Energy Sources

Question 1

1. Define energy.

Answer 1

• The ability or capacity to work

Question 2

2. What is the 1st law of thermodynamics?

Answer 2

• Energy cannot be created or destroyed, only converted from one form to another

Question 3

3. Define a calorie.

Answer 3

• Calorie: The amount of heat energy needed to raise the temperature of 1kg of water by 1°C

Question 4

4. What is the conversion from kcal to kJ?

Answer 4

• 1 kcal = 4.184 kJ

Question 5

5. What are the main energy sources from food?

Answer 5

• Carbs > Fat > Protein

Question 6

6. Describe the use of a bomb calorimeter to determine the energy content of food.

Answer 6

• An electrical element is used to ignite the food, within a combustion chamber. Food is oxidised to produce water, CO2 and nitrogen oxides. The released energy is measured as a heat rise in a water jacket surrounding the combustion chamber.
• Total energy of the foodstuff = Gross Energy (GE) which is the heat given off from this full oxidation

Question 7

7. Describe the food energy flow through the body.

Answer 7

• Ingested Energy (IE)/ Gross Energy (GE) –> Digestible Energy (due to loss via faecel energy and combustible gases from microbial fermentation) –> Metabolizable Energy (ME) due to loss of urinary and surface energy –> Net ME (due to obligatory thermogenesis) –> Net Energy for Maintenance (used for basal metabolism and physical activity)

Question 8

8. How do you calculate ME? And what is the easier alternative?

Answer 8

• GE - [energy in urine and faeces], easier to use Atwater factors

Question 9

9. What are the Atwater factors for CHO, Protein, Fat and Alcohol and what is their purpose?

Answer 9

• CH0 = 3.75 kcal/g, Protein = 4 kcal/g, Fat = 9kcal/g, Alcohol = 7kcal/g. These adjust for the losses in energy due to faeces and urine, they correct the ME value.

Question 10

10. What two factors does the Atwater system depend on?

Answer 10

• The variable heat of combustion for protein/CHO/fat and food processing & fibre content

Question 11

11. What is the main determinant of energy density and dry energy density?

Answer 11

• Water content and dietary fat

Question 12

12. Give examples of high energy density foods.

Answer 12

• Walnuts, pastry, butter, fat spreads

Question 13

13. Give examples of low energy density foods.

Answer 13

• Fruit and veg

Question 14

14. Define palaeolithic diets.

Answer 14

• Diets of early homo sapiens, there was probably more energy from protein than carbs, compared to the modern Western diet