• 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
• Energy budget can be calculated in the following way

energy input - energy output = energy budget

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

Energy imbalances can affect weight gain

• Energy budgets should be balanced
  • The amount of energy taken in should equal the amount of energy used, or transferred
• A difference in energy taken in and energy used will affect an individual's weight
• Weight gain
  • If?energy intake is higher than the energy output?excess energy will be converted into fats by the body so the person will?gain weight
  • If the energy output remains less than intake over a sustained period of time the individual may become overweight and eventually obese
• Weight loss
  • If?energy intake is less than energy output?the body will need to take energy from elsewhere and fat reserves will be converted into energy, the person will?lose weight
  • If the energy difference is large over a sustained period of time the individual may become underweight
• Just as an aside here; current research suggests that 'not all calories are equal', so the balancing of the energy budget in the way described here is likely to be more complicated in reality
  • The body is thought to process different foods in different ways, e.g. fats vs sugars, sucrose vs fructose, etc.