lymphatic Transport

Chylomicrons travel through the lymphatic system and eventually enter the bloodstream via the thoracic duct.

Chyle

Lymph fluid passing through the lymph vessels, containing chylomicrons.

Metabolism

A central theme in biochemistry that keeps cells and organisms alive. It provides energy and building blocks for growth and multiplication.

Catabolic (degradative) reactions

Break down complex biomolecules (carbohydrates, proteins, fats) into simpler ones, releasing energy. Examples: Glycolysis, Fatty Acid Spiral.

Anabolic (synthetic) reactions

Synthesis of small molecules and building blocks not sufficiently available in food; Synthesis of macromolecules (proteins and nucleic acids). Requires energy. Examples: Glycogenesis, Lipogenesis, Tissue Protein Synthesis.

Central metabolic hub

Acetyl Co-A, where the catabolism of carbohydrates, fats, and proteins converges.

Energy usage in metabolism

Energy is used in the formation of high-energy phosphate compounds (like ATP) or enters the Krebs Cycle (TCA Cycle) to produce H+ and CO2.

ATP (Adenosine Triphosphate) Structure

Adenine, ribose, and 3 phosphate groups connected by covalent bonds.

Stability

Unstable molecule.

Hydrolysis

Yields a more stable product.

Phosphorylation

When a molecule is phosphorylated (receives a phosphate group from ATP), it becomes more reactive and stable.

Electron Transport Chain (ETC) Function

The final common pathway for electrons from various fuels to oxygen, converting electrochemical energy into chemical energy in the form of ATP.

Electron Transport Chain (ETC) Location

Inner mitochondrial membrane.

Electron Transport Chain (ETC) Components

Complex I (NADH dehydrogenase), Complex II (Succinate dehydrogenase), Coenzyme Q (ubiquinone), Complex III (Cytochrome bc1 complex), Cytochrome c, Complex IV (Cytochrome oxidase), ATP Synthase.

Proton pumping

Complexes I, III, and IV pump protons (H+) from the mitochondrial matrix into the intermembrane space, creating an electrochemical proton gradient.

ATP Synthase

Uses the energy from the proton gradient to synthesize ATP from ADP and Pi.

Inhibitors of ETC - Rotenone

Inhibits Complex I.

Inhibitors of ETC - Antimycin

Inhibits Complex III.

Inhibitors of ETC - Cyanide/CO

Inhibit Complex IV.

Inhibitors of ETC - Oligomycin

Inhibits ATP Synthase.

Inhibitors of ETC - Uncoupling agents (e.g., DNP)

Dissipate the proton gradient, allowing electron transport to continue without ATP synthesis, releasing energy as heat.

Fatty Acid Synthesis Location

Cytoplasm.

Fatty Acid Synthesis Precursors

Glucose (via Glycolysis and Pyruvate Metabolism to Acetyl CoA), Pyruvate.

Citrate Shuttle

Acetyl CoA (from mitochondria via TCA cycle) is transported into the cytoplasm as citrate. Citrate is then cleaved back into Acetyl CoA and oxaloacetate in the cytoplasm.

Saturated fatty acids

No double bonds.

Unsaturated fatty acids

Contain one or more double bonds.

Polyunsaturated fatty acids

Contain multiple double bonds (>2).

Essential fatty acids

Polyunsaturated fatty acids that are NOT synthesized by the body and must be obtained from the diet. They decrease the risk of Cardiovascular Disease. Examples: Linoleic Acid (Omega 6), Alpha-Linolenic Acid (ALA), Eicosapentaenoic Acid (EPA), Docosahexaenoic Acid (DHA).