Nutrition & Digestion: Structure, Function, and Human Health

Nutrition & Digestion

Introduction

Nutrition and digestion are fundamental biological processes that enable animals, including humans, to obtain, process, and utilize food for energy, growth, and maintenance. This chapter explores feeding behaviors, digestive system structures, the biochemical breakdown of food, essential nutrients, and the impact of diet on health.

Feeding Behaviors and Ingestion Methods

Main Feeding Behaviors

• Herbivores: Animals that primarily consume plants.

• Carnivores: Animals that primarily consume other animals.

• Omnivores: Animals that consume both plants and animals.

• Detritivores: Animals that feed on dead organic matter.

Methods of Food Acquisition

• Filter Feeders: Strain small food particles from water (e.g., clams, baleen whales).

• Substrate Feeders: Live in or on their food source (e.g., caterpillars, earthworms).

• Fluid Feeders: Suck nutrient-rich fluids from a host (e.g., mosquitoes, hummingbirds).

• Bulk Feeders: Eat relatively large pieces of food (e.g., humans, lions).

Food Processing and Digestion

Stages of Food Processing

• Ingestion: Taking in food.

• Digestion: Mechanical and chemical breakdown of food into absorbable units.

• Absorption: Uptake of nutrients by body cells.

• Elimination: Removal of undigested material from the body.

Mechanical vs. Chemical Digestion

• Mechanical Digestion: Physical breakdown of food (e.g., chewing, churning in stomach).

• Chemical Digestion: Enzymatic hydrolysis of macromolecules into monomers.

Major Food Biomolecules and Their Digestion

• Carbohydrates: Broken down by amylases into monosaccharides.

• Proteins: Broken down by proteases (e.g., pepsin, trypsin) into amino acids.

• Lipids: Broken down by lipases into fatty acids and glycerol.

• Nucleic Acids: Broken down by nucleases into nucleotides.

Digestive System Structures

Types of Digestive Compartments

• Food Vacuoles: Intracellular digestion in single-celled organisms.

• Gastrovascular Cavities: Single opening for ingestion and elimination (e.g., cnidarians).

• Alimentary Canals: Complete digestive tracts with separate mouth and anus (e.g., earthworms, humans).

Human Digestive System Overview

• Oral Cavity: Mechanical (chewing) and chemical (salivary amylase) digestion begins.

• Pharynx and Esophagus: Swallowing and peristalsis move food to the stomach.

• Stomach: Stores food, secretes gastric juice (acid and enzymes), initiates protein digestion.

• Small Intestine: Major site of chemical digestion and nutrient absorption; receives bile and pancreatic enzymes.

• Large Intestine: Reclaims water, compacts feces, houses beneficial bacteria.

Accessory Organs

• Liver: Produces bile, processes nutrients, detoxifies substances.

• Gallbladder: Stores and releases bile.

• Pancreas: Secretes digestive enzymes and bicarbonate.

Digestive Enzymes and Their Functions

Summary Table of Digestive Enzymes

Enzyme	Source	Substrate	Product
Salivary Amylase	Salivary Glands	Starch	Maltose
Pepsin	Stomach	Proteins	Small polypeptides
Pancreatic Amylase	Pancreas	Starch, Glycogen	Maltose
Trypsin, Chymotrypsin	Pancreas	Polypeptides	Smaller peptides
Lipase	Pancreas	Fats	Fatty acids, Glycerol
Nucleases	Pancreas	DNA, RNA	Nucleotides
Disaccharidases (e.g., maltase, lactase, sucrase)	Small Intestine	Disaccharides	Monosaccharides
Aminopeptidase, Carboxypeptidase	Small Intestine	Peptides	Amino acids

Absorption and Transport

Small Intestine Structure and Function

• Villi and Microvilli: Increase surface area for absorption.

• Capillaries and Lacteals: Transport absorbed nutrients to the bloodstream and lymphatic system.

Liver's Role in Nutrient Processing

• Hepatic Portal Vein: Transports absorbed nutrients from the intestine to the liver for processing and detoxification.

Digestive Disorders

Common Disorders

• Acid Reflux: Backflow of stomach acid into the esophagus.

• Ulcers: Erosions in the stomach lining, often caused by Helicobacter pylori infection.

• Liver Disorders: Includes hepatitis, fatty liver, and cirrhosis.

Evolutionary Adaptations of Digestive Systems

Adaptations to Diet

• Herbivores: Longer digestive tracts, specialized stomachs (e.g., ruminants with four-chambered stomachs) for cellulose digestion.

• Carnivores: Shorter digestive tracts, simple stomachs.

• Omnivores: Intermediate digestive tract length and complexity.

Nutrition: Essential Nutrients and Dietary Requirements

Three Nutritional Needs

• Energy: Sufficient calories for cellular respiration and activity.

• Essential Nutrients: Nutrients that cannot be synthesized by the body and must be obtained from the diet.

• Organic Building Blocks: For biosynthesis of macromolecules.

Essential Nutrients

• Essential Amino Acids: Nine amino acids must be obtained from food (e.g., leucine, lysine, tryptophan).

• Essential Fatty Acids: Linoleic and linolenic acids, required for making certain lipids.

• Vitamins: Organic molecules required in small amounts for metabolism (e.g., vitamin C, B vitamins).

• Minerals: Inorganic nutrients (e.g., calcium, iron, potassium, sodium).

Table: Major Minerals and Their Functions

Mineral	Dietary Sources	Functions	Deficiency Symptoms
Calcium	Dairy, dark green vegetables	Bone and tooth formation, nerve and muscle function	Impaired growth, loss of bone mass
Phosphorus	Dairy, meats, grains	Bone and tooth formation, acid-base balance	Weakness, bone pain
Sulfur	Proteins	Component of amino acids	Impaired growth
Potassium	Meats, dairy, fruits, vegetables	Acid-base balance, nerve function	Muscle weakness, paralysis
Sodium	Table salt	Acid-base balance, nerve function	Muscle cramps, reduced appetite
Magnesium	Whole grains, leafy vegetables	Enzyme cofactor, ATP processing	Nervous system disturbances
Iron	Meats, eggs, legumes	Component of hemoglobin	Anemia, weakness
Fluorine	Water, tea, seafood	Tooth structure	Tooth decay
Iodine	Seafood, iodized salt	Thyroid hormone component	Goiter

Energy Content and Dietary Balance

Measuring Food Energy

• Calorie (kcal): The amount of energy required to raise 1 kg of water by 1°C.

• Basal Metabolic Rate (BMR): The minimum energy expenditure for basic physiological functions.

• Recommended Daily Intake: ~55% carbohydrates, 20-25% fat, 15% protein.

Obesity and Human Health

Obesity: Causes and Consequences

• Obesity: Excessive accumulation of body fat, often due to energy intake exceeding expenditure.

• Possible Causes: Genetic predisposition, hormonal regulation (e.g., leptin), lifestyle factors, evolutionary adaptations.

• Health Risks: Increased risk of cardiovascular disease, diabetes, certain cancers.

Leptin and Appetite Regulation

• Leptin: A hormone produced by adipose tissue that helps regulate appetite and energy balance.

• Leptin Resistance: May contribute to obesity when the body does not respond properly to leptin signals.

Diet and Disease Risk

• Cardiovascular Disease: Linked to high intake of saturated fats, cholesterol, and imbalances in serum lipoproteins (e.g., LDL, HDL).

• Cancer: Certain dietary patterns may increase or decrease cancer risk.

Summary

• Animals require a balanced diet for energy, essential nutrients, and biosynthesis.

• The digestive system is specialized for the breakdown and absorption of food.

• Dietary choices and nutrient intake have significant impacts on health and disease risk.