Nutrients and their Digestion, Absorption and Metabolism

To comprehend the impact of eating on the human body, it is essential to understand:

The basic chemical composition of foods.
How dietary chemicals are processed within the body (digestion, absorption, and metabolism).
The specific functions of these chemicals and their metabolites (products formed in the body).

Nutrients are dietary chemicals that fulfill several vital roles in the body:

Energy Provision: Supplying energy measured in kilocalories (kcal) or kilojoules (kJ).
- 1 ext{ kcal (large calorie)} = ext{energy to warm } 1 ext{ g of water } 1 ^ ext{o} ext{C} = 4.2 ext{ kJ}
Heat Generation: Contributing to thermogenesis.
Structural Components: Providing building blocks for tissues like tendons, cells, organs (e.g., muscles, bone).
Functional Roles: Enabling metabolic pathways that drive various physiological processes with functional consequences.
Regulatory/Signaling Molecules: Acting as precursors or components for hormones, cytokines, and adipokines.

Food contains a variety of dietary chemicals, including nutrients and other constituents:

Nutrients: Essential chemicals required for proper body function.
- Macronutrients: Needed in large quantities.
  - Protein
  - Carbohydrates
  - Fat/Lipids
  - Water
- Micronutrients: Needed in small quantities.
  - Vitamins
  - Minerals
Other Constituents of Foods:
- Fiber: Complex non-digestible carbohydrates.
- Phytochemicals: Trace chemicals with biological activity that may impact health, but are not classified as macronutrients, micronutrients, or fiber.

Composition: All proteins are polymers of amino acids (AA) linked end-to-end. There are 20 common amino acids.
Classification of Amino Acids:
- Essential: The human body cannot synthesize these, so they must be obtained through diet. If the diet lacks essential amino acids, body proteins cannot be synthesized.
- Conditionally Essential: The body may not be able to synthesize sufficient quantities, especially depending on factors like growth rate or specific dietary conditions.
- Non-essential: Human cells can synthesize these amino acids.
Functions: Involved in growth, maintenance, and repair by supplying amino acids. Proteins carry out most bodily functions and are conserved whenever possible (e.g., during starvation or calorie deficiency).
Energy Content: Proteins provide 4 ext{ kcal/gram} as energy.
Sources: Meat, fish, poultry, dairy products, legumes, and grains are rich sources.

Types:
- Simple Sugars: Typically comprise two attached sugar units (disaccharides).
  - Sucrose = Glucose-Fructose
  - Lactose = Glucose-Galactose
- Complex Carbohydrates: Polymers of many sugar units, such as those found in grains, beans, peas, and potatoes. Some (e.g., glycogen) are branched structures. These are absorbed more slowly.
Blood Glucose Control: Certain complex carbohydrates promote better blood glucose (glycemic) control compared to simple sugars.
Energy Content: Carbohydrates provide 4 ext{ kcal/gram}.

Classification: Fat belongs to the family of compounds known as lipids.
Structure: Dietary fat is primarily triacylglycerol (TAG) or triglyceride (TG), consisting of fatty acids (FA - acyl groups with a carboxyl function) bound to glycerol.
Energy Content: Fatty acids provide concentrated energy, 9 ext{ kcal/gram}. This high energy density allows for efficient energy storage without requiring water for hydration.

Body Composition: The body is predominantly water, approximately 60-70 ext{%} in children and 50 ext{%} in adults. Body water percentage decreases with age due to less lean tissue.
Distribution: Water distributes throughout the body without barriers; water balance is tightly regulated.
Functions:
- Solvent: The primary solvent of living organisms; life processes occur