Nutrition for Health & Health Care - Chapter 2: Digestion and Absorption

To function properly, a healthy gastrointestinal (GI) tract needs: three essential elements (details not provided).

The digestive tract is selective:
- Materials of nutritive value are broken down into smaller particles and absorbed.
- Most nonnutritive materials are left undigested and excreted.
The gastrointestinal (GI) tract is described as:
- A flexible, muscular tube extending from the mouth to the anus.
- Nutrients enter the rest of the body through the cells of the digestive tract.
- Principal organs include the stomach and intestines.
- The term "Gastro" refers to the stomach.
- Nutrients must pass through the cells of the digestive tract wall to enter the body.

Digestive process defined:
- The breakdown of complex food particles into smaller, absorbable particles.
Digestive system includes:
- All organs and glands involved in the ingestion and digestion of food.
Path of food through the digestive tract in sequential order:
1. Mouth
2. Esophagus
3. Stomach
4. Small Intestine
5. Large Intestine (Colon)
6. Rectum

Mouth to the Esophagus:
- The bolus (chewed food) slides across the epiglottis.
- The epiglottis blocks the trachea to prevent choking.
- The upper esophageal sphincter allows the bolus to slide into the stomach.
Esophagus to Stomach:
- The lower sphincter prevents the bolus from slipping back up.
- In the stomach, the bolus is transformed into chyme.
Small Intestine:
- Chyme is released bit by bit through the pyloric sphincter into the small intestine.
- Chyme passes by the common bile duct, which secretes digestive fluids from the gallbladder and pancreas.
- It then travels through segments of the small intestine: duodenum, jejunum, ileum.
Large Intestine:
- Remaining contents pass through the ileocecal valve into the large intestine (colon).
- Contents pass the opening leading to the appendix and then to the rectum.
- The colon withdraws water, leaving semisolid waste for the rectum.
- Rectal muscles hold back waste until defecation, and the anus allows waste to pass.

Muscles and glands perform digestive functions without conscious effort:
- Gastrointestinal motility: Spontaneous movement in the digestive tract via involuntary muscular contractions.
- Peristalsis: Successive waves of contractions that move contents along the GI tract.
- Segmentation: Periodic squeezing or partitioning by circular muscles that mix and slowly push contents along.

The rate of digestion depends on meal composition:
- High in simple sugars: rapid digestion.
- Rich in fat: slower digestion.
Energy-yielding nutrients (carbohydrates, fats, proteins) are disassembled into basic building blocks before absorption.
Most nutrients (vitamins, minerals, water) are absorbed as-is.
Undigested residues, such as fibers, continue through as a semisolid mass; excreted waste contains little value post digestion.

Villi: Fingerlike projections from small intestine folds; singular form is villus.
Microvilli: Tiny projections on villus cells that trap and transport nutrient particles.
Nutrient molecules small enough to be absorbed are caught among microvilli, where they are drawn into cells.
Intestinal tract cells are specialized for different nutrient absorption.
The myth of food combining (separating food groups) is debunked; together foods can enhance each other's use.

Lymphatic System:
- A system of vessels and ducts conveying digestion products toward the heart.
- Lymph consists of constituents of blood except red blood cells.
Triglycerides:
- A class of lipids; chief form of fat in foods and body.
- Composed of glycerol and three fatty acids.
Lipoproteins:
- Transport vehicles for lipids in lymph and blood.
- Chylomicrons: Lipoproteins that transport lipids from intestinal cells into the body.
- Body cells remove needed lipids from chylomicrons, leaving remnants for liver cells.

Nutrients enter blood or lymphatic system and are transported throughout the body to any cell.
Blood is carried to the digestive system via an artery that branches into capillaries.
Blood leaving the digestive system returns through veins:
- Hepatic Portal Vein: Collects blood from the GI tract to the liver.
- Hepatic Vein: Collects blood from liver capillaries back to the heart.
The blood lipid profile includes the types of fat and lipoproteins present; factors that influence LDL and HDL concentrations include:
- Weight management
- Reduced saturated fats
- Soluble fibers
- Physical activity

Homeostasis: Maintenance of constant internal conditions by control systems in the body.
Hormones: Chemical messengers secreted by glands that travel to target organs and invoke responses for maintaining homeostasis.
- Example: Gastrin - a hormone from stomach cells prompting gastric acid secretion.
Microbes: Include bacteria, viruses, fungi, protozoa; referred to as microbiota or gut flora, and their collective genes are called the microbiome.
pH: The concentration of hydrogen ions; lower pH indicates stronger acid (e.g., pH 2 = strong acid).
Prebiotics: Indigestible substances promoting beneficial bacteria growth.
Probiotics: Live microorganisms in foods/dietary supplements with health benefits.
Various lifestyle factors influence GI health:
- Sleep aids tissue repair.
- Physical activity protects against colon cancer.
- Mental state impacts digestion via nerves and hormones.
- Adequate nutrition is essential for GI tract health.

The Food and Drug Administration (FDA) identifies foodborne illness as the primary food safety concern in the U.S., with incidents of food poisoning dominating other contamination types.
Causes of foodborne illness include infections and intoxications.
Four strategies to ensure food safety:
- Clean: Wash hands and surfaces often.
- Separate: Avoid cross-contamination.
- Chill: Refrigerate foods promptly.
- Cook: Ensure proper cooking temperatures for food.