Lecture 19: Energy from food

Question 1

What is the first law of thermodynamics?

Answer 1

That the total energy within a system is constant

- energy can neither be created or destroyed

Question 2

What are the units of energy ?

Answer 2

Joule (J) = energy required to push against 1 Newton of force for 1 meter
1 cal = 4. 184 J
1 Cal = 4. 184 kJ

Question 3

How is energy content measured

Answer 3

Energy content is measured by oxidising food samples in a bomb calorimeter. A measured amount of dry food is placed inside the calorimeter in an atmosphere of oxygen is ignited.
- The amount of heat released is measured by the increase in temperature of the water jacket and thus the heat of combustion can be calculated

Question 4

What is basal metabolism and what factors affect it?

Answer 4

BMR is the energy required for maintenance of life: for muscle contractions, nerve conduction, ion transport, synthesis of macromolecules and maintenance of body heat
= energy expenditure at rest
BMR is affected by:
Gender, age, body size, genetics, hormonal status, stress levels, disease status and certain drugs

Question 5

What increases basal metabolism?

What decreases basal metabolism?

Answer 5

Increased by:
- Athletic training  - Late pregnancy 
- Fever    - Drugs i.e caffeine
- Hyperthyroidsim
Decreased by:
- Malnutrition     - Sleep
- Drugs i.e blockers     - Hypothyroidism

Question 6

What is direct calorimetry?

What does it do?

Answer 6

• Relies on measuring heat output from an individual in a whole body calorimeter
• Good from determining BMR

= Measure energy expenditure

Question 7

What is indirect calorimetry?

What does indirect calorimetry allow you to calculate ?

Answer 7

• Based on O2 consumption and CO2 production measured using a respirometer
• Certain amount of energy is associated with every litre of O2 consumed
  = Allows calculation of energy expenditure for a range of activities
  = Allow for the respiratory exchange ratio (RER)

Question 8

What are atwater factors?

Answer 8

Not all the energy in food is available to humans

• as there are some losses in faeces
• nitrogen is not oxidised

Question 9

What equations are used to determine Energy balances in the body?

Answer 9

E intake = E expended + E stored

E expended = Basal metabolism + activities

• a small amount of E is lost in faeces urging and sweat