Nutrition, Metabolism, and Energy Balance

Introduction

This chapter explores the essential concepts of nutrition, metabolism, and energy balance, which are fundamental to understanding how the human body acquires, processes, and utilizes nutrients for growth, maintenance, and energy production.

Nutrients

Definition and Types of Nutrients

• Nutrient: A substance in food needed for growth, maintenance, and repair of the body.

• Macronutrients: The three major nutrients that make up the bulk of ingested food:

  • Carbohydrates

  • Lipids (fats)

  • Proteins

• Micronutrients: Nutrients required in small amounts, including:

  • Vitamins

  • Minerals

• Water is also required and considered a nutrient.

• Essential nutrients: Nutrients that must be obtained from the diet because the body cannot synthesize them from other nutrients. Approximately 45–50 nutrients are considered essential.

Energy Value of Nutrients

• Energy value is measured in kilocalories (kcal).

• One calorie is the amount of heat needed to raise the temperature of 1 kg of water by 1°C.

• One dietary "Calorie" (with a capital C) equals 1 kcal.

Dietary Guidelines

• MyPlate: A visual guide representing dietary guidelines as portions on a dinner plate.

• Food groups represented:

  • Fruits

  • Vegetables

  • Grains

  • Protein

  • Dairy

• Key dietary principles:

  • Eat only what you need (eat less overall).

  • Eat plenty of fruits, vegetables, and whole grains.

  • Avoid junk food.

Metabolism

Definition of Metabolism

• Metabolism: The sum of all biochemical reactions inside a cell involving nutrients.

Anabolism and Catabolism

• Anabolism: Synthesis of large molecules from small ones (e.g., synthesis of proteins from amino acids).

• Catabolism: Hydrolysis of complex structures to simpler ones (e.g., breakdown of proteins into amino acids).

Cellular Respiration and Phosphorylation

• Cellular respiration: Catabolic breakdown of food fuels to form ATP in cells. The goal is to trap chemical energy in ATP.

• Energy can also be stored in glycogen and fats for later use.

• Phosphorylation: Enzymes shift high-energy phosphate groups of ATP to other molecules, activating them to perform cellular functions.

Stages of Nutrient Processing

1. Stage 1: Digestion, absorption, and transport to tissues.

2. Stage 2: Cellular processing (in cytoplasm):

   • Synthesis of lipids, proteins, and glycogen, or

   • Catabolism (glycolysis) into pyruvic acid and acetyl CoA

3. Stage 3: Oxidative breakdown of intermediates into , water, and ATP (occurs in mitochondria).

Additional info: Cellular respiration consists of glycolysis (stage 2) and all of stage 3.

Carbohydrate Metabolism

Glucose Utilization

• When glucose enters a cell, it is phosphorylated to glucose-6-phosphate.

• Most cells lack enzymes for the reverse reaction, so glucose becomes trapped inside the cell.

• Only cells in the intestine, kidney, and liver can reverse this reaction and release glucose.

• This mechanism keeps intracellular glucose concentration low, ensuring continued glucose uptake.

Oxidation of Glucose

• Glucose is catabolized via the following reaction:

• Complete glucose catabolism requires three pathways:

  • Glycolysis

  • Krebs cycle

  • Electron transport chain and oxidative phosphorylation

Summary Table: Major Nutrient Types and Functions

Example: ATP Production from Glucose

• During cellular respiration, one molecule of glucose can yield up to 32 molecules of ATP.

• This process involves glycolysis (in the cytoplasm), the Krebs cycle (in the mitochondria), and the electron transport chain (in the mitochondria).