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1.2.4Topic 1 · Lifestyle, Health and Risk

Energy Budgets and Diet — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 8 parts Lesson 16 of 23 Unit quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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1.2.4 Energy Budgets and Diet — Edexcel A Level Biology A (SNAB) 9BI0 notes: Explain the concept of an energy budget, calculate body mass index and energy requirements, and describe how diet affects health.The body needs a continual supply of energy for metabolic reactions, activ…

  • kilojoules (kJ)
  • basal metabolic rate (BMR)
  • Body mass index (BMI)
  • waist-to-hip ratio
  • Being underweight
  • Saturated fats

Part 1 of 6Energy in and energy out

The body needs a continual supply of energy for metabolic reactions, active transport, movement, growth and maintaining body temperature. This energy comes from the chemical energy of food, measured in kilojoules (kJ) or kilocalories (kcal), where 1 kcal is about 4.2 kJ. The balance between intake and expenditure is the energy budget:

Energy in and energy out
If energy intake equals energy expenditure, body mass stays constant.
If intake exceeds expenditure, the surplus is stored mostly as fat in adipose tissue and body mass rises.
If intake is lower than expenditure, stores are broken down and mass falls.

Energy density differs between food types

  • lipids supply roughly 37 kJ per gram, carbohydrates and proteins about 17 kJ per gram.
  • A diet rich in fat therefore provides a lot of energy in a small mass.

Part 2 of 6Energy requirements

Total daily energy expenditure includes the basal metabolic rate (BMR), the energy needed at rest to maintain body functions, plus the energy cost of digesting food and of physical activity.

Energy requirements
BMR varies with age, sex, body mass (muscle uses more energy than fat), hormones and body temperature.
Needs change with life stage: growing children, pregnant and breastfeeding women and very active people need more than the average.
Energy used in an activity can be measured with a calorimeter, or with a respirometer by oxygen uptake.

Part 3 of 6Body mass index and waist-hip ratio

Body mass index (BMI) = mass in kg / (height in m)2. For a person of mass 80 kg and height 1.75 m, BMI = 80 / 3.0625 = 26.1 kg m-2.

BMI (kg m-2)Classification
Below 18.5Underweight
18.5 to 24.9Healthy range
25 to 29.9Overweight
30 or aboveObese

BMI is simple but does not distinguish muscle from fat, so a muscular athlete may be classed as overweight. The waist-to-hip ratio indicates where fat is stored; fat around the abdomen is more strongly linked to cardiovascular disease and type 2 diabetes than fat on the hips.

Part 4 of 6Energy values and measuring energy in food

The energy content of a food can be measured by burning a known mass under a boiling tube of water and measuring the rise in temperature.

1
Energy transferred = mass of water (g) x 4.2 J per g per degree C x temperature rise.
2
If burning 1.0 g of a snack raises 20 g of water by 25 degrees C, the energy is 20 x 4.2 x 25 = 2100 J, or 2.1 kJ per g.
3
The result underestimates the true value because heat is lost to the surroundings and combustion may be incomplete.
4
Improvements include shielding, a calorimeter and stirring the water.

Food labels give energy per 100 g and per portion, and they may use traffic-light colours for fat, saturated fat, sugar and salt.

Energy values and measuring energy in food
Interpreting a label involves calculation: if a serving of 40 g has 800 kJ per 100 g, then it supplies 40 / 100 x 800 = 320 kJ.
A person who increases intake by 1000 kJ per day without raising activity will gain mass, since about 37 kJ is stored in each gram of fat.

Part 5 of 6Diet and health

Obesity
Obesity increases the risk of CVD, type 2 diabetes, some cancers and joint problems.
Being underweight
Being underweight can cause weakened immunity, loss of bone density and menstrual problems.
Saturated fats
Saturated fats raise LDL and CHD risk; trans fats are particularly harmful; salt raises blood pressure.
Dietary fibre
Dietary fibre aids the movement of food through the gut and lowers cholesterol.
Sugar
Sugar consumed in excess is easily stored as fat and contributes to dental decay.

A balanced diet contains the right quantities of carbohydrates, lipids, proteins, vitamins, minerals, fibre and water. Evidence from epidemiological studies forms the basis of dietary recommendations, such as guidelines to limit saturated fat and salt, and to eat at least five portions of fruit and vegetables a day.

Underlying all of this is the idea of homeostasis of body mass: appetite hormones such as leptin from fat cells, and ghrelin from the stomach, influence hunger, but environment, habit and genetics also matter, so weight management is rarely simple. Health advice should therefore be based on evidence and delivered sensitively.

Part 6 of 6Exam-style questions

  1. Calculate the BMI of a person of mass 92 kg and height 1.80 m, and classify them. [3]3 marks

  2. Explain why BMI may be an unreliable indicator of health for a professional rugby player. [2]2 marks

  3. A snack contains 12 g of lipid, 30 g of carbohydrate and 5 g of protein. Using 37 kJ per g for lipid and 17 kJ per g for the others, calculate its energy content. [3]3 marks

Key terms

TermMeaning
Energy budgetThe balance between energy intake and energy expenditure.
Basal metabolic rateEnergy used per unit time at rest to maintain body functions.
Body mass indexMass in kg divided by the square of height in metres.
ObesityHaving a BMI of 30 or above (excess body fat).
Waist-hip ratioWaist circumference divided by hip circumference.
Energy densityAmount of energy per gram of a food.
KilojouleUnit of energy equal to 1000 joules.
Dietary fibrePlant material not digested by humans that aids gut function.

Avoid these mistakes

1
Using height in cm in the BMI formula.
2
Saying that fat is only stored when you eat fat. Any excess energy can be stored as fat.
3
Mixing kJ and kcal.
4
Saying BMR is the energy used during exercise.
5
Stating that BMI above 25 always means unhealthy.

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