AS 2: Diet, Lifestyle and Health — Energy and Energy Balance
Welcome to your study notes for Energy and Energy Balance! Energy is the fuel that powers everything our bodies do, from running a sprint to simply keeping our hearts beating while we sleep. In this chapter, we will break down where energy comes from, how our bodies use it, how to calculate energy requirements, and how energy imbalance links directly to body weight and health.
Don't worry if the calculations or definitions seem tricky at first — we will break every concept down into clear, manageable steps!
---1. Components of Energy Expenditure
Our bodies expend (use up) energy throughout the day in three main ways: Basal Metabolic Rate (BMR), Physical Activity Level (PAL), and the Thermic Effect of Food (TEF).
A. Basal Metabolic Rate (BMR)
Definition: The minimum amount of energy required to maintain vital body functions (such as heartbeat, breathing, cellular repair, and body temperature) while the body is at complete rest in a temperate environment.
• Think of BMR as your body's "idling engine." Even if you stay in bed all day without moving, your body still burns a substantial amount of energy just to keep you alive.
• BMR accounts for the largest proportion of total daily energy expenditure for most people.
B. Physical Activity Level (PAL)
Definition: A numerical value used to express a person's daily physical activity, used to estimate total energy expenditure.
• While BMR is about survival at rest, PAL represents the energy used during movement and physical tasks (e.g., walking, exercising, working).
• A sedentary person has a lower PAL value, whereas an active athlete has a much higher PAL value.
C. Thermic Effect of Food (TEF)
Definition: The energy required for the digestion, absorption, and disposal of nutrients from the food we consume.
• When you eat, your digestive tract has to do work: breaking down macronutrients, transporting them across gut walls, and metabolising them.
D. Total Energy Expenditure (TEE)
To calculate a person's estimated Total Energy Expenditure (TEE), we multiply their baseline metabolic needs by their physical activity:
\(\text{Total Energy Expenditure (TEE)} = \text{BMR} \times \text{PAL}\)
Key Takeaway: Energy expenditure is made up of vital survival functions (\(\text{BMR}\)), daily movement (\(\text{PAL}\)), and digestion (\(\text{TEF}\)). Total energy used is estimated as \(\text{BMR} \times \text{PAL}\).
---2. The Energy Balance Equation
Energy Balance is the scientific relationship between energy intake (food and drink consumed) and energy expenditure (energy used by the body).
The Three States of Energy Balance
1. Neutral Energy Balance:
\(\text{Energy Intake} = \text{Energy Expenditure}\)
• Body weight remains stable over time.
2. Positive Energy Balance:
\(\text{Energy Intake} > \text{Energy Expenditure}\)
• More energy is consumed than the body uses.
• The excess energy is stored as adipose tissue (body fat), leading to weight gain and, over time, an increased risk of becoming overweight or obese.
3. Negative Energy Balance:
\(\text{Energy Intake} < \text{Energy Expenditure}\)
• The body expends more energy than it consumes.
• The body must draw upon stored energy reserves (glycogen and adipose tissue) to meet its needs, leading to weight loss.
Exam Alert: Avoid describing energy balance simply as "eating a healthy diet." In the exam, always state the scientific balance between energy intake and energy expenditure.
Key Takeaway: Weight changes are driven by the energy balance equation: eating more energy than you expend leads to a positive balance (weight gain), while expending more than you take in leads to a negative balance (weight loss).
---3. Energy Density
Definition: The amount of energy (measured in kilocalories, \(\text{kcal}\), or kilojoules, \(\text{kJ}\)) per gram of food.
• High Energy-Dense Foods: Contain a large amount of energy per gram. These foods are typically high in fats and oils (e.g., fried foods, butter, pastries). Because fat provides more energy per gram than protein or carbohydrate, high-fat foods pack significant calories into small serving sizes, making it easy to consume an excess of energy and enter a positive energy balance.
• Low Energy-Dense Foods: Contain fewer calories per gram. These foods are usually high in water and dietary fibre (e.g., vegetables, fruits, broth-based soups). They provide bulk and volume in the stomach, promoting fullness (satiety) with fewer total calories.
Key Takeaway: Understanding energy density helps explain why diets rich in high-fat, energy-dense foods contribute rapidly to positive energy balance and weight gain.
---4. Measuring Body Composition & Health Risks
To evaluate whether an individual's energy balance has led to a healthy weight or an increased risk of chronic diseases, health professionals use standard assessment tools.
A. Body Mass Index (BMI)
Body Mass Index (BMI) is a standard calculation used to assess whether an adult is a healthy weight for their height.
Formula:
\(\text{BMI} = \frac{\text{Weight (kg)}}{(\text{Height (m)})^2}\)
BMI Classifications:
• Underweight: \(< 18.5\)
• Healthy weight: \(18.5 - 24.9\)
• Overweight: \(25.0 - 29.9\)
• Obese: \(30.0+\)
Step-by-Step BMI Calculation Example:
A person weighs \(75\text{ kg}\) and is \(1.75\text{ m}\) tall.
Step 1: Square the height: \(1.75 \times 1.75 = 3.0625\)
Step 2: Divide the weight by the squared height: \(\frac{75}{3.0625} \approx 24.49\)
Step 3: Interpret the result: A BMI of \(24.49\) falls into the Healthy weight category (\(18.5 - 24.9\)).
Watch Out for Maths Traps:
1. Always convert height to metres (e.g., \(175\text{ cm} = 1.75\text{ m}\)).
2. Always remember to square the height (\(\text{height} \times \text{height}\)) before dividing.
B. Waist-to-Hip Ratio (WHR)
While BMI measures overall body weight relative to height, it does not assess where fat is stored. Waist-to-Hip Ratio (WHR) evaluates fat distribution around the abdomen (central adiposity).
Formula:
\(\text{WHR} = \frac{\text{Waist circumference}}{\text{Hip circumference}}\)
Health Risk Thresholds:
• Men: Health risk increases significantly if \(\text{WHR} > 1.0\)
• Women: Health risk increases significantly if \(\text{WHR} > 0.85\)
Central fat storage around vital abdominal organs is strongly linked to higher risks of cardiovascular disease and type 2 diabetes.
Key Takeaway: BMI compares weight to height squared to place individuals into weight bands (\(< 18.5\) underweight, \(18.5 - 24.9\) healthy, \(25 - 29.9\) overweight, \(30+\) obese). WHR measures abdominal fat distribution, with increased risk above \(1.0\) for men and \(0.85\) for women.
---5. SACN Energy Reference Values
The Scientific Advisory Committee on Nutrition (SACN) sets Estimated Average Requirements (EAR) for energy in the UK (2011 guidelines).
• Adult Women: Approximately \(2000\text{ kcal/day}\) (or \(\approx 8.4\text{ MJ/day}\))
• Adult Men: Approximately \(2500\text{ kcal/day}\) (or \(\approx 10.5\text{ MJ/day}\))
Note: These values represent population averages and vary according to age, gender, body composition, and physical activity levels.
Key Takeaway: Remember the benchmark SACN 2011 EAR values for energy: \(\approx 2000\text{ kcal}\) for women and \(\approx 2500\text{ kcal}\) for men.
---6. Examiner Pitfalls & Revision Checklist
Make sure you avoid these common mistakes highlighted in CCEA Chief Examiner reports:
• Don't confuse BMR and PAL: BMR is the energy for basic survival and organ function at rest; PAL is the multiplier for physical movement and activity.
• Use precise definitions: Always define energy balance in terms of energy intake versus energy expenditure, rather than general "healthy eating."
• Check your BMI maths: Convert centimetres to metres first, square the height, and write down your units.
• Use technical vocabulary: Use the term energy density to explain why high-fat foods contribute quickly to positive energy balance.
• Link obesity to energy imbalance: When explaining how obesity develops, clearly explain the sustained positive energy balance (intake exceeding expenditure over time) that causes excess adipose tissue storage.