Energy Budgets & Diet
- Every cell needs a constant supply of energy to fuel metabolic processes; this energy is released during the process of respiration
- Organisms therefore need to take in enough food to fuel respiration
- Food contains stored energy in the form of biological molecules such as carbohydrates and lipids
- The term energy budget describes the amount of energy taken in by an organism minus the amount of energy the organism transfers during life processes such as growth, movement, reproduction, and respiration
- The transfer of energy to the environment during the life process is sometimes referred to as energy 'used'
Analysing data on energy budgets and diet
- Data on energy budgets may be given in Calories, kilocalories, or kilojoules
- A Calorie (Cal) is the same as a kilocalorie (kcal)
- The average adult needs around 2 000 kcals per day to maintain a healthy weight
- A kilojoule (kJ) is an alternative energy unit
- The average adult needs about 8 700 kJ a day to maintain a healthy weight
- A Calorie (Cal) is the same as a kilocalorie (kcal)
- Energy budget can be calculated in the following way
energy input - energy output = energy budget
Worked example
A student's recommended daily intake of energy is 12 400 kJ.
Mark is a male student who takes in the recommended daily intake of energy; he is in school for six hours a day, goes for a run for an hour after school, sleeps for eight hours a night, and spends two hours a day eating meals.
Use the graph below to calculate his energy budget over a 24 hour period.
Step 1: Use the graph to find the energy expenditure for each activity for males
School = 4 kJ/min
Running = 17 kJ/min
Sleep = 3 kJ/min
Meals = 5 kJ/min
Step 2: Convert values into the number of hours the student spends on each activity
School = (4 x 60) x 6 = 1 440 kJ in 6 hrs
Running = 17 x 60 = 1 020 kJ in 1 hr
Sleep = (3 x 60) x 8 = 1 440 kL in 8 hrs
Meals = (5 x 60) x 2 = 600 kJ in 2 hrs
Step 3: Add up the energy expenditures to find the energy output
1440 + 1020 + 1440 + 600 = 4 500 kJ
Step 4: Substitute numbers into the energy budget equation
energy input - energy output = energy budget
12 400 - 4 500 = 7 900 kJ
This tells us the student has an excess of 7 900 kJ of energy every day