Principles of Nutrition Week 2: Sources of Energy

Sources of Energy

Introduction

  • Presented by: Dr. Yemisi Latunde-Dada

  • Department: Nutritional Sciences

  • School: Life Course & Population Sciences

  • Faculty: Life Sciences & Medicine

Learning Objectives

  • Learn and describe the units of energy used to measure food energy content.

  • Define the main sources of energy in the UK diet.

  • Explain how the gross energy content of foods may be measured.

  • Calculate energy contents of foods using Atwater factors based on their known macronutrient composition.

  • Recognize the main factors that influence the energy density of foods.

Key Concepts

  • Units of energy and sources of energy

  • Determination of energy contents of foods

  • Gross energy (GE) and metabolizable energy (ME) content of foods

  • Atwater factors

  • Energy density of foods

Understanding Energy

What is Energy?

  • Different forms of energy:

    • Solar

    • Thermal

    • Mechanical

    • Chemical

    • Electrical

First Law of Thermodynamics

  • Definition: Energy cannot be created or destroyed, only converted from one form to another.

  • Examples:

    • Car engine: Chemical → Mechanical

    • Battery: Chemical → Electrical

    • Photosynthesis: Solar → Chemical

    • Food: Chemical → Heat, Mechanical

Uses of Energy in the Body

  • Energy is utilized for:

    • Growth, pregnancy, lactation, and tissue repair.

    • Essential body functions: keeping warm, respiration, circulation.

    • Physical movement and muscle function.

Measuring Energy in Food

Units of Measurement

  • Energy is typically measured in:

    • Kilocalories (kcal)

    • Kilojoules (kJ)

    • Conversion:

    • 1 kcal = 4.184 kJ

  • Examples of energy contents:

    • 1 slice of bread = 70 kcal = 293 kJ

Problem-Solving Exercise

  • Calculate energy in kJ for a packet of crisps (175 kcal):

    • Result: 732 kJ

  • If a sausage roll contains 285 kJ, calculate calories in 3 sausage rolls:

    • Result: 273 kcal

Sources of Energy in the Diet

Macronutrients

  • Energy primarily derived from macronutrients:

    • Carbohydrates > Fats > Proteins

    • Alcohol also serves as a source.

  • NDNS Results (2008-2012):

    • Average daily intake:

    • 8.88 MJ/day (2111 kcal/day)

    • Breakdown by years indicating fluctuation in intake

Carbohydrates

  • Types:

    • Simple (sugars)

    • Complex (starches, fibres):

    • Monosaccharides: glucose, fructose, galactose

    • Disaccharides: sucrose, lactose, maltose

    • Polysaccharides: starch, non-starch polysaccharides (fibre)

  • Stored as glycogen in liver and muscle - limited energy supply.

  • Sources: cereals, beans, tubers, fruits, honey, syrup.

Fats

  • Structure: Triglycerides (3 fatty acids on glycerol backbone)

  • Functions: energy storage (unlimited), insulation, protection of organs, and cellular structure.

  • Sources: meat, oils, dairy products.

Proteins

  • Composition: Nitrogen-containing macromolecules of amino acids.

  • Functions: muscle structure, connective tissue, skin, enzymes for growth and maintenance

  • Essential amino acids: 8 out of 20 must be consumed through diet.

  • Sources: meat, dairy, beans, and pulses.

Determining Energy Content of Food

Method: Bomb Calorimetry

  • Oxidation of food in a bomb calorimeter measures energy released.

  • Procedure: Food is ignited with oxygen under high pressure, producing heat.

  • Measurement: Heat rise in water jacket around combustion chamber leads to calculation of gross energy (GE).

Energy Flow through the Body

  • Formula:

    • Ingested Energy (IE) = Gross Energy (GE) - Faecal Energy (FE) - Urinary Energy (UE)

    • Metabolizable Energy (ME) also includes gaseous losses and various energy expenditures.

Metabolizable Energy (ME)

  • Formula: ME = Gross Energy - (Energy in Faeces + Energy in Urine)

  • A comprehensive measurement may require bomb calorimetry of food, faeces, and urine or the use of Atwater factors.

Atwater Factors

General Overview

  • Atwater Factors allow for energy content calculation of foods based on macronutrients.

  • Table of Atwater Factors:

    • Glucose: 15.6 kJ/g (3.7 kcal/g)

    • Starch: 17.5 kJ/g (4.2 kcal/g)

    • Fat: 39.1 kJ/g (9.4 kcal/g)

    • Protein: 22.9 kJ/g (5.5 kcal/g)

    • Ethanol: 29.7 kJ/g (7.1 kcal/g)

Specifics of Atwater Factors

  • Energy produced during metabolism is lower than combustion due to losses.

  • Energy yield per gram:

    • 1 g CHO → 3.75 kcal (16 kJ)

    • 1 g Protein → 4 kcal (17 kJ)

    • 1 g Fat → 9 kcal (37 kJ)

    • 1 g Alcohol → 7 kcal (29 kJ)

Applications

  • Carbohydrates yield different energy amounts based on their category (monosaccharides, disaccharides).

  • Atwater factors may vary based on food processing and type.

Applications in Food Composition

  • Key resources include McCance & Widdowson and USDA databases for food composition.

  • Evaluation methods include bomb calorimetry and various food intake methods (diaries, AIM).

Energy Density of Foods

Definition

  • Energy Density: The amount of energy per gram of food, often expressed in calories (kcal) or kilojoules (kJ).

Factors Influencing Energy Density

  • Determined primarily by water content:

    • Foods with high water content generally have low energy density (e.g., fruits and vegetables).

    • Example calculations demonstrate varying energy values based on water content, e.g., 1.4 kcal/g for boiled rice (70% water) vs. 3.8 kcal/g for raw rice (11% water).

Categories of Energy Density in Foods

  1. High Energy Density Foods:

    • Example: Walnuts (6.9 kcal/g), butter (7.5 kcal/g)

    • Characteristics: Low water content and high fat.

  2. Low Energy Density Foods:

    • Characteristics: High water and fiber content leading to lower caloric values.

    • Example: Fruits and vegetables with ~80-95% water.

Practical Implications

  • Low energy density (LEDF) foods can promote satiety and appetite control, contributing to weight management.

Sample Menus

  • High Energy Density Menu:

    • Total: 2584 kcal

  • Low Energy Density Menu:

    • Total: 1686 kcal

  • Examples demonstrate the difference in calorie intake from meals with contrasting energy densities.

Conclusion

Importance of Understanding Energy Sources

  • The dietary energy mix influences health outcomes and disease risk factors.

  • Awareness and calculations help individuals adhere to dietary goals, especially low-fat diets.

Summary

  • Food energy is quantified in calories or joules.

  • Main dietary energy sources are carbohydrates, fats, proteins, and alcohol.

  • Energy content and density affect meal planning and nutritional adequacy.