1.2.4 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
• 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.